United States 

Department of 

Agriculture

Forest 

Service

North Central 

Research Station

General Technical 

Report NC-243

BLACK WALNUT INA

NEW CENTURY

PROCEEDINGS OF THE

6TH WALNUT COUNCIL RESEARCH SYMPOSIUM
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Cover Photo by: James McKenna

Published by:

North Central Research Station

Forest Service–U.S. Department of Agriculture

1992 Folwell Avenue

St. Paul, MN 55108

2004

Web site: www.ncrs.fs.fed.us
Black Walnut in a New Century

Proceedings of the 

6th Walnut Council Research Symposium

Lafayette, Indiana

July 25-28, 2004

Editors:

C.H. Michler

P.M. Pijut

J. W. Van Sambeek

M. V. Coggeshall

J. Seifert

K. Woeste

R. Overton

F. Ponder, Jr.

Table of Contents i

Black Walnut in a New Century

Contents

TREE IMPROVEMENT

History of Black Walnut Genetics Research in North America ..................................................... 1

Erin Victory, Keith Woeste, and Olin E. Rhodes, Jr.

Walnut Genetic Improvement at the Start of a New Century ..................................................... 9

Keith E. Woeste and James R. McKenna

Cultivar Evaluation and Development for Black Walnut Orchards ............................................. 18

William Reid, Mark V. Coggeshall, and Kenneth L. Hunt

Persian Walnut Breeding in California .......................................................................................... 25

Charles A. Leslie and Gale H. McGranahan

PROPAGATION AND GENETIC MODIFICATION

Effect of Seed Position and Media on Germination of Black Walnut and Northern 

Red Oak: Implications for Nursery Production and Direct Seeding ..................................... 31

Anthony S. Davis, Barrett C. Wilson, and Douglass F. Jacobs

Vegetative Propagation of Butternut (Juglans cinerea) Field Results ....................................... 37

Paula M. Pijut

Genetic Transformation of Black Walnut (Juglans nigra) ............................................................ 45

Michael J. Bosela, Gurpreet S. Smagh, and Charles H. Michler

NURSERY PRODUCTION AND PLANTATION ESTABLISHMENT

Production and Early Field Performance of RPM® Seedlings in Missouri Floodplains ............. 59

Daniel C. Dey, Wayne Lovelace, John M. Kabrick, and Michael A. Gold

Facilitating Nutrient Acquisition of Black Walnut and Other Hardwoods at Plantation 

Establishment ........................................................................................................................... 66

Douglass F. Jacobs and John R. Seifert

Soils and Nutrition Management for Black Walnut ..................................................................... 71

Felix Ponder, Jr.

NATURAL STAND MANAGEMENT

Site Relationships and Black Walnut Height Growth in Natural Stands in Eastern 

Kansas ....................................................................................................................................... 77

Wayne A. Geyer and Felix Ponder, Jr.

Managing Black Walnut in Natural Stands: The Human Dimension ......................................... 81

H.E. “Hank” Stelzer
ii Table of Contents

Proceedings of the 6th Walnut Council Research Symposium

AGROFORESTRY AND NUT PRODUCTION

Ground Cover Management in Walnut and Other Hardwood Plantings ................................... 85

J.W. Van Sambeek and H.E. Garrett

Girdling Eastern Black Walnut to Increase Heartwood Width ...................................................101

Larry D. Godsey, W.D. “Dusty” Walter, and H.E. “Gene” Garrett

Use of Herbicide to Reduce Stump-sprouting Following Thinning of an Eastern 

Black Walnut Agroforestry Planting .......................................................................................106

W.D. “Dusty” Walter, H.E. “Gene” Garrett, and Larry D. Godsey

WALNUT PEST MANAGEMENT

Diseases of Intensively Managed Eastern Black Walnut ...........................................................110

Manfred E. Mielke and Michael E. Ostry

Spread of Butternut Canker in North America, Host Range, Evidence of Resistance 

Within Butternut Populations and Conservation Genetics .................................................114

M.E. Ostry and K. Woeste 

Insects Attacking Black Walnut in the Midwestern United States .......................................... 121

Steven Katovich

Control of Deer Damage with Chemical Repellents in Regenerating Hardwood Stands ...... 127

Brian J. MacGowan, Larry Severeid, and Fred Skemp, Jr.

INTERNATIONAL WALNUT RESEARCH

A New Tree Improvement Programme for Black Walnut in the United Kingdom ................... 134

Karen Russell and Gabriel E. Hemery

Genetic and Silvicultural Research Promoting Common Walnut (Juglans regia) for 

Timber Production in the United Kingdom .......................................................................... 138

Gabriel E. Hemery

Walnut Tissue Culture: Research and Field Applications .......................................................... 146

Jose M. Lopez

Current Research in Spain on Walnut for Wood Production ..................................................... 153

Neus Aletà

ECONOMICS OF WALNUT PRODUCTION

Beyond theWild Nut: Moving Toward Profitable Black Walnut Nut Crops .............................. 156

Brian Hammons, Felix Ponder, Jr., and John Rickman

Factors Affecting the Quality of Walnut Lumber and Veneer .................................................... 161

Daniel L. Cassens

The Black Walnut Resource in the United States ...................................................................... 168

Stephen R. Shifley
Table of Contents iii

Black Walnut in a New Century

SHORT NOTE

Performance of Black Walnut in the Yellow River Watershed of the People’s 

Republic of China .....................................................................................................................177

J. W. Van Sambeek, Sheng-Ke Xi, William A. Gustafson, and Mark V. Coggeshall

POSTER SESSION

Finding a Walnut in a Haystack: How Molecular Biology Can Help Rediscover Lost 

“Elite” Black Walnuts ............................................................................................................. 183

Rodney L. Robichaud, James McKenna, and Keith Woeste

Relationships Between Tree Size and Nut Production for Eastern Black Walnuts .................. 184

David Brauer, Adrian Ares, and Andrew Thomas

Sugar-bushing Black Walnut ........................................................................................................ 185

G. G. Naughton, W. A. Geyer, and D. L. Chambers

Arthropod Communities in Monocropped and Black Walnut-Intercropped Forages ............. 186

W. Terrell Stamps, Terry L. Woods, Robert L. McGraw, and Marc J. Linit

Climatic Factors and Black Walnut Nut Production .................................................................. 187

Neal Sullivan, Felix Ponder, Jr., and Viniece Jennings

How Much Regeneration is Needed to Minimize Genetic Erosion via Timber Harvest? ....... 188

Jeffrey C. Glaubitz, Rodney Robichaud, Keith Woeste, and Olin E. Rhodes, Jr.

Tree Improvement 1

Black Walnut in a New Century

HISTORY OF BLACK WALNUT GENETICS RESEARCH IN NORTH AMERICA

Erin Victory; Keith Woeste; and Olin E. Rhodes, Jr.1

ABSTRACT—Eastern black walnut (Juglans nigra L.) is an economically and 

ecologically important hardwood species that has been used throughout the history of 

settlement in North America. It was a resource that helped Native Americans in their 

everyday life, it helped European settlers carve a living out of the wilderness, and it 

has helped rural farmers and private landowners subsist and invest in the future. 

Described here is a brief history of black walnut breeding and molecular genetics 

research. Current genetic research may ultimately lead to the domestication of black 

walnut, an event that would be a hallmark for forest tree species.

Eastern black walnut (Juglans nigra L.) has been 

a component of North American forests since the 

Middle to Upper Cretaceous (Elias 1980) and has 

been used by humanity for centuries. This species 

has never held a predominant position in temperate 

forests, but it has been a consistent feature in the 

Central Hardwood Region. Almost every part of 

the tree has played some valuable role in helping 

people subsist, including consumption of the nuts 

and the use of nut husks for dyes, and use of the 

wood for fencing, railroad ties, firewood, gunstocks, 

and fine furniture. Black walnut also is an 

important tree for forest animals, providing shelter 

and forage especially for squirrels. 

In the 20th century, black walnut researchers 

began to study the ecology, reproductive biology, 

and phenotypic variation of this valuable species. 

Eventually breeding programs were initiated to 

find black walnuts of sufficient growth rate and 

architecture to meet the demands of industry. 

Although much is now known about black walnut 

biology and natural history, this quest continues 

today, after over 60 years of effort. The purpose 

of this is to provide a summary of improvement 

programs, investigations of vegetative propagation, 

advances in taxonomic relationships within the 

Juglandaceae, parentage, gene flow, and studies of 

genetic diversity made possible by molecular genetics.

NUT CULTIVARS

Landowners have identified and selected walnut 

trees for propagation since the late 1800s (Zarger 

1969). Emphasis on cultivation in the early 20th

century was focused on nut tree culture and 

propagation (MacDaniels 1933, Talbert 1942). 

Gradually, an awareness grew that quality walnut 

trees were becoming uncommon due to the clearing 

of land for agriculture and overharvesting. A 

tree planting movement began with the goal of 

restoring the diminishing supply of quality black 

walnut (Record 1986, U.S. Forest Service 1966). 

Contests sponsored by the Northern Nut Growers 

Association and State agencies were conducted 

with the intent of discovering new cultivars (Crane 

and Reed 1937). 

The Tennessee Valley Authority spent many years, 

from 1934-1960, identifying trees and cultivars 

that were well adapted to the valley (Zarger 1969). 

McKay (1971) published his lifelong breeding 

program to develop a Persian walnut and eastern 

black walnut hybrid with superior nut production, 

with little success. By the end of the 1960s, there 

were 18 nut cultivars that were widely used for nut 

production in the Central Hardwood Region (Zarger 

1969), but efforts towards an improved timber 

variety were just beginning.

1

Graduate Student (EV), Professor (OER), Department of Forestry and Natural Resources, 195 Marstellar St., Purdue University, West 

Lafayette, IN 47906; and Research Molecular Geneticist (KW), Hardwood Tree Improvement and Regeneration Center, North Central Research 

Station, United States Department of Agriculture Forest Service, 170 South University St., Purdue University, West Lafayette, IN, 47906. EV is 

corresponding author: to contact, call (765) 494-3997 or email at erin@fnr.purdue.edu. 

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., eds. 

2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. Tech. Rep. NC243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p. 
2 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

TREE IMPROVEMENT

In the 1960s there was a shift from “mining” 

walnut trees from natural stands to a conscientious 

effort towards improving black walnut as a crop. 

The impetus for this was a perceived decrease in 

the availability of high quality walnut timber in 

natural stands, a condition attributed to the longstanding practice of high-grading (Clark 1965, 

Elias 1980). Reduced supplies of high quality 

trees increased the price of walnut timber leading 

growers and industry to recognize the need for 

high quality, fast-growing trees to meet increasing 

market demands. The increasing demand for black 

walnut led the USDA Forest Service to initiate their 

Black Walnut Genetics Project at Carbondale, IL 

in 1965 (Funk 1966), a research effort focused on 

progeny testing, seed sources, and the heritability 

of apical dominance. The mission was to gather 

and provide genetic information with which to base 

a breeding program for superior trees (Caraway 

1976). 

By the mid-1960s, several provenance studies 

were already underway, with plans to begin 

hybridization research (Clark 1965). However, 

while information gathered at the beginning of 

the century regarding pollination, vegetative 

propagation, and seed germination provided a 

sound foundation for basic research, there were 

virtually no data on the genetics of the species 

(Wright 1966). To address questions related to 

regional adaptability and variability, in the absence 

of genetic data, seed zones were proposed for the 

Central States based upon temperature, annual 

precipitation, occurrence of frost in the spring and 

fall, the number of frost-free days, latitude, and 

summer day length (Deneke and others 1981). Seed 

zones are regions representing a limited range 

of environmental variance such that the seeds 

collected anywhere in a zone could be planted in 

the same zone with some assurance of success. 

By the 1970s, black walnut research had been 

focused on tree improvement for almost a decade, 

but there remained a lack of high quality product 

(seeds or seedlings) to meet the demands of a 

growing industry. In a summarization of what 

was known about taxonomy, evolution, sexual 

reproduction, and breeding and improvement 

programs of black walnut, Funk (1970) stated that 

black walnut breeding programs should focus on 

climatic adaptation, vigor and growth rate, form, 

fecundity, and pest resistance. In describing the 

importance of the natural phenotypic variation in 

walnut, both Funk (1973) and Bey (1970, 1973) 

found that there were more differences among seed 

sources than within stands or areas. For instance, 

Bey’s research on six year-old provenance studies 

from six States in the Midwest indicated that the 

origin of seed collection affected the timing of 

leaf flush and leaf fall, and tree height, with trees 

from southern sources flushing earlier, losing 

their leaves later, and being generally taller and 

of larger diameter. Bey (1980) again used these 

provenances to determine that geographic origin 

was an important component when considering 

seed sources and recommended choosing trees up 

to 200 miles south of the planting site as the tallest 

20% of the trees in his study were from sources 

south of the planting site. His results indicated 

that though sources beyond 200 miles south of 

the planting site were taller and bigger than trees 

from northern sources, they also were more prone 

to injury or death due to frost and cold winters. 

This finding was again confirmed by Geyer and 

Rink (1998) when they took diameter, height, 

and survival measurements on a 17-year-old 

provenance test in southern Illinois. 

An outcome of Bey’s (1973) research was his 

conclusion that future research needed to 

determine the importance of trade-off’s in 

trait selection, how successful silvicultural 

manipulations were in addressing problems, and 

the effectiveness of selecting wild trees based on 

phenotype. 

By 1982, 11 States within the walnut commercial 

range had established walnut improvement 

programs, from provenance and progeny testing, 

to second-generation seed orchards (Rink and 

Stelzer 1982). For example, nine timber varieties 

(thought to be superior in growth rate, form, 

or seed yield [Beineke 1984]) were selected and 

propagated at Purdue University. Moreover, in 

the same decade, several regional cooperatives 

were established: the NC-99 program, and the 

eight-state Fine Hardwood Tree Improvement 

Cooperative. NC-99 began in 1960 (then known as 

NC-51) and focused on determining the range and 

pattern of genetic diversity, breeding, selection and 

seed production, breeding zones, and advanced 

generation breeding for species of economic 

importance. NC-99 contributed significantly to the 

knowledge accumulated on breeding black walnut 

(Stier 1986). The eight-state Fine Hardwood Tree 

Improvement Cooperative began in 1986 to combine 

efforts towards genetic improvement of primarily 

black walnut and northern red oak in the north 

central region. The optimism that was a hallmark 

of the early efforts towards black walnut timber 

breeding was tempered in later years as it became 

clear that walnut was extremely site sensitive and 

that significant environment-genotype interactions 

would confound genetic effects and complicate their 

interpretation (Geyer and Rink 1998). Today, efforts 

are still underway to breed a fast-growing, straightboled, black walnut timber variety. For more detail, 

see McKenna and Woeste (2004, this volume).
Tree Improvement 3

Black Walnut in a New Century

DISEASE MANAGEMENT

Of all the diseases and damaging agents that 

affect black walnut, only walnut anthracnose, 

caused by the fungus Gnomonia lepostyla, has 

been the subject of genetic research. Walnut 

anthracnose can severely damage a nut crop and 

hinder tree growth. Beginning in the 1970s, Black 

and Neely published a series of articles on walnut 

anthracnose and the factors influencing its spread 

(Neely 1979, Black and Neely 1978a). Resistance 

to anthracnose was considered an important 

goal of tree improvement programs, and Black 

and Neely (1978b) determined that while eastern 

black walnut was less susceptible than the Hinds 

(Juglans hindsii), hybrids between eastern black 

walnut and California black walnut varied in their 

resistance. For example, in a study of the disease 

susceptibility of walnut hybrids, black walnut 

crossed with the Hinds was the most susceptible 

(had the most lesions), while black walnuts crossed 

with Japanese walnut (Juglans ailantifolia) were 

the most resistant (Black and Neely 1978b). This 

research suggested that Persian and Japanese 

walnut were potential sources for resistance genes 

to anthracnose. 

Funk and others (1980) evaluated 62 black walnut 

families over 4 years for anthracnose incidence by 

visually estimating incidence of leaflets with one 

or more anthracnose-caused spots, estimating 

premature crown defoliation due to anthracnose, 

and determining the electrical resistance of 

cambial tissue. They also evaluated individual 

and family correlations among disease ratings and 

growth rates. Their results indicated that year to 

year correlations for anthracnose incidence were 

stronger for individual trees than family means, 

and that selecting for faster growth resulted in 

a 13% decrease in anthracnose incidence, while 

selecting for low anthracnose incidence resulted in 

only a 7% growth increase. 

TISSUE CULTURE

Vegetative propagation of black walnut became 

emphasized in the 1980s as a mechanism 

for circumventing problems associated with 

conventional propagation (Coggeshall and Beineke 

1997). Walnut trees produce fewer seeds per tree 

than many other species, and once those seeds are 

harvested, only a portion is viable. Poor seed yield 

and losses to predation prior to nut collection often 

result in an undersupply of seeds to sell or plant 

(Beineke 1982). Vegetative propagation potentially 

permits the almost limitless multiplication of 

selected genotypes without additional recourse 

to seeds. Grafting had long been practiced as a 

method for propagating black walnut, and a high 

rate of grafting success provided factors as scion 

wood storage, rootstock condition, and grafting 

technique and aftercare were optimized (Beineke 

1983). The multiplication of microshoots in vitro

was pursued as a promising method for producing 

ramets on their own roots and in association with 

research into the development of a transformation 

and regeneration system for black walnut (Van 

Sambeek and others 1997). Efficient rooting of 

micropropagated walnuts proved difficult, but 

by the late 1980’s somatic embryogenesis looked 

promising, and genetic engineering was being 

considered for black walnut (Rink 1989). Somatic 

embryogenesis experiments were in progress by 

1993 (Van Sambeek and others 1990, Long and 

others 1995, Neuman and others 1993, Khan 

1995). Since then, considerable progress has been 

made in tissue culture, for more detail, see Michler 

and Bosela (2004, this volume). 

MOLECULAR GENETICS

One of the first molecular-genetic studies of black 

walnut was the discovery of its chromosome 

number (n=16, Woodworth 1930). The development 

of allozyme markers in the 1980s made the study 

of black walnut population genetics feasible. 

Kung and Rink (1987) used eight allozyme loci 

to estimate the outcrossing rate and fixation 

index for trees sampled within one county in 

Illinois. The use of molecular tools to answer 

population level questions and questions related 

to tree improvement expanded in the 1990’s when 

researchers began to correlate isozyme allele 

frequencies with traits such as height and diameter 

(Kung and others 1991, Rink 1997). Additionally, 

throughout the 1990s allozyme markers were used 

to examine mating parameters, outcrossing rates, 

and fixation indices in black walnut (Rink and 

others 1989, 1994; Zhang 1990; Kung and others 

1991; Zuo 1994; Zuo and others 1995; Busov 1996). 

By the late 1990s questions focused on overall 

levels of genetic diversity and the partitioning of 

genetic variation among black walnut populations 

(Busov 1996, Busov and others 2002). 

The 1990s also marked a period in which 

morphological studies of walnut taxonomy were 

re-evaluated using molecular techniques. For 

example, Smith and Doyle (1995) found congruence 

between taxonomy of the Juglandaceae based on 

morphological (from literature including Manning’s 

1978 paper along with subsequent papers) and 

molecular data (chloroplast restriction site data). 

Their findings supported Manning’s (1978) 

classifications within the Juglandaceae (based 

upon inflorescences, flowers, and fruits) with the 
4 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

exception of the subfamilial status of Platycarya. 

Smith and Doyle’s findings suggested that either 

Engelhardieae should be moved to subfamilial 

status, or that Manning’s two subfamilies, 

Juglandoideae and Platycaryoideae, should be 

reduced to the tribal level. However, it was clear 

that Platycarya was grouped with Juglans, Carya, 

and Pterocarya, and was not a separate subfamily.

In 1995, Fjellstrom and Parfitt used species-level 

phylogenetic trees based on RFLP’s (restriction 

fragment length polymorphisms) to detect clear 

distinctions between the sections of Juglans

(e.g., old world walnuts vs. new world walnuts). 

Fjellstrom and Parfitt (1994b) also used RFLP’s 

to estimate the genetic diversity of 13 Juglans

species worldwide. Though no linkage maps exist 

specifically for the eastern black walnut, Fjellstrom 

and Parfitt (1994a) used the inheritance and 

linkage of 48 RFLP loci to establish a linkage map 

for Persian walnut. This linkage map for walnut 

was expanded by Woeste and others (1996) to total 

107 markers in 15 linkage groups. 

In the 21st century, Stanford and others (2000) 

described the phylogeny of Juglans using MATK 

(a chloroplast maturase-encoding gene) and ITS 

(Internal Transcribed Spacer) sequence data. They 

found all four sections of the genus branched out 

as commonly assumed. Rhysocaryon (i.e., old world 

walnuts vs. new world walnuts such as Persian 

walnut), to which eastern black walnut belongs, 

showed eastern and western species distinctions 

(e.g., J. nigra vs. J. hindsii), and also a distinction 

between temperate and tropical species (e.g., J. 

nigra vs. J. neotropica). Microsatellite loci were first 

published for black walnut by Woeste and others 

(2002); it is clear that microsatellites (SSRs) will 

replace isozymes as the molecular tool of choice 

to answer questions pertaining to walnut genetic 

diversity across regions, studies of parentage, and 

cultivar identification (Potter and others 2002, 

Ninot and Aleta 2003, Robichaud and others 2004, 

this volume). 

THE FUTURE?

Domestication, according to Kass (1993), is 

associated with seven activities: silvicultural 

manipulation, site enhancement, pest control, 

natural selection, semi-natural selection, 

anthropogenic selection, and a correlated response 

to selection. If Kass is correct, black walnut is 

well on its way to domestication. The first four 

activities are undoubtedly common factors to black 

walnut management, whether on plantations, or in 

managed woodlots. Semi-natural selection refers to 

a response by the tree to artificial factors brought 

about by socio-agricultural circumstances, such as 

tolerance to grazing animals, local site conditions, 

and ease of propagation. Human selection, the 6th 

criterion, involves actively pursuing an ideotype 

that is a culmination of the desired characteristics 

in a particular species. In black walnut, this 

would be traits such as fast growth, straightness, 

and a high proportion of heartwood in timber 

varieties, or early nut bearers, consistent yield, and 

a high proportion of kernel in nut tree varieties. 

It also includes a component of predictability, 

which would lead to an increase in crop yield, a 

characteristic still under development in black 

walnut. Site conditions and genetic factors are still 

too unpredictable to strive for, and consistently 

maintain, a black walnut ideotype. Persian walnut, 

on the other hand, is much farther along in this 

process than black walnut and there is probably an 

ideotype for that species that has more or less been 

attained. 

Kass’ last criterion (a correlated response to 

selection by the organism) also has not been 

met, as tree breeding and selection programs are 

too recent. Furthermore, although black walnut 

responds somewhat to management, it is not 

dependent upon it for survival. Black walnut has 

not yet evolved any dependence upon human 

manipulation to survive (Reid 1997). Persian 

walnut, though not dependent upon anthropogenic 

influence for survival, has been cultivated for 

centuries and the best varieties only grow well 

in highly modified environments (Ramos 1998). 

For Weirsum (1997), domestication is realized 

as increased people-plant interactions that 

result in morphological and genetic changes in 

the organism. He makes a distinction between 

biological and comprehensive definitions for 

domestication, the former referring to a gradual 

change in morphological and genetic characters 

for specific uses and environments, and the 

latter referring to changes in management and 

exploitation of the plant that brings about changes 

in the morphology and genetics of the species. 

So far, black walnut has been recognized as a 

valuable tree species and has been harvested 

out of natural forests, propagated on farms, in 

experiments, and in provenance studies and 

plantations. Its biology, and especially reproductive 

biology, has been examined, and it has been 

clonally propagated via tissue culture. The next 

step in domestication is to genetically modify black 

walnut (this does not necessarily imply genetic 

transformation) in some way to make it a better 

crop species, and more responsive to management 
Tree Improvement 5

Black Walnut in a New Century

activities. This has not yet been accomplished, but 

it has been on the horizon for some time. 

LITERATURE CITED

Beineke, W.F. 1982. New directions in genetic 

improvement: grafted black walnut plantations. 

In: Black walnut for the future: 3rd walnut 

symposium; 1981 August 10-14; West 

Lafayette, IN. Gen. Tech. Rep. NC-74. St. Paul, 

MN: U.S. Department of Agriculture, Forest 

Service, North Central Forest Experiment 

Station: 64-68. 

Beineke, W.F. 1983. The genetic improvement of 

black walnut for timber production. In: Janick, 

J., ed. Plant breeding reviews. Volume 1. 

Westport, CT: AVI Publishing Co.: 236-266.

Beineke, W.F. 1984. Characteristics of Purdue 

University’s patented black walnut trees. West 

Lafayette, IN: Purdue University, Cooperative 

Extension Service. 4 p. Available online: http://

www.ces.purdue.edu/extmedia/FNR/FNR-115.

html.

Bey, C.F. 1970. Geographic variation for seed 

and seedling characters in black walnut. Res. 

Note NC-101. St. Paul, MN: U.S. Department 

of Agriculture, Forest Service, North Central 

Research Station. 4 p.

Bey, C.F. 1973. Genetic variation and selection. 

In: Black walnut as a crop: 2nd black walnut 

symposium; 1973 August 14-15; Carbondale, 

IL. Gen. Tech. Rep. NC-4. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, North 

Central Forest Experiment Station: 62-65.

Bey, C.F. 1980. Growth gains from moving black 

walnut provenances northward. Journal of 

Forestry. 78(10): 640-641, 645. 

Black, W.M.; Neely, D. 1978a. Effects of 

temperature, free moisture, and relative 

humidity on the occurrence of walnut 

anthracnose. Phytopathology. 68: 1054-1056.

Black, W.M.; Neely, D. 1978b. Relative resistance 

of Juglans species and hybrids to walnut 

anthracnose. Plant Disease Reporter. 62(6): 

497-499.

Busov, V.B. 1996. Allozyme variation and mating 

system of black walnut (Juglans nigra L) across 

the central hardwood region. Carbondale, IL: 

Southern Illinois University. 68 p. M.S. thesis.

Busov, V.B.; Rink, G.; Woeste, K. 2002. Allozyme 

variation and mating system of black walnut 

(Juglans nigra L.) in the central hardwood region of 

the United States. Forest Genetics. 9(4): 319-326.

Caraway, C. 1976. A forestry sciences laboratory...

and how it grew. St. Paul, MN: U.S. Department 

of Agriculture, North Central Forest Experiment 

Station. 77 p. 

Clark, F.B. 1965. Forest service research in black 

walnut. In: Annual report of the Northern Nut 

Growers Association. 56: 84-87.

Coggeshall, M.V.; Beineke, W.F. 1997. Black walnut 

vegetative propagation: the challenge continues. 

In: Van Sambeek, J.W., ed. Knowledge for 

the future of black walnut: 5th Black walnut 

symposium; 1996 July 28-31; Springfield, MO. 

Gen. Tech. Rep. NC-191. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, North 

Central Forest Experiment Station: 70-77.

Crane, H.L.; Reed, C.A. 1937. Nut breeding. In: 

Yearbook of agriculture. Washington, DC: U.S. 

Department of Agriculture, 75th Congress, 1st

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Tree Improvement 9

Black Walnut in a New Century

WALNUT GENETIC IMPROVEMENT AT THE START OF A NEW CENTURY

Keith E. Woeste and James R. McKenna1

ABSTRACT—Since the early 1960s, researchers in programs to improve black walnut 

for timber have struggled to meet some basic breeding objectives. Promising avenues 

of research had been identified by the early 1950s, and conventional methods to 

breed forest trees were widely adopted as suitable for walnut. Progress has been slow 

however, due to practical problems related to field plot establishment and maintenance. 

Many plantings are of no scientific value because of poor site selection, insufficient 

experimental blocking, and a lack of long-term care. From good test plantings, we have 

learned that regional adaptation exists for walnut, and that traits such as growth rate 

and timber form are genetic and may be improved through selection and breeding. The 

pace of walnut improvement in the next forty years should be much greater because of 

lessons learned from the past and because of the availability of new molecular tools to 

modify genetic backgrounds and track genetic gains. Scientists at the Hardwood Tree 

Improvement and Regeneration Center are seeking to overcome practical problems in 

tree improvement research so that we can produce high quality research plots that will 

provide improved walnut for Midwestern hardwood forests throughout the next century.

Walnut improvement has been an important area of 

forest genetics research in the Central Hardwoods 

region for over 40 years. However, many of the 

central issues in walnut breeding identified by 

researchers in the 1950s remain unresolved. This 

paper will review the areas of research suggested 

by scientists working at the start of hardwood 

improvement in the 1950s and 1960s, and review 

important issues related to future prospects for 

improving walnut. Our goal is to evaluate which 

approaches have yielded valuable data, to learn 

from past mistakes, to identify future genetic 

improvement goals and successful methods to 

achieve these goals. 

Black walnut has a natural tendency to grow 

crooked and forked, without strong apical 

dominance, and with a great deal of variability 

in form and growth rate. To have high timber 

value, a walnut must be straight and have a bole 

free of branches. Thus, trees with straight boles 

and sparse lateral branches that naturally prune 

themselves; relatively fast growth, strong apical 

dominance, high percent heartwood, heartwood 

with good color, and little or no pin knots are the 

goals of black walnut improvement. 

Naturally, walnut grows in rich moist sites with 

deep, well-drained soils. Foresters have long 

recognized that walnut is site-sensitive. Thus, 

wider site adaptability and regional adaptation are 

additional goals for walnut timber improvement. If 

the objective of walnut improvement is to produce 

trees that can be grown anywhere within its range 

under any level of management, then fast growth 

and other important traits will be sacrificed for 

broad adaptability. Few private landowners want 

these type of trees and it is not possible to breed 

a tree that will grow well without management 

(Zobel 1984). It might be theoretically possible to 

“select specific families for specific sites” (Rink and 

Clausen 1989), but such an approach is impractical 

without more information about site variation in 

the Central Hardwoods region.

OLD LESSONS—NEW PROSPECTS

Plantation Management

Many of the past walnut genetic improvement 

trials failed to accomplish their goals because of 

poor site selection and management of field plots 

1

Research Molecular Geneticist (KEW) and Biologist (JRM), Hardwood Tree Improvement and Regeneration Center, Department of Forestry 

and Natural Resources, 195 Marsteller St., Purdue University, West Lafayette, IN, 47907. KEW is corresponding author: to contact, call (765) 

496-6808 or email kwoeste@fs.fed.us.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., eds. 

2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. Tech. Rep. NC243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
10 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

(Table 1). Regardless of seed source, walnut can 

only be grown adequately on a good walnut site 

and tree improvement is not capable of altering the 

basic biology of a species. Many genetic trials have 

yielded little data because they were planted on 

sites that should not have been used. Furthermore, 

good site preparation, control of annual weeds, and 

removal of vines and perennial competition are 

indispensable if a study is to succeed. It is critical 

to have clearly drawn maps that are updated 

(track the loss of trees) with many backup copies. 

Navigating around many old research plots is 

difficult because of uncertainty about where rows 

or blocks begin or end. Under such circumstances, 

a permanent stake is needed to mark corners of 

the plot. The use of a species other than walnut 

to mark border rows and separate blocks is a 

practice that can keep plots clearly marked for 

the long-term. Today, GPS technology gives us the 

opportunity to locate plantings and soon, because 

of developing technology, every tree can be mapped 

within a planting. Decisions about which tree 

tag to use are a compromise among the needs for 

permanence, low cost, and ease of use. 

Study Size and Experimental Design

Since mature walnut trees are large, walnut 

genetic studies have to be large (comprised of 

many trees) to be certain that differences among 

families can be statistically determined. In 

natural stands, walnut does not grow in large, 

uninterrupted stands, as do many conifers. 

Good walnut sites are often irregularly shaped, 

long and narrow or even discontinuous. These 

features complicate walnut research in particular 

because such sites are not easily blocked and 

they may be difficult to manage as experimental 

plots. Experimental blocking needed to account 

for the large variability common in good walnut 

sites is critical, but was rarely undertaken. 

Experimental methods such as single-tree plots 

and discontinuous plots can help to remove 

environmental effects from experimental analysis.

Walnut genetic tests were typically planted using 

a randomized complete block design with four to 

six blocks, and four seedlings per family planted 

together in each block. If all trees survived and 

began as uniform seedlings, this meant that 16 

to 24 trees defined each family. This design was 

practical and sufficient to detect large genetic 

differences in provenance tests (Clausen 1983), 

but it was not appropriate for discriminating small 

differences among families growing on variable sites. 

Hardwoods are most valuable to landowners when 

they are mature, thus most study sites cannot be 

re-used. As a consequence, it is difficult to know 

the extent to which results at one site will be 

applicable to other similar sites. Beineke (1989) 

found that on good walnut sites, families from one 

location also did well in other locations, suggesting 

that genotype by environment interactions should 

not be debilitating to a breeding program. This 

conclusion needs further testing.

Minckler (1953) suggested that because long-term 

genetic research plots are subject to failure, many 

small studies should be established to minimize 

the risk. However, Rink (1989) suggested that 

most studies were too small to provide statistically 

significant results. Increasing the number of study 

sites rapidly increases management and labor 

costs. Large studies are more efficient to manage 

when programs have small budgets and few staff. A 

survey of the results of previous walnut plantings 

(Table 1) suggests that small studies are prone 

to abandonment, perhaps because it is easy to 

find a higher priority than the management of 

a small stand that will ultimately only produce 

a small amount of information. Zobel (1984) 

guessed that as many as half of all genetic studies 

are abandoned. Test sites that are targeted to 

committed landowners is the best way to avoid 

such problems. By including private landowners 

and forest industries as active participants and 

cooperators in hardwood research, we are hopeful 

that new walnut genetic plantings will have better 

long-term care. 

SELECTION AND EVALUATION OF 

ELITE WALNUT

Selection Theory

Selection theory and practice are well developed 

for many plant breeding systems, but not yet for 

black walnut. The fundamental equation that 

underlies genetic selection and evaluation of plants 

and animals states that: P = G + E + (GxE); where 

(P) is the phenotype or the outward appearance 

of the plant; (G) is the genotype, the underlying 

genes; (E) represents the environmental factors 

that affect how a plant looks; and (GxE) represents 

the interaction of genetic and environmental factors 

that affect the appearance of a plant. Phenotypic 

mass selection (picking the best trees based on 

a comparison with their neighbors) and progeny 

tests have been the most common tools for the 

evaluation and selection of superior walnut trees. 

The efficacy of mass selection when trees are 

growing in native conditions depends on the traits, 

species, care taken by the evaluators (Morgenstern 

1975), and good fortune. Only by rigorous study 

of open-pollinated progeny, controlled-pollinated 
Tree Improvement 11

Black Walnut in a New Century

progeny, and replicated clonal trials, can we 

determine if a given tree really is genetically 

superior.

Open-pollinated Progeny Tests

Walnut progeny tests are typically based on seed 

produced from open-pollination of plus trees. Rink 

(1989) suggested that a 25% improvement could 

be obtained by selecting the best trees in the best 

50% of the families in a progeny test, but this 

suggestion is based on a small amount of data 

and is not likely to be the outcome in practice. 

Beineke (1989) published more modest results. 

Our preliminary evaluation of the elite selections 

at Purdue University indicated that 72 of the 427 

trees selected from the wild and past progeny tests 

are above average and genetically superior. If these 

values hold true, then phenotypic mass selection 

was a reliable selection method for walnut 17% of 

the time. In other words, only 17 out of 100 selected 

walnut trees from natural stands are actually 

genetically superior.

For the most part, we do not know what test 

environments are best for which traits, and at 

what age should selection take place. There has 

been no empirical test in walnut of gain from 

early selection for high-heritability traits such 

as diameter growth (Beineke 1989). Several 

researchers have suggested that selection among 

progeny can begin as early as age five (Rink 

1984, Coggeshall and Pennington 1982, Beineke 

1989). In practice, both Coggeshall and Beineke 

preferred to wait until at least age 10 (personal 

communication). The best examinations of 

selection age indicate that early selection (before 

age 10) is not likely to be effective despite the 

hopeful enthusiasm of some authors (Wright 

1966). It is probably best to select at age 15 or 

longer (Rink and Clausen 1989, Bresnan and 

others 1992). 

Traits, aside from DBH and height, that should 

be evaluated and the best methods to determine 

how they are best quantified are not known. 

Traits such as improved percent heartwood, selfpruning, wound healing, apical dominance, and 

straightness make walnut more valuable, but many 

of these traits are hard to quantify, and traits that 

are difficult to measure accurately are difficult 

or costly to improve (Zobel 1984). Black walnut 

genomics and genetic modification technologies 

should improve our accuracy for incorporating 

genes for quantitative traits into improved families 

although this research will still be costly. Rink 

(1989) found that heartwood color is primarily 

under environmental control and as such not 

subject to genetic improvement. As past grafted 

trees reach sufficient size, evaluation of clonal trees 

from different sites will provide a direct test of the 

genetics of heartwood color.

Controlled Cross Progeny Tests

Controlled cross pollinations are necessary to 

efficiently combine traits into a single genotype and 

to study the genetic regulation of both simple and 

complex traits. Controlled crosses that produce 

full-sibling families are expected to be almost twice 

as efficient as open-pollinated half-sib families 

for meeting improvement goals (Kung and others 

1974). Unfortunately, the biology of walnut makes 

the use of controlled crosses expensive (Beineke 

1989). A problem with using controlled crosses 

is that the cost of producing a large number of 

progeny is high, and if only a relatively small 

number of progeny are produced the rate at which 

genes are fixed is much greater than might be 

expected from selection alone. Even genes favored 

by natural selection are at an increased risk of 

loss in small full-sibling populations (Namkoong 

1979). Open-pollinated half-sib families permit 

the retention of beneficial genes that have been 

naturally selected into the breeding population, but 

at the cost of maintaining large numbers of progeny 

and large numbers of individuals from each family. 

Controlled crossing of elite individual trees is an 

important method to understand both the specific 

genetic combining ability of trees, and to develop 

new superior individuals for seed orchards or 

clonal propagation. There have been some full 

sib walnuts produced and planted over the past 

40 years, but too few to provide meaningful 

information. Conventional control-pollination of 

walnut is labor intensive and on average results 

in only 0.5 to 1 nut per bagged shoot. Walnut seed 

has an average of 50% viability, thus many bagged 

branches and pollinated flowers do not produce 

seed. Currently, the cost of controlled crosses is 

so much greater than that of collecting an equal 

number of open pollinated seed that the value of 

controlled crosses in walnut improvement is limited 

to specific applications in research and breeding. 

Artificial control-pollination will remain necessary 

to cross individual trees that do not overlap with 

time of pollen shed and receptivity of female 

flowers. DNA markers allow researchers to 

improve accuracy of selection (marker-assisted 

selection) and determine propagation mistakes. 

If the performance of certain full-sib families is 

significant, then production of select crosses could 

be accomplished by planting isolated seed orchards 

with clones that will cross-pollinate naturally. 
12 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

Clonal Tests

Clonal trees, whether grafted or own-rooted, can 

be used to measure and understand local and 

regional environmental effects. The importance of 

genotype by environment interactions (GxE) can 

be determined by placing genetically comparable 

studies on multiple sites. Unfortunately, grafted 

trees have been used in only a few black walnut 

genetic studies. The most important grafted walnut 

timber breeding collection in the U.S. is at Purdue 

University. While these plantations were not 

designed as experimental test plots, they do show 

that many traits are consistent from grafted tree to 

grafted tree. 

Additional research on vegetative propagation might 

make rooted clones possible. Own-rooted trees are 

superior to grafted trees for certain types of genetic 

tests because the roots and stems of own-rooted 

trees are genetically identical. An evaluation of the 

effects of environment on walnut growth will be best 

answered by long-term trials using clonal (rooted) 

selections on sufficiently large numbers of sites 

so that within-region variance can be determined. 

Own-rooted clones would be particularly valuable 

as controls in progeny tests for quantifying 

block and site effects, for quantifying phenotypic 

plasticity, and for determining the value of various 

management techniques. 

THE OUTPUT OF WALNUT IMPROVEMENT

Seed Orchards

Black walnut is subject to alternate bearing and 

low yield (on a pounds of nuts per acre basis), 

thus seeds are always in short supply. This is 

especially true of genotypes selected for timber 

traits rather than nut yield. Seed orchards are a 

fundamental part of most tree improvement plans 

(Rink and Stelzer 1981), and they may be the most 

tangible product of the last 40 years of black walnut 

improvement. Funk (1966) reviewed some of the 

practical aspects of seed orchard establishment. 

Research on black walnut nut production (Jones 

and others 1998) addressed important issues 

related to nut orchard management, but research 

is still needed on treatments for early bearing and 

seed orchard design. Zobel (1984) suggested that 

as few as 30 – 40 trees might be sufficient for a 

production population. Much larger populations 

(about 300) are needed as genetic reserves. These 

might be maintained as sublines (see below) and 

progeny tested over time. Selections from the 

larger population can be used to supplement 

genetic deficiencies in the most advanced breeding 

population or as a long-term buffer against 

inbreeding. 

The debate over the use of clonal versus seedling 

seed orchards divided walnut geneticists for a 

generation, but in hindsight, clonal orchards seem 

better when there is no commitment to rogue 

seedling orchards (Beineke 1989). Beineke (1982) 

observed gains of 5 to 10 percent for height and 

diameter using clonal seed orchards containing the 

best 20% of the families. When the seedlings of the 

entire seed orchard were compared with nurseryrun stock the results were much less promising. 

These results are based on extremely small 

sample sizes, and Beineke suggested that gains 

in form were more evident than those for height 

and diameter. Perhaps the most important reason 

orchards produce no gain compared to random 

trees is that the clones in most of the orchards 

in the Central Hardwoods were not subject to 

rigorous progeny testing (Beineke 1982, Rink and 

Stelzer 1981). More recently, a study comparing 

‘Purdue #1’ grafted trees, Tennessee State Nursery 

improved seedlings, and Missouri State common 

seedlings showed that after 12 years, improved 

seedlings were no different in height growth and 

timber form than the clonal trees (Hammitt 1996). 

Both improved sources were significantly better 

than the common stock for height and form, but 

they were the same for diameter growth. The 

HTIRC is planting similar studies to provide data 

on the gains landowners will receive from planting 

improved walnut.

Seed orchards at two State-owned nurseries in 

Indiana produce over 200,000 walnut seeds in 

a good year; about 25% of the total number of 

walnut seeds needed by the nursery. Seed from 

the nurseries’ seed orchards is mixed with seed 

purchased from local collectors, expanding the 

genetic diversity in Indiana hardwood plantations. 

At present, few private nurseries maintain their 

own improved seed orchards, but the list of those 

that do is growing. 

It is important for black walnut improvement 

to improve seed yield from select trees because 

large progeny tests are necessary to produce 

the greatest gain. Management techniques that 

improve the seed yield need additional research. 

Some of the methods being tested at HTIRC include 

tree training systems and water and fertility 

management. Other methods to reduce seedling 

juvenility, the time period in which a tree cannot 

flower, will accelerate breeding by permitting 

more rapid testing of generations. Research on 

seed handling and storage might provide new 

possibilities for making seed available to breeders 

in years when late frosts leave many trees fruitless, 

and in the years of alternate bearing when seed 

production is low.
Tree Improvement 13

Black Walnut in a New Century

Seed Zones

Proposed black walnut seed zones (Deneke and 

others 1981) are based on an educated guess rather 

than the analysis of experimental data. They seem 

reasonable based on years of field observations. 

The existence of locally adapted walnut populations 

in discrete ecological habitats seems possible, but 

it has yet to be demonstrated. It also seems likely 

that for any two seed zones or provenances, the 

possible variance within the zone could easily 

exceed the variance between adjoining zones. Seed 

zones complicate nursery operations by limiting the 

bulking of seeds and sometimes by limiting seed 

availability. 

While several studies have investigated stand and 

provenance level variance (Bey and Williams 1975, 

Wendel and Dorn 1984, Bresnan and others 1992, 

Geyer and Rink 1998), the terms do not signify a 

clear, precise, repeatable type of sample. We were 

not able to identify any data that indicate exactly—

or even approximately—how many trees from how 

many locations would adequately characterize the 

genetic variance of walnut growing in a region. 

These studies did establish that trees from seed 

moved a relatively short distance northward and 

often grew significantly faster than trees from local 

seed sources, supporting the 200 mile guideline. 

Sublines and Genetic Conservation

Sublines can be used for the avoidance of 

inbreeding depression and the retention of genetic 

diversity (Namkoong 1979; McKeand and Beineke 

1980). Inbreeding depression is a serious long-term 

concern for certain types of breeding programs. 

The basic idea behind sublines is that at least 

two separate breeding populations (or sublines) 

of trees are maintained, and the best individuals 

from each subline are included in more advanced 

breeding populations or seed orchards. Breeders 

have recourse to the sublines when they need 

new sources of genetic variability and new genes. 

Because we do not know which traits will be 

important in the future, sublines can be used for 

genetic conservation. 

Hybrids

Interest in the use of interspecific hybrids (hybrids 

between two different walnut species) to improve 

vigor dates back to the early 20th century (Wright 

1966). Most first-generation (F1) interspecific 

hybrids in the genus Juglans show dramatic hybrid 

vigor, but only small plantings of hybrids have been 

made because of the difficulty in obtaining F1 seed 

(Bey 1969). Hybrids have been most frequently 

made for nut production (e.g., Persian walnut x 

butternut) rather than for timber. In the last 20 

years in Europe, the black walnut x Persian walnut 

hybrid has received considerable attention as a 

potential timber tree (http://www.walnuttrees.

co.uk/timber_home.htm). There are a few examples 

of other hybrids with potential as a timber crop. 

Juglans nigra x J. hindsii (Northern California 

black walnut or NCB) is known as the “Royal 

hybrid” and is sold in the Pacific Northwest. Several 

J. nigra x J. major (Arizona black walnut) hybrids 

are growing at the former Tree Improvement Center 

Arboretum associated with Southern Illinois 

University. The wood qualities and timber potential 

of most of these hybrids is not well known yet. 

HTIRC has identified a few mature hybrids for 

analysis of their wood quality. 

Walnut growers in California purchase 90% of 

their grafted Persian walnut trees that have a 

hybrid rootstock called “Paradox.” This F1 hybrid 

is a cross between NCB × Persian walnut. On 

very good walnut soils in California, nut yields 

are similar between NCB and Paradox rootstock. 

On more marginal sites, Paradox rooted trees out 

perform NCB. Paradox rootstock is also particularly 

useful on heavy soils and for fields that are often 

saturated. In the Midwest, it may be possible to use 

hybrids to expand the soil types and sites on which 

black walnut can be grown. Several of the western 

walnut species may be drought tolerant, and they 

may be useful as rootstocks for seed orchards. 

SUMMARY

The potential for large-scale development of 

improved walnut seed has never been greater. 

In the next decade, we will be available to revisit 

and evaluate many 30- to 40-year-old plantings 

and begin to assess the value and potential of 

select stock at mid rotation. O lder plantings 

are more reliable sources of data for selec ting 

outstanding parents. We have propagated several 

new seed orchards at Purdue University and we are 

planning to expand seed orchards for the Indiana 

Department of Natural Resources in the near 

future. In addition, we can provide material and 

advice to private companies and individuals that 

want to establish improved seed orchards. HTIRC 

orchards will contain better parents than were 

available in the past, and they will be planted and 

managed to maximize seed production to provide 

a steadily increasing quantity of improved walnut 

seed for landowners through the next century. In 

addition, molecular genetic tools will be used to 

access genetic improvement and to speed the rate 

that genetic improvements are made.
14 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

ACKNOWLEDGMENT

The authors thank Mark Coggeshall and Ron 

Overton for their critical and helpful reviews of the 

manuscript.

LITERATURE CITED

Beineke, W.F. 1982. New directions in genetic 

improvement: grafted black walnut plantations. 

In: Black walnut for the future: 3rd walnut 

symposium; 1981 August 10-14; West 

Lafayette, IN. Gen. Tech. Rep. NC-74. St. Paul, 

MN: U.S. Department of Agriculture, Forest 

Service, North Central Forest Experiment 

Station: 64-68. 

Beineke, W.F. 1989. Twenty years of black walnut 

genetic improvement at Purdue University. 

Northern journal of Applied Forestry. 6: 68-71.

Beineke, W.; Stelzer, H.E. 1985. Genetic 

improvement of black walnut: Is it working? 

Annual report of the Northern Nut Growers 

Association. 76: 26-31. 

Bey, C. 1969. The Forest Service black walnut 

genetics project: a progress report. In: 

Schreiner, E.J., ed. Proceedings, 15th 

northeastern forest tree improvement 

conference; 1967 July 25-26; Morgantown, 

WV. Upper Darby, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station: 31-34.

Bey, C.F.; Williams, R.D. 1975. Black walnut trees 

of southern origin growing well in Indiana. 

Indiana Academy of Science Proceedings. 84: 

122-127.

Bresnan, D.; Geyer, W.A.; Lynch, K.D.; Rink, G. 

1992. Black walnut provenance performance in 

Kansas. Northern Journal of Applied Forestry. 

9: 41-43.

Clausen, K.E. 1983. Performance of black walnut 

provenances after 15 years in 7 Midwestern 

plantations. Proceedings, 3rd North central 

forest tree improvement conference; 1983 

August 17-19; Wooster, OH. Madison, 

WI: Deptartment of Forestry, University of 

Wisconsin: 24-33.

Coggeshall, M.V.; Pennington, S.G. 1982. Indiana 

Division of Forestry black walnut tree 

improvement program In: Black walnut for the 

future: 3rd walnut symposium; 1981 August 10-

14; West Lafayette, IN. Gen. Tech. Rep. NC-74. 

St. Paul, MN: U.S. Department of Agriculture, 

Forest Service, North Central Forest Experiment 

Station: 61-63.

Deneke, F.J.; Funk, D.T.; Bey, C. 1981. Preliminary 

seed collection zones for black walnut. Gen. Tech. 

Rep. NA-FB/M-4. St. Paul, MN: U.S. Department 

of Agriculture, Forest Service, Northeastern Area 

State and Private Forestry. 5 p.

Funk, D.T. 1966. Seed orchards. In: Black walnut 

culture: walnut workshop; August 2-3. U.S. 

Department of Arigriculture, Forest Service, North 

Central Forest Experiment Station: 62-65.

Geyer, W.A.; Rink, G. 1998. Interaction effects of 

seed source, culture, and pest management on 

growth of Juglans nigra in plantations. Silvae 

Genetica 47: 51-58.

Hammitt, W.E. 1996. Growth differences among patented 

walnut grafts and selected seedlings 12 years 

after establishment. In: Van Sambeek, J.W., ed. 

Knowledge for the future of black walnut: 5th Black 

walnut symposium; 1996 July 28-31; Springfield, 

MO. Gen. Tech. Rep. NC-191. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, North 

Central Forest Experiment Station: 63-68.

Jones, J.E.; Mueller, R.; Van Sambeek, J.W., eds. 

1998. Nut production handbook for eastern 

black walnut. Southwest Missouri Resources, 

Conservation & Development (RC&D), Inc. 

http://www.ncrs.fs.fed.us/epubs/walnut/.

Kung, F.H.; Bey, C.F.; Larson, J.L. 1974. Nursery 

performance of black walnut twins. In: Garrett, 

P.W. ed. Proceedings, 22nd northeastern forest 

tree improvement conference; 1974 August 7 – 9; 

Syracuse, NY. Upper Darby, PA. U.S. Department 

of Agriculture, Forest Service, Northeastern Forest 

Experiment Station: 184-190.

McKeand, E.; Beineke, W.F. 1980. Sublining for 

half-sib breeding populations of forest trees. 

Silvae Genetica. 29: 14-17.

Minckler, L.S. 1953. Recent advances in the field 

of forest genetics. Illinois Academy of Science 

Transactions. 46: 56-62.

Morgenstern, E.K. 1975. Review of the principles of 

plus-tree selection. In: Morgenstern, E.K.; Lolst, 

A.H.; Teich, A.H.; Yeatman C.W., eds. Plus-tree 

selection: review and outlook. Publ. 1347. Ottawa, 

Canada: Canadian Forestry Service: 1-27.

Namkoong, G. 1979. Introduction to quantitative 

genetics in forestry. Tech. Bull. 1588. 

Washington, DC: U.S. Deparement of 

Agriculture, Forest Service.
Tree Improvement 15

Black Walnut in a New Century

Table 1.—Characteristics of 33 typical walnut genetic studies and the factors that affected their success.

Planting Planter1 Location Type2 Year

Family/

No. Trees Comments and Recommendation3

Salamonie

Reservoir

Funk Wabash

Co., IN

PP 1963 24/ 840 Study CG-369 (NC-1402), good 

uniform site, good drainage, no 

slope, good early management, 

several companion plantings 

abandoned very early. [M]

Hoosier NF Bey Lawrence

Co., IN

IP

PR

1967 15/ 360 Fair to good site, flood prone six 

parents per provenance, pruned for 

5 years, fair survival to year 7 (61%), 

root rot in planting stock, herbicides 

used for six years, stakes not found 

in 2003, alder overtopped walnut. 

[M]

Shidler

(Martell)

Beineke Tippecanoe

Co., IN

PT 1968 13/ 170 Good site, no records after age five, 

growth fair, [A]

C.B. Stems Beineke Harrison

Co., IN

PT 1969 8/ 80 Measured in 1978, 1982 and 1986. 

Two of three reps abandoned for 

poor survival, excellent growth. 

[MM, A]

Pierson

Hollowell

Richmond

Beineke Wayne Co.,

IN

PT 1969 — [MM], [A]

Pierson

Hollowell

Beineke Parke Co.

IN

PT 1971 9/ 300 Not thinned, vines, slow growth, 

PVC pipe with rebar at corners, 

some stakes with tags. [A]

SIPAC

Flick

Beineke Dubois Co.,

IN

PT 1971 8/ 108 Demonstration plot, excellent site 

and management. [M]

(Table 1 continued on next page)

Rink, G. 1984. Trends in genetic control of juvenile 

black walnut height growth. Forest Science. 30: 

821-827.

Rink, G. 1989. Recent advances in walnut 

tree improvement. In: Phelps, J.E., ed. 

The continuing quest for quality: 4th black 

walnut symposium; 1989 July 30-August 

2; Carbondale, IL. Indianapolis, IN: Walnut 

Council: 140–144.

Rink, G.; Stelzer, H. 1981. The status of black 

walnut tree improvement programs in the 

north central region. Northern Nut Growers 

Association Annual Report. 72: 57-60.

Rink, G.; Clausen, K.E. 1989. Site and age effects 

on genotypic control of juvenile Juglans nigra L. 

tree height. Silvae Genetica. 38(1): 17-21.

Wendel, G.W.; Dorn, D.E. 1985. Survival and 

growth of black walnut families after 7 years in 

West Virginia. Res. Pap. NE-569. Broomall, PA: 

U.S. Deparment of Agriculture, Forest Service, 

Northeast Forest Experiment Station.

Wright, J. 1966. Breeding better timber varieties. 

In: Black walnut culture: 1st walnut symposium; 

1966 August; Carbondale, IL. St. Paul, MN: 

U.S. Department of Agriculture, Forest Service, 

North Central Forest Experiment Station: 5. 

Zobel, B.; Talbert, J. 1984. Applied forest tree 

improvement. New York, NY: John Wiley and 

Sons, Inc. 505 p.
16 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

(Table 1. continued) 

Planting Planter1 Location Type2 Year

Family/

No. Trees Comments and Recommendation3

Hillenbrand

Industries

Beineke Franklin

Co., IN

PT 1972 11/ 150 Good site, poor care, hard to find 

corners and border rows, fair survival. 

[A]

Jasper

Free Tract

Beineke Orange

Co., IN

PT 1972 4/ 158 Good site and growth, good early 

management, visited 1988 and 

corners blazed. [A]

Pierson

Hollowell

Beineke Parke Co.

IN

CC 1973 16/ 250 Never thinned, could not find corners 

in 2000. [A] 

Pleasant Valley, 

Sexton Creek

Rink Alexander

Co., IL

PT

SO

1973 54/ 2700 FS-NC-1151, CG-425, trees were 

thinned, disked, vetch seeded in year 

11, excellent site, thinned at age 10, 

three blocks lost to flood. [M]

Martell Beineke Tippecanoe

Co., IN

PT 1975 12/ 168 Good site, map upside down, good 

growth, selections made, harvested 

2001.

Missisinewa 

Reservoir

Beineke Wabash

Co., IN

PT

SO

1976 11/ 285 Poor site, no maintenance, variable 

growth, no thinning, grasses 

suppress growth, IDNR seed source. 

[M]

IDNR

HarrisonCrawford SF

Beineke Crawford

Co., IN

PT

CC

1977 36/ — Site also known as “Cave” Poor site, 

poor survival and growth. [A]

Pierson

Hollowell

Beineke Morgan.

Co., IN

PT 1977 18/ 440 Good site, PVC with rebar at corners, 

map upside down. [M]

Rattlesnake

Ferry

Geyer Jackson

Co., IL

IP

PP

1977 5/ 1500 Flood prone, fair drainage corrective 

pruning, good early management, 

follow-up study published in 2003. 

[M

Clark State 

Forest

Beineke Clark Co., IN PT 1978 12/ 534 Fair site, well managed in excellent 

condition, borders removed, PVC 

pipe with rebar placed inside corner 

at row 2 tree 2. [M]

Missisinewa 

Reservoir

Beineke Wabash Co., IN PT

SO

CC

1978 29/ 802 Off site, growth better than 1976 

progeny test, poor management. 

[CC], [M]

Parsons Dorn Tucker Co., WV PP

SO

1978 34/ 1000 Fir site, top dieback, poor growth 

first 7 years, survival fair to good, no 

published records since 1985. [A]

Jasper-Pulaski Beineke Jasper, Co., IN PT

SO

1979 9/ 200 Off site, high site variability 4’ x 5’ 

planting too close, no maintenenace, 

IDNF nursery. [A]

(Table 1 continued on next page)
Tree Improvement 17

Black Walnut in a New Century

(Table 1.—continued) 

Planting Planter1 Location Type2 Year

Family/

No. Trees Comments and Recommendation3

Martell Beineke Tippecanoe 

Co., IN

PT 1979 40/ 800 OK site, good map, fair growth, 

thinned 1987, 1991, 10 selections 

made. [M]

Vallonia Nursery Beineke Jackson Co., IN PT

SO

1979 9/ 200 Managed by IDNR, good site, 

thinned, good seed source, paired 

with Jasper-Pulaski test. [M]

IDNR Beineke Harrison Co., 

IN

PT 1980 44/ 876 Off site, poor growth. [A]

Jasper-Pulaski Beineke Jasper, Co., IN PT

SO

1980 30/ 496 Off site, high site variability, 4’ x 5’ 

planting too close, no maintenance, 

IDNR nursery. [A]

Vallonia Nursery Beineke Jackson Co., IN PT

SO

1980 36/ 740 Managed by IDNR, good site, 

thinned, good seed source,paired 

with Jasper-Pulaski test. [M]

Big Creek Van 

Sambeek

Hardin Co., IL PP 1981 131/ 2500 Flood prone site, good drainage, 

low survival in some blocks, corners 

clear in 2003, fair to good growth. 

4-tree plots could be discerned, 

good maps, but access to the site 

contested. [M]

SEPAC Seifert Jennings Co., 

IN

PP 1981 80/ 900 Companion to Kellogg Forest 

Planting (CG-369). Good site and 

management. [M]

Merry Lee 

Nature Center 

Goshen College

Beineke Noble Co., IN PT 1984 15/ 308 High mortality. [MM], [A]

Spurgeon 

Hollow Lake

Beineke Jackson Co., IN PT 1984 58/ 1180 Excellent site and management, 

thinned twice, frequently visited, 

collaboration with IDNR. [M]

Clark State 

Forest

Beineke Clark Co., IN PT

CS

1988 10/ 546 Site extremely variable, fair growth, 

collaboration with OSU, primarily 

potting media study. [M]

Mount Tabor Beineke Monroe, Co., 

IN

PT

IP

1990 53/ 3180 Interplanted pine and European 

black alder, good site, good growth, 

corners and row end markers 

missing. [A]

Wolf Creek Beineke Lenawee Co., 

MI

CT 1993 5/ 800 Survivial 25%, poor growth repeated 

deer browse, poor site, great initial 

management. [A[

1

Only one investigator is listed although most plantings involved several investigators and collaborators. 

2

PP= Provenance/progeny test, IP = Interplanting trial, PR= Provenance study, PT = Progeny test,

 CT= Clone test, CS= container study, CC= Controlled crosses, SO= Seed orchards.

3

[M]= Maintained as valuable, [A]= Abandoned, [MM]= Missing maps.
18 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

1Associate Professor (WR), Pecan Experiment Field, Kansas State University, Chetopa, KS 67336; Research Associate (MVC), Center for 

Agroforestry, Univ. of Missouri, 10 Research Center Rd., New Franklin, MO 65274; and Research Associate (KLH), Center for Agroforestry, 

Univ. of Missouri, 203 ABNR, Columbia MO 65211. WR is the corresponding author (620-597-2972 or wreid@oznet.ksu.edu).

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., eds. 

2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. Tech. Rep. NC243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.

CULTIVAR EVALUATION AND DEVELOPMENT FOR BLACK WALNUT ORCHARDS

William Reid, Mark V. Coggeshall, and Kenneth L. Hunt1

ABSTRACT—Black walnut is an underdeveloped orchard crop. Hundreds of cultivars 

have been named but a commercial orchard industry has not developed. The 

horticultural characteristics of currently available black walnut cultivars are reviewed. 

Important cultivar traits include: leaving date, flowering date, growth habit, disease 

susceptibility, yield, and nut quality. Breeding program priorities for developing 

horticulturally superior cultivars are presented.

Eastern black walnut (Juglans nigra L.) trees 

produce a uniquely flavored nut that is a favorite in 

many Midwestern kitchens. Each year, American 

consumers use 2 million pounds of black walnut 

kernels in cookies, cakes, and ice cream products 

(Hammons 1998). The current level of black walnut 

production is supported by the collection and 

processing of nuts from wild walnut trees. During 

the fall, rural residents harvest nuts by hand and 

deliver the nuts to hulling and buying stations set 

up across the Midwest. An average of 25 million 

pounds of hulled seedling walnuts are purchased 

per year at an average price of 10 cents per pound. 

The continued reliance of the black walnut 

industry on the hand harvest of seedling trees will 

not allow for the future expansion of this nut crop. 

The wild crop is inherently variable, with wide 

swings in both total production and nut quality. 

This unpredictability of supply undermines the 

industry’s ability to increase utilization. Further, 

increasing family incomes and a continuing 

population shift from rural to suburban areas is 

slowly eroding the workforce that has traditionally 

harvested seedling walnuts. The future of the 

black walnut industry lies in the establishment of 

orchards of high-quality, high-yielding cultivars.

Since ‘Thomas’ was named as America’s first 

black walnut cultivar in 1881 (Corsa 1896), 

cultivar development has proceeded haphazardly. 

Throughout the 20th century, black walnut 

enthusiasts discovered new cultivars in the wild or 

planted open pollinated seeds of previously named 

cultivars just to “see what happens”. Even at this 

slow pace of crop development, over 700 cultivars 

have been named (Berhow 1962, Reid 1997a) and 

the percent of edible kernel has improved two-fold 

(seedling walnuts average 17% kernel while some 

cultivars consistently produce over 34% kernel). 

But these improvements in kernel quality have not 

been enough to stimulate large-scale development 

of black walnut orchards.

In recent years, we have initiated a program to 

systematically improve black walnut as a nut 

producing tree. We have started this process 

by evaluating the horticultural characteristics 

of existing cultivars, striving to understand the 

key components that determine yield and kernel 

quality. This work has also led us to develop 

key objectives for a systematic breeding effort to 

improve black walnut as an orchard crop.

HORTICULTURE CHARACTERISTICS OF 

BLACK WALNUT CULTIVARS

Many of today’s black walnut cultivars have been 

selected based on a single trait, percent kernel 

(Reid 1990). This has lead to the propagation of 

hundreds of low-yielding, disease-susceptible 

cultivars that are able to produce enough nuts 
Tree Improvement 19

Black Walnut in a New Century

to satisfy a hobbyist’s craving for easy to crack 

walnuts. In contrast, designing and implementing 

a commercial black walnut orchard will require 

detailed cultivar information, enabling growers 

to choose productive cultivars adapted to their 

specific growing conditions. Key cultivar traits 

include: leafing date, flowering dates, growth habit, 

disease resistance, precocity, productivity, and 

shelling quality. 

Leafing and Flowering Dates

Black walnut trees are among the latest leafing 

trees in eastern deciduous forests. This late-leafing 

characteristic ensures emerging leaves, along with 

developing flowering structures, avoid late spring 

frosts. Temperatures below -2o C will kill emerging 

green tissues, destroy floral initials, and eliminate 

the potential for nut production. Among the 

cultivars in our collections, ‘Davidson’ is the first 

cultivar to break bud. In contrast, ‘Thomas/Myers’ 

initiated new growth more than three weeks after 

‘Davidson’ (Table 1). We have also noted significant 

tree-to-tree variation in bud break date within each 

cultivar. Although we cannot document the cause 

of this effect at this time, we have initiated trials to 

determine rootstock effects on tree phenology and 

nut productivity. 

As expected, late-leafing cultivars flowered later in 

the season regardless of dichogamy. The variation 

in flowering date was similar to the variation 

measured for bud break. (Table 1). The vast 

majority of cultivars in our collections exhibited 

the protogynous flowering habit (Table 1). These 

observations suggest that novel strategies may 

be needed to ensure pollination in black walnut 

orchards. In commercial pecan or Persian walnut 

orchards, adequate pollen is ensured throughout 

the flowering season by simply planting a 

protandrous cultivar and a protogynous cultivar 

in the same block (Grauke 1997, Hendricks and 

others 1998). In black walnut, providing pollen 

throughout the pollination season is made more 

difficult because of a critical lack of protandrous 

black walnut cultivars. To solve this problem, 

early-leafing protogynous cultivars could be used 

to pollinate late-leafing protogynous cultivars. 

However, at least one early-leafing protandrous 

cultivar must be included in the orchard to provide 

pollen for early-leafing protogynous clones. 

Growth Habit

Several black walnut cultivars exhibit a branching 

habit that can be most accurately described as 

spur-type growth. Previous work has described 

this growth pattern as lateral bearing (Reid 1997); 

Table 1.—Leafing date, flowering type and fruit 

ripening season for black walnut growing at 

New Franklin, MO, 2001-2003. 

Cultivar

Leafing 

Date1

Flowering 

Type2

Ripening 

Season3

Beck 17.6 ± 2.1 II mid

Christianson 21.5 ± 4.0 II mid

Clermont 19.0 ± 5.2 II mid

Cranz 19.3 ± 4.0 II late

Daniel 14.6 ± 6.5 II early

Davidson 0 II early

Dot 22.5 ± 3.5 II mid

Drake 17.0 ± 4.4 II mid

Elmer Myers 18.0 ± 5.4 II late

Emma K 5.3 ± 2.5 II mid

Football 11.0 ± 2.1 I early

Grundy 10.7 ± 2.3 II late

Hay 22.0 ± 5.3 II late

Knuvean 8.3 ± 1.2 I early

Krouse 5.7 ± 0.6 II late

Kwik Krop 18.3 ± 1.5 II mid

McGinnis 5.0 ± 1.7 I mid

Mintle 7.0 ± 1.0 II mid

Rowher 17.3 ± 2.3 II mid

Rupert 8.3 ± 1.5 II late

Sarcoxie 17.0 ± 1.7 II mid

Sauber 14.7 ± 6.0 II early

Schessler 2.0 ± 1.1 II early

Scrimger 16.7 ± 4.0 II late

Sparks 127 17.0 ± 2.9 II early

Sparks 129 17.7 ± 3.8 II late

Sparks 147 22.7 ± 6.4 II mid

Sparrow 16.8 ± 3.4 II early

Stambaugh 19.7 ± 3.8 II mid

Surprise 13.0 ± 5.1 II late

Ten Eyck 20.3 ± 4.5 II late

Thatcher 18.3 ± 5.8 II late

Thomas 21.0 ± 5.8 II late

Thomas/

Myers 22.7 ± 6.4 II early

Victoria 15.7 ± 3.8 I late

1 Mean Days after Davidson ± std. dev. 

 (Davidson average leaf burst = Apr. 12)

2 I = protandrous, II = protogynous 

3

 Ripening dates

Early = Sept. 1-14

Mid = Sept 15-28

Late = Sept. 29 - Oct 12
20 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

however, the growth and bearing habit of spurtype black walnuts is not analogous to the lateral 

bearing habit found in Persian walnut (Juglans 

regia L.). Lateral bearing in J. regia occurs when 

lateral shoots arise from the current season’s new 

growth and terminate in a pistillate flower cluster 

(Hendricks and others 1998). This type of bearing 

habit has not been observed in black walnut. 

However, several black walnut cultivars develop 

short, compact branches (compressed internode 

lengths) along primary branches in a growing habit 

comparable to spur-type apples. Black walnut 

spurs are multi-year-old shoots that grow to a 

length of 8 to 12 inches. These spurs dramatically 

increase the number of potential fruiting sites on 

the tree and can spread fruiting throughout the 

canopy. Spur-type black walnut cultivars are listed 

in Table 2. 

Anthracnose Susceptibility

Anthracnose is a major limiting factor to annual 

nut production in black walnut (Reid 1995). 

Although anthracnose can infect stems and 

fruit, leaf infections prove most damaging to tree 

performance. The anthracnose fungus (Gnomonia

leptostyla [Fr.] Ces. & De Not.) can infect leaves 

shortly after full leaf expansion and completely 

defoliating the tree by mid-August (Neely 1979). 

Early defoliation decreases nut quality (Reid 1986) 

and enhances the trees natural tendency towards 

alternate bearing (Worley 1979). 

To evaluate black walnut cultivars for anthracnose 

susceptibility, we rate the defoliation of fruiting 

shoot leaves in mid-August (expressed as a 

percentage of leaflets lost). By evaluating fruiting 

shoots only, we ensure that our assessments 

are made on leaves from all the same age cohort 

(Reid 1995). We have not found cultivars with 

complete resistance to this disease, but have noted 

wide variation in susceptibility. Among the most 

susceptible clones in our trials are such popular 

cultivars as ‘Football’ and ‘Surprise’. In contrast, 

‘Sparrow’ and ‘Thomas’ demonstrated excellent 

leaf retention. Initial evaluations of anthracnose 

susceptibility for black walnut cultivars can be 

found in Table 3. 

Yield and Alternate Bearing

The lack of emphasis in the selection process on 

nut yield, especially yield of edible kernel/acre, is 

the primary impediment to the commercialization 

of black walnut. Potential black walnut growers 

must be confident that income from nut sales will 

return a fair profit for investments made in land, 

labor, trees, supplies, and equipment. 

Our initial investigation of black walnut cultivar 

performance was initiated in 1987 when 21 

cultivars were propagated on black walnut 

rootstocks in a replicated study. Trees were 

fertilized with nitrogen (100 lbs. N/acre/year) and 

anthracnose was controlled with fungicides. Nut 

production began 4 years after propagation but 

the first significant crop wasn’t recorded until 

1997. Yield data for these cultivars for the five year 

period, 1997-2001, is given in Table 4. In this young 

orchard, Rupert had the greatest in-shell yield 

but Drake produced the most kernels (Table 5). 

Emma K and Football exhibited perfect alternate 

Table 2.—Black Walnut cultivars exhibiting the spurtype growth habit. Data taken from Reid (1997a, 

1997b).

Cultivar Exhibiting Spur-type Growth

Beck Kwik Krop

Cranz McGinnis

Davidson Rupert

Emma K Sparks 127

Football Sparks 147

HPC-120 STW-13

HPC-148 Surprise

Table 3.—Anthracnose ratings for black walnut cultivars 

growing at New Franklin, MO during 2003.

Cultivar

Anthracnose 

Defoliation 

Potential1 Cultivar

Anthracnose 

Defoliation 

Potential1

Bowser higth Patterson med

Crosby low Rupert low

Christianson med Sauber med

Clermont med Schessler med

Cochrane high Scrimger low

Davidson med Sparks 127 high

Elmer Myers high Sparks 147 med

Emma K med Sparrow low

Football high Stambaugh med

Hare med Surprise high

Jackson high Thomas low

Kwik Krop med Thomas/Myers med

Mintle high Victoria med

1

 High > 30% leaflets defoliated by mid-August

 Med = 16-30% leaflets defoliated by mid-August

 Low< 16% leaflets defoliated by mid-August
Tree Improvement 21

Black Walnut in a New Century

bearing, producing nuts one year followed by zero 

production the next year. In contrast, low yielding 

Cranz produced a small crop of nuts each year, 

while Rupert demonstrated both high yield and low 

alternate bearing tendency. 

Nut production during this time period was 

comparatively low even for the highest yielding 

clones. The black walnut trees in this study were 

established at a spacing of 30 ft. x 30 ft. or 48 

trees/acre. Production of 20 lbs. (a high averages 

yield in our trial) of hulled and dried nuts per tree 

would result in a yield of 960 lbs/acre. In contrast, 

young Persian walnut orchards (years 10-15) 

yield over 4000 lbs/acre of hulled and dried nuts 

(Hendricks 1999). 

Fruit and Nut Characteristics

Each fall, black walnut enthusiasts collect nut 

samples of each cultivar in their planting in 

order to enter nut judging contests held in most 

Midwestern states. Nuts are weighed, cracked, and 

then kernels weighed to determine average nut 

weight and percent kernel. These basic nut quality 

parameters have been determined for hundreds 

of black walnut clones and provide much of our 

current knowledge base for black walnut cultivar 

performance. The results of 43 years of nut sample 

evaluation in Kansas are presented in Table 6. 

Since black walnut is not marketed to the consumer 

as an in-shell product, large nut size has not been an 

important cultivar selection criterion. Ease of shelling 

has been and will continue to be one of the most 

important nut quality traits measured. Important 

nut characters that influence ease of shelling include 

shell thickness, inner wall partition thickness, and 

openness of kernel cavity. These characteristics are 

easily observed by cutting open the nut in cross 

section with a band saw. We are currently working on 

methods to quantify these nut traits. 

Missing from nut evaluations are critical 

determinations of husk characteristics. In black 

Table 4.—Yield, average yield and alternate bearing index for black walnut cultivars growing at 

Chetopa, KS.

Cultivar

Yield (lbs./tree) Alternate 

Bearing Index1 1997 1998 1999 2000 2001

5 Year 

Average

Bowser 1.8 6.1 5.1 1.1 10.6 4.9 0.52

Cranz 10.8 5.5 7.2 7.1 6.8 7.5 0.12

Davidson 23.0 0.0 17.0 27.3 28.9 19.2 0.57

Drake 23.0 6.0 27.8 5.9 36.1 19.8 0.65

Dubois 8415 3.1 0.7 3.8 0.0 3.3 2.2 0.83

Emma K 2.5 0.0 8.1 0.0 13.8 4.9 1.00

Farrington 4.0 2.4 9.6 1.5 11.1 5.7 0.59

Football 24.0 0.0 22.0 0.0 13.0 11.8 1.00

Kwik Krop 2.4 0.3 6.9 0.0 3.4 2.6 0.92

McGinnis 6.0 0.8 12.1 0.0 28.3 9.4 0.91

Mintle 18.9 0.0 6.7 1.5 16.3 8.7 0.87

Rupert 30.9 11.8 19.2 23.8 27.4 22.6 0.22

Scrimger 2.2 2.0 8.7 2.5 15.6 6.2 0.49

Sparks 127 10.0 0.0 4.2 0.9 5.4 4.1 0.84

Sparks 147 2.7 3.4 6.9 0.6 2.4 3.2 0.47

Sparks 177 21.6 7.8 9.8 15.8 1.4 11.3 0.41

Sparrow 15.2 4.2 24.4 7.6 28.2 15.9 0.59

Stabler 4.1 7.6 20.5 0.0 12.4 8.9 0.69

Surprise 7.5 16.1 12.6 4.3 6.2 9.3 0.29

Vander Sloot 12.7 8.6 20.1 1.8 14.5 11.5 0.55

Victoria 22.5 2.3 10.0 0.3 5.4 8.1 0.82

 1 Alternate bearing index = 1/(n-1) Σ |(yn

-yn-1)/(yn+yn-1)|, where y= yield and n=year.
22 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

walnut, kernel color is strongly affected by the 

length of time the husk remains on the nut 

following fruit ripening (Chase 1941). As the husk 

begins to soften (break down), black-staining 

alkaloids are released from the husk and soak 

through the shell to darken the nut meat and 

change the flavor of the nut. This results in the 

‘strong’ flavor of which most consumers of black 

walnut are familiar. Removing the husk and 

washing the nut as soon as the fruit becomes ripe 

results in walnut kernels that are lighter in color 

and milder flavored. Uniformity of fruit ripening 

has a major impact on kernel quality. Since most 

walnuts are harvested following a single tree 

shaking, trees with long ripening periods will 

appear to produce kernels that vary widely in 

color. This stems from the fact that, at the time 

of harvest, nuts are collected at varying stages of 

husk ripening and hence varying stages of kernel 

staining. 

Black walnut husks are indehiscent and must be 

removed mechanically following harvest. However, 

it has been noted that some cultivars are easier 

Table 5.—Average yield (1997-2001), percent kernel, 

and kernel yield for black walnut cultivars 

growing at Chetopa, KS.

Cultivar

Yield 

(lbs./tree)

% 

Kernel

Kernel 

(lbs./tree)

Bowser 4.9 34.03 1.67

Cranz 7.5 32.87 2.47

Davidson 19.2 27.45 5.27

Drake 19.8 30.97 6.13

Dubois 8415 2.2 29.10 0.64

Emma K 4.9 34.28 1.68

Farrington 5.7 32.13 1.83

Football 11.8 32.62 3.85

Kwik Krop 2.6 31.03 0.81

McGinnis 9.4 32.24 3.03

Mintlev 8.7 29.86 2.60

Rupert 22.6 25.55 5.77

Scrimger 6.2 30.94 1.92

Sparks 127 4.1 32.58 1.34

Sparks 147 3.2 38.04 1.22

Sparks 177 11.3 33.46 3.78

Sparrow 15.9 31.51 5.01

Stabler 8.9 30.32 2.70

Surprise 9.3 32.63 3.03

Vander Sloot 11.5 25.70 2.96

Victoria 8.1 24.93 2.02

Table 6.—Average nut weight and percent kernel of black 

walnut cultivar samples entered in the Kansas Growers 

Association Annual Nut Show from 1959-2001.

Cultivar

Nut Weight (g) % Kernel

Mean 

Std. 

Deviation Mean

Std. 

Deviation

Beck 15.87 0.23 27.97 1.51

Bowser 15.51 1.42 32.27 3.93

Clermont 16.40 5.70 30.27 3.32

Cochrane 14.05 0.42 32.47 1.94

Cranz 14.18 1.51 30.46 3.62

Davidson 22.31 1.90 26.55 1.57

Drake 19.31 1.82 30.09 2.44

Emma K 16.32 2.57 34.57 1.91

Eureka 15.86 1.96 25.94 0.05

Farrington 19.03 5.93 26.78 5.20

Football 20.29 2.66 29.96 2.30

Hare 21.10 2.31 27.21 1.66

Jackson 14.29 1.85 35.43 1.25

Kwik Krop 17.10 2.89 30.55 2.50

McGinnis 16.66 0.12 31.19 2.90

Mintle 16.06 1.61 30.99 2.30

Myers 14.17 2.53 25.90 8.03

Ogden 19.71 5.93 21.24 5.34

Ohio 16.52 2.21 27.19 4.04

Peanut 19.53 0.32 26.08 5.20

Perry 19.55 1.82 27.41 1.20

Rowher 22.38 3.13 24.30 1.07

Rupert 18.29 1.54 25.12 0.91

Sauber 15.11 2.27 32.03 2.57

Scrimger 19.11 2.56 29.44 2.22

Sol 19.34 2.37 24.13 1.46

Sparks 127 15.00 2.91 32.64 2.99

Sparks 147 16.69 1.51 36.45 4.23

Sparks 177 20.90 2.15 33.07 1.60

Sparrow 18.59 2.25 29.92 2.07

Stabler 16.97 2.91 25.40 4.94

Stangle 18.98 3.30 27.07 3.34

Stark 18.82 3.11 26.56 3.89

Surprise 20.28 2.27 33.20 2.15

Thomas 21.79 4.88 24.39 4.54

Tom Boy 21.66 1.02 27.38 2.15

Vander Sloot 24.35 2.53 27.54 1.77

Victoria 17.73 3.22 24.37 3.60

to hull than others. We have observed two types 

of walnuts: cultivars that could be termed ‘clingstone’, where the husk is tightly held by the shell 

and cultivars that are ‘free-stone’, where the husk 

is loosely held or separated from the nut shell at 
Tree Improvement 23

Black Walnut in a New Century

ripening. We are in the process of collecting data on 

these important husk characteristics. 

SETTING PRIORITIES FOR A BLACK WALNUT 

BREEDING PROGRAM

Black walnut remains an emerging crop. Will black 

walnut cultivars be found with the yield potential 

to cover the high costs of harvest, hulling, washing, 

and drying nuts? As we work with this crop, we 

may discover new cultural techniques that promote 

yield, but a systematic genetic improvement 

program holds the greatest potential for developing 

cultivars able to sustain a commercial walnut 

orchard industry. 

Many important genetic traits can be found in 

today’s black walnut cultivars. By producing nuts 

that average 38% kernel, ‘Sparks 147’ demonstrates 

that black walnut can be pushed towards a truly 

thin shelled nut. Many cultivars produce walnuts 

on spurs increasing the potential nut bearing 

surface of the tree. Still other cultivars seem less 

susceptible to anthracnose. The genetic variation 

necessary for an effective breeding program is 

available, but what are our genetic goals?

Goals for Improving Yield

Almost every walnut enthusiast has seen a walnut 

tree so heavily laden with fruit that the limbs are 

breaking. Crack open some nuts from one of these 

over-producing trees and you will find poorly filled 

kernels. In addition, the tree will not bear another 

nut crop for at least two years. Do these common 

observations indicate that black walnut trees are 

already producing near their genetic limit? We 

think not.

Black walnut trees expend a huge amount for 

biological energy to produce both husk and shell. 

The husk is a waste product that stains many 

things it touches black. While the shell has some 

commercial value (Hammons 1998), its value has 

a minimal impact on the economics of a walnut 

orchard. In breeding walnuts for nut production, 

reducing the tree’s investment in husk and shell 

should allow more energy to be directed towards 

kernel production. The advantage of this strategy 

can be easily seen by comparing the husk and shell 

of a modern Persian walnut cultivar to that of a 

black walnut cultivar, then comparing the yields 

of these two species (see above). The hybridization 

of J. regia with J. nigra has been recorded with few 

positive results (McKay 1971). Although additional 

hybridization work may be warranted, we feel that 

we can meet our objectives of minimal husk and 

thin shells within J. nigra. 

Preliminary work with different seed sources 

for rootstock trees seems to indicate that 

rootstock may have a dramatic influence on scion 

performance. We have already observed a rootstock 

effect on leafing date (above). We have yet to 

document the impact rootstock has on yield but 

trials are in place. We are also interested in the 

potential of interspecific hybrids for rootstocks in 

black walnut orchards. 

Selecting trees that develop fruiting spurs at a 

young age will enhance precocity, increasing 

yield during the establishment phase of a walnut 

orchard. But maintaining the fruitfulness of spurs 

on mature trees may be the key to maximizing 

yield. Optimum light penetration into the tree 

canopy is critical for the stimulation of flowering 

on spurs (Van Sambeek 1998). By selecting trees 

exhibiting open tree architecture, we can ensure 

nut production throughout the canopy.

We have already discussed the critical lack of 

protandrous black walnut cultivars. Is a lack of 

early shedding pollen in the orchard contributing 

to low yields? For some early leafing cultivars, 

lack of pollen may be a problem; however, we have 

not documented this or other potential pollination 

problems in black walnut. In any case, the 

development of protandrous cultivars should be 

important to the commercial success of this crop. 

Goals for Improving Disease Resistance

As mentioned earlier, anthracnose is a major 

limiting factor to fruit production in black walnut. 

Although immunity to this disease has not been 

identified, we will strive to incorporate some 

level of natural defense against this disease. 

Anthracnose can be controlled with well-timed 

fungicide applications; however, the propagation 

of extremely susceptible clones will only result 

in increasing production costs (more fungicide 

applications) for nut producers. All seedlings in the 

breeding program will be screened for anthracnose 

susceptibility. Seedlings demonstrating severe 

susceptibility to the disease will be culled. 

Goals for Improving Nut and Kernel Quality

‘Thomas’, our oldest black walnut cultivar, averages 

24% kernel. Today, we hardly consider planting 

a new walnut cultivar unless it produces at least 

30% edible kernel. Several cultivars regularly 

produce nuts containing over 35% nut meat. The 

genetic improvement of percent kernel in black 
24 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

walnut is the one significant advancement made 

thus far in the evolution of this tree as an orchard 

crop. But additional improvements can be made. 

Our thinnest shelled walnuts are only one or two 

generation removed from wild ancestors. With 

controlled crosses and additional selection pressure 

we will strive to develop walnuts with over 40% 

kernel. 

Kernel quality is heavily influenced by husk 

characteristics. Uniformity in fruit ripening is 

critical for harvesting uniformly light colored 

kernels. Ease of hull removal enhances a 

producer’s ability to remove all remnants of kernelstaining hull from the nut. Yet to be discovered are 

the negative husk characteristics that promote dark 

veins in the kernel integument. As we continue to 

work with the collection of cultivars we currently 

have available, recording husk parameters will 

provide us with the valuable information we will 

need to incorporate husk traits into the breeding 

program. 

LITERATURE CITED

Berhow, S. 1962. Black walnut varieties—a 

reference list. Northern Nut Growers Association 

Annual Report. 53: 63-69.

Chase, S.B. 1941. Effect of handling methods on 

the color and flavor of black walnut kernels. 

Northern Nut Growers Association Annual 

Report. 32: 34-37.

Corsa, W.P. 1896. Nut culture in the United States. 

Special report to the Secretary of Agriculture. 

U.S. Deptartment of Agriculture, Division 

of Pomology. Washington, DC: Government 

Printing Office: 144 p.

Grauke, L.J. 1997. Pecan pollination. In: McEachern, 

G.R.; Stein, L.A., eds. Texas pecan handbook. 

TAEX Hort. Handbook. College Station, TX: Texas 

A&M University: 105(3): 7-13.

Hammons, B.K. 1998. Status report on the Eastern 

black walnut nut industry, nut markets, byproducts, and future challenges. In: Jones, 

J.E.; Mueller, R.; Van Sambeek, J.W., eds. Nut 

production handbook for Eastern black walnut. 

Republic, MO: Southwest Missouri Resource 

Conservation & Development, Inc.: 25-28.

Hendricks, L.C.; Coats, W.W.; Elkins, R.B.; 

McGranahan, G.H.; Phillips, H.A.; Ramos, 

D.E.; Reil, W.O.; Syder, R.G. 1998. Selection of 

varieties. In: Ramos, D.E., ed. Walnut production 

manual. University of California, Division of 

Agriculture and Natural Resources: 3373: 84-89.

Hendricks, L.C. 1999. Walnut variety test planting 

in Merced County. In: Walnut research reports 

1999. Sacramento, CA: Walnut Marketing 

Board: 107-109.

McKay, J.W. 1971. Walnut hybrids at Beltsville, 

Maryland. Northern Nut Growers Association 

Annual Report. 62: 34-38.

Neely, D. 1979. Etiology, epidemiology, and control 

of black walnut anthracnose. In: Kessler, K.J.; 

Weber, B.C., eds. Walnut insects and diseases. 

Gen. Tech. Rep. NC-52. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, North 

Central Forest Experiment Station: 58-62.

Reid, W. 1986. Reasons for poor kernel filling. 

Northern Nut Growers Association Annual 

Report. 77: 37-38.

Reid, W. 1990. Eastern black walnut: potential for 

commercial nut producing cultivars. In: Janick, 

J.; Simon, J., eds. Advances in new crops. 

Portland, OR: Timber Press: 327-331.

Reid, W. 1995. Understanding black walnut 

anthracnose. The Nut Kernel. 47(3): 13-14.

Reid, W. 1997a. Black walnuts. In: Brooks, R.; Olmo, 

H.P., eds. Registry of new fruit and nut varieties. 

3rd ed. Alexandria, VA: ASHS Press: 156-160.

Reid, W. 1997b. Black walnut. In: Okie, W.R., ed. 

Register of new fruit and nut varieties. Brooks 

and Olmo List 38. HortScience. 30: 785-805.

Reid, W. 1997. Evaluation and management of 

black walnut for nut production. In: Van 

Sambeek, J.W., ed. Knowledge for the future 

of black walnut: 5th Black walnut symposium; 

1996 July 28-31; Springfield, MO. Gen. Tech. 

Rep. NC-191. St. Paul, MN: U.S. Department 

of Agriculture, Forest Service, North Central 

Forest Experiment Station: 211-216.

Van Sambeek, J.W. 1998. Carbohydrate 

assimilation, translocation, and utilization. In: 

Jones, J.E.; Mueller, R.; Van Sambeek, J.W., 

eds. Nut production handbook for eastern black 

walnut. Republic, MO: SW Missouri RC&D: 73-

78. <www.ncrs.fs.fed.us/epubs/walnut/>

Worley, R.E. 1979. Pecan yield, quality, nutlet set, 

and spring growth as a response of time of fall 

defoliation. Journal of the American Society for 

Horticultural Science. 104: 192-194.
Tree Improvement 25

Black Walnut in a New Century

1Staff Research Associate (CAL) and Pomologist (GHM), Pomology Department, University of California Davis, One Shields Ave., Davis, CA 

95616 CAL is corresponding author: to contact call (530) 752-5604 or e-mail at <caleslie@ucdavis.edu>.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., eds. 

2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. Tech. Rep. 

NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.

PERSIAN WALNUT BREEDING IN CALIFORNIA

Charles A. Leslie and Gale H. McGranahan1

ABSTRACT—For over 50 years the University of California Davis Walnut Breeding 

Program has worked to address the needs of California walnut growers by 

identifying genetic approaches to problems and developing improved cultivars. The 

breeding program is a cooperative endeavor that draws on the efforts and resources 

of university researchers and facilities, USDA germplasm programs, the statewide 

Cooperative Extension program, grower, and nursery funding sources, and the 

extensive participation of individual growers, processors, and nurserymen in all 

aspects of the program.

CALIFORNIA WALNUT INDUSTRY

Production

Persian walnut production in the United States 

is concentrated almost entirely in the State 

of California. California growers account for 

99% of all domestic Persian walnut production 

(Walnut Marketing Board 1997). There are now 

approximately 200,000 bearing acres of Persian 

walnuts in California, an increase during the last 

15 years of about 20%. In that same time period 

production per bearing acre has increased about 

20% to a current statewide average of 1.5 tons per 

acre and total production of Persian walnuts has 

increased about 30% to nearly 300,000 in-shell 

tons annually (California Agricultural Statistics 

Service 2003).

About 60% of this production is consumed 

domestically but exports are an important 

component of marketing the crop and maintaining 

price to the grower (Siebert 1998). Concerted 

marketing by grower organizations and processors, 

in some cases with the assistance of government 

programs supporting development of export 

markets (Siebert 1998), have helped increase the 

proportion of the crop exported from only 25% 20 

years ago to about 40% in the last 4 years.

Growers

California walnut growers farm an average of 50 

acres of walnuts but farm size varies widely (Hasey 

1994). Large growers farming 100 or more acres 

account for 60% of the acreage but constitute 

only 12% of the growers. Nearly 60% of growers 

farm fewer than 20 acres while producing only 

10% of the crop. Production over the last several 

years has averaged 1.5 tons/acre. Prices can vary 

widely from year to year but recently the crop has 

returned about $1,000/ton to growers (California 

Agricultural Statistics Service 2003).

About half of the California walnut crop is 

produced by growers who belong to Diamond 

Walnut, the largest growers’ cooperative (Beede and 

Hasey 1998). Diamond has field representatives 

statewide who work with the member growers, 

a central shelling and processing facility, and 

its own product development and marketing 

programs. Approximately 70 independent shellers 

and handlers, varying widely in size and services, 

market the other half of the production. Economic 

conditions are contributing to increasing numbers 

of vertically integrated grower-handlers who market 

their own production.

The ability of cooperative members and 

independent growers to collaborate on issues 

of mutual interest such as product standards, 
26 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

development of export markets, and investment in 

research has contributed greatly to the success of 

the industry. These issues are coordinated through 

two grower organizations, the California Walnut 

Commission, responsible for export development 

(Siebert 1998), and the Walnut Marketing Board.

The Walnut Marketing Board oversees 

implementation of the Walnut Marketing Order 

established in 1949 under the 1937 Federal 

Agricultural Marketing Act (Hasey 1994). The 

marketing order was established and approved 

by growers in response to low prices and absence 

of enforceable quality standards. By law, the 

board’s activities are conducted under USDA 

oversight but board members are walnut growers 

and handlers elected by the growers. The board 

oversees quality and grade standards, monitors 

collection of industry production statistics, and 

regulates commodity volume. In addition to these 

functions, the order established a mechanism 

for funding marketing and production research. 

Growers assess themselves at the handler level 

on merchantable crop tonnage and the board 

allocates a portion of these funds annually to fund 

horticultural and post-harvest research (Beede and 

Hasey 1998).

Nurseries

Commercial walnut nurseries are another key 

element of the industry. Nurseries vary widely 

in size and scope of activities from large multicrop operations with international sales to small 

family walnut operations and growers who produce 

seedlings only for their own use. There are no State 

or other public nurseries.

Persian walnuts are a clonally propagated crop 

in which production is almost entirely from scion 

varieties grafted to seedling rootstocks. Rootstocks 

in California are either seedlings of California 

black walnut (Juglans hindsii) or hybrids derived 

from black walnuts pollinated with Persian pollen 

(J. hindsii x J. regia). These hybrids, known 

as Paradox, have superior vigor but are more 

susceptible to crown gall disease (Catlin 1998). 

Nurseries collect seed from trees known to 

produce Paradox seed, establish seed orchards 

for this purpose, or purchase seed from suppliers. 

Seedlings are either sold ungrafted after one year 

for growers to graft on their own or they are grafted 

or budded in the nursery to the desired scion 

variety and sold as 2-year-old grafted trees. Recent 

advances in rooting methods have contributed to 

very limited production of Persian cultivars on their 

own roots and clonal rootstock.

Nurseries are regulated by the California 

Department of Agriculture (CDFA). The CDFA sets 

standards for nursery stock, oversees production 

and distribution of certified stocks, and addresses 

some nursery interests in research. The nurseries, 

through their organization, the California Fruit 

Tree, Nut Tree, and Grapevine Improvement 

Advisory Board (IAB), assess themselves a fee 

based on plant materials sold and dedicate a 

portion of these funds to research. 

WALNUT RESEARCH FUNDING

Research and technological development is an 

important component in maintaining a growing, 

profitable, and competitive walnut industry. 

Funding for walnut research is derived from a 

variety of sources. 

The State of California, through the University of 

California, particularly the UC Davis Pomology 

department, has long contributed support through 

its funding of faculty, research staff, extension 

specialists, and county-based farm advisors, 

infrastructure including research farm operations, 

greenhouses and laboratories, and funding for 

equipment. 

The USDA-ARS has contributed funding for 

research positions, some as adjunct faculty, 

research programs, and particularly funding 

through the National Clonal Germplasm Repository 

for an extensive walnut germplasm collection. 

Some research is also funded through State and 

university competitive grants intended to support 

biotechnology development and foster technology 

transfer to industry. 

As State and university funding for agricultural 

research has come under pressure in recent 

years, direct grower funding through the Walnut 

Marketing Board has become increasingly 

important. Through the Board the growers provide 

substantial annual funding of research proposals 

and have endowed a permanent staff position for 

the Walnut Breeding Program. Nurseries, through 

the IAB, fund research that addresses their 

concerns including clean stock, trueness-to-type, 

and propagation. Contributions from individuals 

in the industry have also been important and 

have included funding for a graduate student 

assistantship specifically for walnut research.

RESEARCH-INDUSTRY INTERACTION

Industry input and participation has been an 

essential element of walnut research success. The 

annual Walnut Research Conference has become 
Tree Improvement 27

Black Walnut in a New Century

a key component in coordinating walnut research 

efforts and industry-researcher interactions in 

California. This conference originated as an 

annual training meeting for the UC Cooperative 

Extension Service county farm advisors. Farm 

advisors, roughly the equivalent of county agents 

in other States, are responsible for working 

directly with growers to solve problems and to 

disseminate research information from university, 

USDA, and other sources. They are also expected 

to conduct some applied research of their own. 

This meeting was originally an opportunity for 

university researchers at all levels and perspectives 

to internally share information, update skills, 

and address new problems and directions. This 

meeting also acquired the function of reviewing 

and recommending proposals to be submitted to 

the Walnut Marketing Board for funding.

As grower funds became an increasingly important 

component of research financing, members of the 

Walnut Marketing Board Research Committee 

were invited to see the presentation of proposals 

first hand. Initially they were invited as observers 

but over the years their role has expanded to 

full participants, speakers, and members of 

panel discussions. As proposals began to involve 

coordinated funding with the nursery industry as 

well, nursery board members were also invited. 

USDA-ARS researchers working on walnuts also 

attend. 

The USDA Juglans Crop Advisory Committee, 

a group with diverse expertise in all species of 

walnuts and which advises the USDA National 

Clonal Germplasm Repository on germplasm 

collection and maintenance issues, generally holds 

its annual meeting in conjunction with the Walnut 

Research Conference and committee members with 

interests in black walnut and butternut have also 

participated in the conference in recent years.

The result is a 3-day conference with a wide 

array of participants assembled to review current 

research results, discuss industry problems, 

and debate directions for new research. The 

combination of formal research presentations, 

structured discussions, and a series of social 

events supported by industry funds, help cement 

relationships and define directions for the coming 

year’s work.

Research discussed at this conference covers 

a broad array of topics. Last year, for example, 

reports, proposals, and panel discussions 

included codling moth control methods, husk fly, 

bacterial blight, crown gall control, integrated 

pest management research, nematodes, blackline 

virus, tree training, mechanical hedging, light 

management, water relations and irrigation 

methods, replacements for post-harvest fumigants, 

rootstock selection, propagation methods, tissue 

culture, genetic engineering, field performance 

of self-rooted plants, DNA finger-printing, and 

development of new varieties.

This cooperation and interaction occurs not 

just at the annual research conference but also 

throughout the research process. Close industry 

and researcher cooperation is essential, not just 

in setting direction, but in implementation and 

transfer to the growers. Information discussed at 

the conference is disseminated statewide to growers 

through a series of annual county grower meetings 

organized by the farm advisors. In addition, 

new information is summarized in extension 

publications and every several years Cooperative 

Extension organizes a weeklong Walnut Short 

Course for interested growers. This course is taught 

by faculty, farm advisors, and innovative growers, 

includes lectures and field tours, and provides 

participants with an updated version of the Walnut 

Production Manual (Ramos 1998), the University of 

California textbook on Persian walnuts.

This entire process ensures a clear understanding 

of industry needs in research, a comfort level 

with progress in the work being funded, and a 

rapid transfer of results back to the growers. An 

examination of how the walnut breeding program 

functions illustrates the importance of this 

interaction and steps where it occurs.

WALNUT BREEDING PROGRAM

History

The Persian Walnut Breeding Program began at UC 

Davis in 1948. In the following 30 years under the 

direction of Gene Serr and Harold Forde crosses 

incorporated t he lateral bearing trait responsible 

for precocious high yield into 15 released cultivars 

(Serr and Forde 1968; McGranahan and others 

1990, 1992; Tulecke and McGranahan 1994). 

Several of these are leading varieties today. 

‘Chandler’, notable for its’ light kernel color, 

high yield of halves, and relatively low input 

requirements is the leading cultivar in the State. 

Objectives

The breeding program has continued since 1982 

under the direction of Gale McGranahan. The 

current breeding program seeks to address a 

variety of industry concerns by developing both 

improved scion varieties and clonally propagated 

rootstocks.
28 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

Growers and handlers are interested in obtaining 

new scion varieties with early harvest dates and 

reduced need for chemical inputs. Over 60% of 

current production is from late harvesting varieties 

(California Agricultural Statistics Service 2003). 

The most popular variety, ‘Chandler’, is relatively 

late harvesting, constitutes 27% of current bearing 

acres, and is increasing as a proportion of total 

plantings. Hartley, another relatively late variety, 

comprises an additional 25% of current acreage. 

Early varieties (Payne, Ashley, and Serr) represent 

only 20% of bearing acres and have declined in 

popularity due to insect and blight susceptibility 

or, in the case of ‘Serr’, poor yield. Concentration 

of crop production at the end of the season results 

in poor utilization of harvesting equipment and 

processing capacity. Late harvest also results in 

increased risk of rain during the harvest operations 

and a tight time-line for drying and shipping 

product to Europe before the holiday season.

Development of scion varieties resistant to cherry 

leafroll virus, the causative agent of blackline 

disease is of particular concern for growers in the 

cooler areas of the State where the disease is most 

prevalent. 

Rootstock qualities of greatest interest include 

vigor, tolerance for blackline virus, and resistance 

to crown gall, nematodes, and Phytophthora 

root and crown rots. Developing commercially 

viable rootstock propagation methods that would 

allow nurseries to replace the industry’s current 

dependence on unimproved seedlings with clonally 

propagated improved rootstock is a high priority.

Germplasm

Collection

A breeding program depends in part on a diverse 

collection of germplasm as a source of raw material 

from which traits of interest can be identified. 

Persian walnuts are native to the mountains of 

Central Asia (Leslie and McGranahan 1998) so 

considerable effort has been directed towards 

collecting material from that area. Funding and 

participation in this work has included a century 

long plant introduction endeavor by USDA plant 

collectors, and more recent trips by USDA and 

university researchers. Collecting has been funded 

in part by California growers, USAID exchanges, 

and USDA-ARS Germplasm exploration funds. 

Material has also become available for use through 

international research contacts, private breeders, 

hobbyists, customs confiscations, and observant 

growers in the State who have noticed useful 

seedling trees.

Maintenance

Both the University of California and the USDA 

National Clonal Germplasm Repository maintain 

walnut germplasm collections. The intent of 

the USDA collections is to include as broad a 

diversity of all walnut species as possible and it 

is maintained for public distribution of material. 

It will not accept proprietary material and is 

managed primarily for wood and nut distribution 

to researchers worldwide. The UC Davis collection 

includes a representation of California commercial 

varieties, advanced selections, and some 

proprietary material and is focused primarily 

on material of interest for breeding purposes. It 

is managed for a variety of activities including 

crossing, breeding evaluations, and graftwood 

distribution of advanced selections. While 

there is some overlap of material, duplication is 

generally avoided and the two collections are used 

cooperatively.

Evaluation

Germplasm in these collections has to be evaluated 

and characterized to determine its useful 

attributes. Some of this work is done specifically 

for the breeding program and is funded through 

grower support of the program. Part of this work, 

particularly evaluation of the broader USDA 

collection has been funded through USDA-ARS 

Germplasm evaluation funds. These data are 

collected and made publicly available on-line 

through the USDA GRIN database.

Crossing Methods

The UC breeding program has used two distinct 

procedures for crossing parent material. In the 

first method, wind-blown pollen is excluded from 

female flowers of interest by covering them with 

tightly secured bags with small plastic windows. 

Pollen is collected from the other parent of interest 

and stored until use. When bagged female flowers 

open and are receptive, pollen is applied through 

the bags with a hypodermic needle. Bags are later 

removed and nuts marked for collection in the fall. 

The male parent is known with this method, but 

the costs are high and seedling production is low.

The second method is to locate geographically 

isolated young trees to avoid wind-blown pollen. This 

requires the cooperation of a grower with a recently 

planted orchard. Any male flowers on these trees are 

removed by hand before opening to prevent selfing. 

Once the female flowers begin to open, pollen of the 

desired male parent or parents is applied by airbrush 

several times during the bloom period. At harvest the 

cooperating grower either donates or is compensated 
Tree Improvement 29

Black Walnut in a New Century

for the nuts. This method produces many more 

seed at lower cost but with low certainty of the male 

parent. Male parents of selections can be determined 

later by DNA analysis. 

Seedling Evaluation

Seed collected from these crosses is then stratified 

and grown to produce the next generation of 

seedlings. These are screened as they mature for 

traits of interest (Forde and McGranahan 1996). 

Commercial walnut nurseries have generously 

donated growing ground, time, resources, and 

expertise to assist this aspect of the program.

After 1 year in the nursery, trees are dug and 

replanted on wider spacing for evaluation. At this 

stage, trees are grown on their own roots, not 

grafted to rootstock. Most commonly these trees are 

planted on UC Pomology department ground and 

farmed by department staff supported by university 

and grower funding. In some cases growers have 

assisted the program by donating orchard space for 

this purpose and have farmed these trees during 

the evaluation process. This has been done by 

planting between rows in an existing widely spaced 

orchard, or more effectively, by interplanting in 

available open space in a newly established orchard 

and then removing the breeding program trees as 

evaluations are completed and the grower’s orchard 

matures to fills the canopy.

As seedling trees mature they are evaluated in 

the field for traits of interest including leafing, 

flowering, and harvest dates, yield, and growth 

habit. When the trees are grown in university 

orchards they are left unsprayed so that variation 

in resistance to insects and disease can be 

observed. When grown within commercial orchards 

this is not normally possible. Nut samples are 

hand collected from each tree at maturity. Samples 

are dried, cracked by hand, and evaluated for 

percent kernel, kernel quality, kernel weight, 

shell characteristics, and yield of halves. Data 

is entered into a database and summarized for 

multiple years. In addition, samples of promising 

individuals are sent to the Diamond Cooperative 

and other interested handlers for their independent 

evaluation.

Collected data is presented to farm advisors, 

growers, and nurserymen in several ways. The first 

is at the annual Walnut Research Conference as 

part of the Walnut Improvement Program’s annual 

report. Data on selections is presented orally to 

attendees and published in the annual proceedings 

of the conference.

The breeding program also holds an annual Crackout 

Meeting in the spring attended by farm advisors, 

handlers, nurserymen, and growers. Growers, 

handlers, and nursery attendees generally have an 

expressed interest in development of new varieties, 

are interested in assisting with evaluation of material, 

or are otherwise active in research activities and the 

marketing board. At this all day meeting the data 

reports are distributed and kernel samples and intact 

nuts of the material under evaluation are displayed. 

Attendees are asked to review the material, examine 

the samples, and provide written comments. In an 

ensuing discussion period they provide valuable 

input on priorities from their varying perspectives, 

help rank material, and suggest which should 

continue in the program.

The program also regularly invites interested 

parties to view selections in the field, either 

through a formal field day or by scheduling 

informal visits at their convenience. Progress in the 

program and information about selections is also 

presented periodically to a wider range of growers 

at annual county grower meetings held around the 

State. 

Selection Trials

Once an individual seedling shows promise 

and is selected for further trials, graftwood is 

collected from the original seedling and grafted 

to rootstocks. Nurseries have often provided 

assistance at this stage by donating rootstock, 

supplying grafters, and in many cases growing the 

grafted trees for the program.

Grafted trees of each selection are then planted in 

test blocks on orchard spacing at diverse locations 

for further evaluation. Currently these tests 

blocks are located at the Chico State University 

Farm in the northern part of the State, on the UC 

Davis campus in the central region and at the UC 

Kearney Field Station in the south. These blocks 

are managed by Cooperative Extension farm 

advisors and are used to evaluate the performance 

of selections on rootstocks under a wide range of 

conditions, obtain a better look at yield, and allow 

farm advisors and growers to see selections in their 

local area.

In addition to the university plots, interested 

growers around the State have volunteered to 

establish trials ranging in size from several trees 

to several acres. Farm advisors assist in identifying 

suitable growers, establishing plots, and observing 

performance. Graftwood is distributed to these 

growers under test agreement and they are asked 

to participate in its evaluation and to attend the 

crackout meeting. This gives the program valuable 
30 Tree Improvement 

Proceedings of the 6th Walnut Council Research Symposium

input on performance under a variety of conditions 

and in commercial settings from observers with 

extensive experience. Growers feel they are assisting 

the process and get an early look at the material 

that is most interesting for their situation. 

As new selections begin to show promise, 

commercial nurseries are encouraged to acquire 

graftwood from the program to test the varieties for 

themselves and to begin increase-blocks of their 

own. This ensures nurseries have adequate input 

into final selection, firsthand knowledge of the 

material, particularly of its grafting performance, 

growth habit, and training requirements, and 

builds an adequate supply of production wood by 

the time the new variety is released. As with grower 

trials, nurseries receive wood under test agreement. 

This allows them to propagate for testing purposes, 

including grower trials, but trees cannot be 

produced for sale until they are patented. 

Release

Selections that continue to show promise in test blocks 

and grower trials become candidates for patent and 

release as new varieties. The patent disclosure process 

requires an extensive description of the selection, 

a summary of available data, and identification of 

attributes distinct from existing varieties. 

Once a selection is patented as a new cultivar, 

nurseries may obtain a commercial license from 

the University of California that allows sale of 

trees. A per-tree royalty is assessed at the time 

of sale from the nursery and returned to the 

university. After patenting costs are recovered, 

part of this fee is assigned for overhead and part is 

returned to the department and breeding program. 

Patenting provides a return to the inventor and the 

university but also seeks to protect the growers 

from unlimited distribution. Patented material 

is not allowed to be sold or grown outside of 

California for five years after release. After that 

period, overseas licensing provides a return to the 

program that would not otherwise occur.

SUMMARY

In an increasingly globalized marketplace, 

technological advancement and improved efficiency 

is essential for survival. The UC Davis Walnut 

Breeding Program is but one aspect of a longterm, highly cooperative, and broadly based 

research effort in California, designed to solve 

problems for the State’s walnut growers and to 

maintain a competitive and profitable industry. 

The contributions and cooperation of all interested 

parties has been the cornerstone of success.

LITERATURE CITED

Beede, R.H.; Hasey, J.K. 1998. The History of 

the walnut in California. In: Ramos, D.E., ed. 

Walnut production manual. Oakland, CA: 

Division of Agriculture and Natural Resources, 

University of California: 8-15.

California Agricultural Statistics Service. 2003. 

2003 California walnut objective measurement 

survey report. 6 p.

Catlin, P.B. 1998. Root physiology and rootstock 

characteristics. In: Ramos, D.E., ed. Walnut 

production manual. Oakland, CA: Division of 

Agriculture and Natural Resources, University 

of California: 119-126.

Forde, H.I.; McGranahan, G.H. 1996. Walnuts. In: 

Moore, J., ed. Fruit breeding. New York, NY: 

John Wiley and Sons: 241-273.

Hasey, J. 1994. Maintaining the competitive edge in 

California’s walnut industry: trends, issues, and 

challenges. Davis, CA: University of California 

Agricultural Issues Center. 86 p.

Leslie, C.A.; McGranahan, G.H. 1998. The origin of the 

walnut. In: Ramos, D.E., ed. Walnut production 

manual. Oakland, CA: Division of Agriculture and 

Natural Resources, University of California: 3-7.

McGranahan, G.H.; Forde, H.I.; Snyder, R.G.; Sibbett, 

G.S.; Reil, W.; Hasey J.; Ramos, D.E. 1992. Tulare 

Persian Walnut. HortScience. 27(2): 186-187.

McGranahan, G.H.; Ramos, D.E; Forde, H.I.; 

Snyder, R.G. 1990. Cisco Persian Walnut. 

HortScience. 25(3): 371-372.

Ramos, D.E., ed. 1998. Walnut production manual. 

Oakland, CA: Division of Agriculture and Natural 

Resources, University of California. 320 p.

Serr, E.F.; Forde, H.I. 1968. Ten new walnut varieties 

released. Califvornia Agriculture. 22: 8-10.

Siebert, J.B. 1998. Marketing California walnuts. In: 

Ramos, D.E., ed. Walnut production manual. 

Oakland, CA: Division of Agriculture and Natural 

Resources, University of California: 23-30.

Tulecke, W.; McGranahan, G.H. 1994. The walnut 

germplasm collection of the University of 

California, Davis: A description of the collection 

and a history of the breeding program of Eugene 

L. Serr and Harold I. Forde, Genetic Resources 

Conservation Program, Division of Agriculture 

and Natural Resources. 39 p.

Walnut Marketing Board. 1997. Walnut Industry 

Fact Sheet. 22 p.
Propagation and Genetic Modification 31

Black Walnut in a New Century

1

Graduate Research Assistants (ASD and BCW) and Assistant Professor (DFJ), Hardwood Tree Improvement and Regeneration Center, 

Department of Forestry and Natural Resources, Purdue University, 195 Marsteller Street, West Lafayette, IN 47907-2033. DFJ is 

corresponding author: to contact, call (765) 494-3608 or e-mail: djacobs@fnr.purdue.edu.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.

EFFECT OF SEED POSITION AND MEDIA ON GERMINATION OF

BLACK WALNUT AND NORTHERN RED OAK: 

IMPLICATIONS FOR NURSERY PRODUCTION AND DIRECT SEEDING

Anthony S. Davis, Barrett C. Wilson, and Douglass F. Jacobs1

ABSTRACT—Germination of black walnut (Juglans nigra L.) and northern red oak 

(Quercus rubra L.) prior to sowing into containers or bareroot nursery beds can help 

maintain desired crop density and reduce nursery costs. Recommended techniques for 

germination of black walnut are labor intensive and require that walnuts be completely 

covered with growing media, periodically removed, and rinsed to identify germination 

prior to excessive elongation of the radicle. The purpose of this study was to determine 

if immersing 50% of a walnut into the soil such that the germinating radicle could be 

readily observed—a successful germination technique for northern red oak acorns—was 

also viable for germinating walnuts. The effect of media type on germination was also 

investigated. Percent germination was not influenced by seed position; however, fewer 

black walnuts germinated in vermiculite than in a soil mix. These findings may benefit 

research projects, small-scale nursery production, and direct-seeding operations.

INTRODUCTION

Throughout the Central Hardwood Forest Region, 

production of fine hardwood trees is an important 

component of the forest industry. In Indiana, 

landowners typically plant for timber production, 

creation of wildlife habitats, and to leave a legacy 

for future generations (Ross-Davis and others in 

press). Northern red oak (Quercus rubra L.) and 

black walnut (Juglans nigra L.) have proven to 

be valuable species for timber production and 

wildlife utilization, making them among the most 

abundantly planted species in Indiana (Jacobs and 

others 2004).

To produce tree seedlings, seeds are collected, 

stored and stratified (if necessary), and sown into 

containers, nursery beds, or the field. Standardized 

nursery practices involve covering the seed with 

soil immediately after sowing to prevent desiccation 

(Landis 1990, Auchmoody and others 1994) and, 

in the case of bareroot stocktypes, to limit seed 

predation (USDA Forest Service 1948, Auchmoody 

and others 1994, Kujawski and Davis 2001). 

Procurement of hardwood tree seed is an expensive 

undertaking, therefore demanding increased 

seed viability to ensure uniform crop production 

and efficient use of nursery growing space. While 

various tests can be used to ensure that only 

viable seeds are planted (e.g., float-test and x-ray), 

germination cannot be guaranteed. For example, 

viable seed sown under insufficient moisture 

or temperature conditions may not germinate. 

Additionally, these tests are expensive and time 

consuming, limiting their value on an operational 

scale for many nurseries.

Germinating seed prior to sowing could reduce 

nursery production costs for bareroot and container 

grown seedlings and increase the success of direct 

seeding operations. Bareroot seedling production 

methods consist of sowing seed in nursery beds 

immediately after collection or removal from 

storage. Germination of seed prior to sowing 

improves the likelihood of maintaining constant 

density in the nursery bed. In containerized 

seedling production, the number of containers 

and the amount of soil media needed are directly 

dependent upon the number of containers sown. 

Therefore, an increase in the percentage of cells 

containing successfully germinated seedlings would 

reduce production costs. The use of pre-germinated 

seed would help to ensure that in each container 

is a viable seed. Direct-seeding operations typically 
32 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

broadcast large volumes of seed across the area 

designated for reforestation (Johnson 1981, 1983). 

Drawbacks associated with the direct seeding of 

black walnut include unpredictable germination 

and seed predation (Van Sambeek 2004). 

Germination of seed prior to sowing resulted in 

successful plantation establishment with uniform 

spacing and survival rates > 90% (Jacobs and 

Severeid 2004). While pre-germination may not be 

operationally feasible for many nurseries, it could 

serve to decrease variation due to germination rate 

in research projects.

An experiment recently conducted by the Hardwood 

Tree Improvement and Regeneration Center called 

for the germination of approximately 10,000 

black walnut seeds (Jacobs and others in press). 

To achieve the required number of germinated 

seeds, walnuts were sown into flats and covered 

with growing media, following the recommended 

procedure for Juglans spp. of covering nuts to a 

depth of up to 7.62 cm (USDA Forest Service 1948, 

Kujawski and Davis 2001, Reid 1998). Seeds were 

examined regularly for germination, and those 

that germinated were removed from the flats and 

stored under moist towels at 2° C until target 

quantities of germinated walnuts were available. 

Minimal development of the radicle of germinated 

seeds was important to maintain uniform seedling 

development across the project (Fig. 1). Therefore, 

early detection was vital and extensive time and 

labor was invested for frequent assessment of seed 

status. 

These intensive requirements raised questions 

about the feasibility of alternative germination 

methods. Successful germination of northern red 

oak results from 50% immersion of acorns into the 

soil (Fig. 2) (Davis 2003), despite recommendations 

that seeds be completely covered by soil (USDA 

Forest Service 1948, Kujawski and Davis 2001). 

Since moisture conditions can be readily controlled 

through a combination of timers and mist-irrigation 

systems, it was hypothesized that germination 

of black walnuts and northern red oak acorns 

would be similar whether they were placed 50% 

immersed in the growing media or fully covered 

by media. Furthermore, given that different media 

have different moisture holding capacities, a soil 

media and coarse vermiculite were both selected as 

variables. Given the ease with which germination 

could be monitored, successful germination 

of walnuts and acorns only 50% immersed in 

the growing media would greatly decrease the 

traditional labor-intensive process of monitoring 

germination. The objective of this study was to 

determine if northern red oak and black walnut 

seed germination was influenced by seed position or 

media type.

Materials and Methods

Northern red oak and black walnut were chosen 

as the trial species based on their importance in 

the Central Hardwood Forest Region. To reduce 

genetic variability, all walnuts and acorns used in 

this study came from a single respective tree. All 

seeds were collected in autumn 2002. The walnut 

hulls were removed prior to storage; prior to sowing, 

acorns were floated for 48 hours with all floating 

seeds removed.

Seeds were sown April 21, 2003 into individual 

75 cm3 pots (5 cm × 5 cm × 5 cm). Each pot 

contained either coarse grade vermiculite (StrongLite® Sunshine®, SungroTM Horticulture, Pine Bluff, 

AR) or Scott’s 366-P ScottsCoirTM growing medium 

(The Scotts Company, Marysville, OH). In their 

respective growing media, walnuts and acorns were 

randomly assigned to one of two treatments: (1) 

50% immersion in the media, or (2) submersion 1 

cm below the surface. To maximize the likelihood 

of germination, acorns were sown on their side 

(Trencia 1996) and walnuts were sown with their 

cleavage line perpendicular to the soil surface 

(Ciccarese 1995). The pots were then watered to 

saturation and placed in a mist propagation bench 

Figure 1.—Minimal development of 

germinated black walnut.

Figure 2.—Northern red oak germinating 

50% immersed in soil.
Propagation and Genetic Modification 33

Black Walnut in a New Century

in the Horticulture and Landscape Architecture 

Plant Growth Facility at Purdue University. Pots 

were misted every 10 minutes for 10 second 

durations for 24 hours per day. After 6 weeks, total 

germination was recorded for each treatment.

The study was established as a completely 

randomized design (2 × 2 factorial [position × 

media]). For each species, analysis of variance was 

used to identify differences in germination percent 

by treatment. The level of significance was set at α

= 0.10. SAS® (SAS Institute, Cary, NC) was used for 

all data analyses.

RESULTS AND DISCUSSION

Neither seed position (p = 0.9756) nor media (p

= 0.9759) had a significant effect on the percent 

germination of northern red oak acorns (Fig. 3). 

Position (50% immersed or completely covered by 1 

cm of media) did not significantly affect the percent 

germination of walnuts (p = 0.7151). Media had a 

significant effect (p = 0.0504) on the germination 

percent of walnuts (Fig. 3). It is possible that fewer 

walnuts germinated in the vermiculite media due 

to the higher porosity of coarse vermiculite. While 

coarse vermiculite was used for this study, different 

grades of vermiculite (i.e., fine or medium textured) 

may be more suitable to walnut germination given 

differences in internal porosity (Landis 1990). There 

was no significant interaction between seed position 

and media type for northern red oak (p = 0.9763) or 

black walnut (p = 0.8014).

Seed position did not influence the germination of 

black walnut or northern red oak acorns. These 

findings concur with those reported for Persian 

walnut (Juglans regia L.) (Ciccarese 1995). When 

the germination of walnuts and acorns prior to 

transplanting is necessary, this method, compared 

to covering with soil, will reduce the time needed to 

identify and collect germinants. 

The concept of using pre-germinated seed is not 

new. DeVelice and Buchanan (1978) recommended 

investigation into the use of pre-germinated 

ponderosa pine (Pinus ponderosa Laws.) as a means 

of increasing the likelihood of successful plantation 

establishment in New Mexico. Germination of 

seed prior to sowing containerized crops has also 

proven beneficial in nursery production in South 

Africa (South and Young 1995). The authors found 

this method ensures that between 98 and 99% of 

container cells are filled, and recommend the use of 

this method in North American nurseries. Adoption 

of this system in containerized seedling production 

would likely increase seed use efficiency and 

crop consistency, decreasing the space needed to 

produce a crop and costs associated with thinning 

and transplanting seedlings. 

Advantages could also be attained in bareroot 

seedling production. Since seedling density 

influences height, root-collar diameter, and dry 

mass (Tomlinson and others 1996), as well as 

the number of first-order lateral roots (Schultz 

and Thompson 1996), using germinated seed 

to maintain consistent density may assist in 

producing more uniform seedlings. Black walnut 

seedlings grown at lower densities were more 

likely to produce more first-order lateral roots, 

which increased seedling survival and growth 

after outplanting (Schultz and Thompson 1996). 

Therefore, using pre-germinated seed would 

Figure 3.—Percent germination of black walnuts and northern red oak acorns for two media 

types. Different letters represent differences within species significant at α = 0.10.
34 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

increase a grower’s control over seedling bed 

density, which could improve outplanting survival 

and growth.

Successful plantation establishment can be 

achieved via direct-seeding (Johnson 1981, 1983; 

Wittwer 1991). Direct-seeded Nuttall oak (Quercus 

nuttallii Palmer) sown at approximately 8,000 

acorns per ha yielded 5,657 trees per ha after 10 

years (Johnson 1981). Mullins and others (1998) 

found that there was no significant difference in 

height or diameter of bareroot, containerized or 

direct-seeded (not pre-germinated) cherrybark oak 

(Quercus falcata Michx. var. falcata) seedlings 5 

years after planting.

While attempts to direct-seed black walnut have 

yielded mixed results (Robison and others 1997), 

the potential exists to do so successfully (Schavilje 

1941, Davidson 1980). Lack of success has been 

attributed to poor germination and growing 

conditions in the field (Robison and others 1997). 

Germination prior to seeding, however, may act 

to improve the success rate of direct-seeding 

attempts, given successful plantation establishment 

with direct-sown pre-germinated seed of 

northern red oak, black walnut, and American 

chestnut [Castanea dentata (Marsh.) Borkh.] 

(Jacobs and Severeid 2004). Further research 

is needed to identify those site conditions and 

tending requirements that will ensure successful 

establishment, as well as determining the economic 

and ecological costs and benefits of this method for 

reforestation. Additionally, research into herbicides 

that are suitable for use with direct seeding 

(Willoughby and others 2003) will lead to improved 

flexibility in field experimentation.

Direct seeding as a means of plantation 

establishment could reduce those stresses related 

to planting. The loss of fine roots of hardwood 

species during wrenching may retard seedling 

development. In addition to potential benefits 

realized in seedling establishment, the cost of 

transporting seed is lower than that of seedlings 

(Van Sambeek 2004). Bullard and others (1992) 

found that on old-field sites in the southern 

United States the cost of direct seeding oak was 

approximately 1/3 of those of planting seedlings 

and that given proper stand management there 

would be no benefit of planting seedlings over direct 

seeding.

The potential to use direct seeding of pregerminated seed as an effective means of enhancing 

existing plantations should not be disregarded. On 

non-industrial private forestland plantations in 

Indiana, survival of hardwood seedlings at age 5 

has been estimated at approximately 65% (Jacobs 

and others 2004). Many landowners establish 

plantations at a desired density. To maintain the 

desired stocking with seedlings would require hand 

planting to fill in where mortality occurred, which 

can be strenuous and may limit seedling vigor 

(Jacobs and others 2004, Russell Jr. and others 

1998). Germinant sowing to replace dead seedlings 

may be a viable means of maintaining stocking at a 

desired level without having to purchase and plant 

additional seedlings.

CONCLUSION

Pre-germination of acorns and walnuts may be 

beneficial by increasing uniformity of research 

projects and small-scale nursery culture and 

direct-seeding operations. The results of this 

study emphasize the need to understand the 

influence of different media types on the production 

of hardwood seedlings. Identification of media 

that maximizes the likelihood of black walnut 

germination will be beneficial to improving walnut 

production efficiency. Easy detection of walnut and 

acorn germination can be achieved with sowing 

seed 50% immersed into soil. This process can 

save money, time, labor, and materials without 

losses associated with non-germinated seeds. As 

consistent seedling density increases the uniformity 

of a crop and maximizes the use of growing space, 

crop production efficiency would benefit from 

this approach. Planting operations that choose 

to employ the direct-seeding method would also 

benefit from pre-germinating seed, as reduced 

sowing density would be needed to achieve the 

desired spacing. Further trials with a larger sample 

size and additional treatments may be warranted 

to affirm our results and identify procedures that 

further enhance germination success.

ACKNOWLEDGMENTS

We would like to thank Jim McKenna (Hardwood 

Tree Improvement and Regeneration Center), Rob 

Eddy (Horticulture and Landscape Architecture 

Plant Growth Facility at Purdue University), 

and Patricio Alzugaray for their assistance 

with seed collection and space management. 

Marcus Selig, Ron Overton, and Amy Ross-Davis 

provided assistance with the preparation of this 

manuscript.
Propagation and Genetic Modification 35

Black Walnut in a New Century

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Propagation and Genetic Modification 37

Black Walnut in a New Century

VEGETATIVE PROPAGATION OF BUTTERNUT (JUGLANS CINEREA) 

FIELD RESULTS

Paula M. Pijut1

ABSTRACT—Juglans cinerea L. is a hardwood species valued for its wood and edible nuts. 

Butternut canker disease (Sirococcus clavigignenti-juglandacearum) threatens its survival. 

Vegetative propagation will be required to produce clones of genotypes selected for resistance 

to butternut canker disease. In 2000, 10 trees were randomly selected from a 6-year-old 

butternut plantation located in Rosemount, MN. Hardwood stem cuttings were collected in 

March, April, and May. Softwood cuttings were collected in June and July. Indole-3-butyric 

acid-potassium salt (K-IBA) at 0, 29, or 62 mM in water and indole-3-butyric acid (IBA) at 

0, 34, or 74 mM in 70% ethanol were tested for root induction on cuttings. The basal end of 

cuttings were dipped in a treatment solution for 10 to 15 s, potted in a peat:perlite mixture 

and placed in a mist bed for 5 to 8 weeks. Rooted cuttings were gradually hardened off 

from the mist bed, allowed to initiate new growth, over-wintered in a controlled cold-storage 

environment, and then outplanted to the field. Rooting was greatest for hardwood cuttings 

taken in mid-May (branches flushed out), 22% with 62 mM K-IBA and 28% with 74 mM 

IBA. Softwood cuttings rooted best when taken in June (current season’s first flush of new 

growth or softwood growth 40 cm or greater) and treated with 62 mM K-IBA (77%) or 74 mM 

IBA (88%). One-hundred and seventy three (173) out of 186 rooted softwood cuttings (93%) 

survived over-wintering and acclimatization to the field. Average heights and stem diameters 

were after 1 year (28.6 cm, 12.4 mm) or 2 years (92.8 cm, 18.6 mm). When plants were 

protected from deer browse, rodent damage, and weed controlled, 91% survived in the field.

INTRODUCTION

Butternut (Juglans cinerea L.), also known as 

white walnut or oilnut, is a relatively slow-growing 

hardwood species found in bottomlands, moist 

uplands, and old fields (Leopold and others 1998). 

Moist, rich loamy soils of hillsides and streambanks 

are the preferred growing sites, although butternut 

can grow quite well on dry, rocky soils. Juglans 

cinerea is considered to be one of the most winter 

hardy (USDA Hardiness Zone range of 3 to 7) of 

the Juglans species (Cathey 2003, Dirr 1998). 

Butternut is seldom found growing in pure stands, 

but rather in association with several other tree 

species such as black cherry, American basswood, 

white and northern red oak, black walnut, white 

ash, red and sugar maple, and American elm. It 

is native to the northeastern United States and 

adjacent Canada, ranging from New Brunswick 

to Georgia, and west to Minnesota and Arkansas 

(Rink 1990). Butternut is valued ecologically and 

economically for its edible nuts and wood (Ostry 

and Pijut 2000). The nuts are oily and sweet, and 

are an important food source for wildlife. Seed 

production begins on trees around 20 years of age, 

with good seed crops occurring every 2 to 3 years, 

based on seedling stands not grafted trees. Because 

of the limited availability of quality butternut wood, 

it commands a high market price for many uses 

including furniture, cabinets, paneling, veneer, and 

fine woodworking. Its light tan heartwood is very 

workable with hand and power tools (for specialty 

carving products) and can be stained to resemble 

black walnut.

Butternut canker disease, caused by the fungus 

Sirococcus clavigignenti-juglandacearum N.B. Nair, 

Kostichka and Kuntz (Nair and others 1979) has 

1

Research Plant Physiologist (PMP), North Central Research Station, USDA Forest Service, Hardwood Tree Improvement and 

Regeneration Center, 195 Marsteller Street, West Lafayette, IN 47907. PMP is corresponding author: to contact, call (765) 496-2162 or 

e-mail at ppijut@fs.fed.us.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p. 
38 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

caused widespread mortality and is threatening 

native tree survival (Orchard and others 1982; 

Tisserat and Kuntz 1984; Cummings 1993; Ostry 

1997, 1998; Nair 1999). The fungus, believed to 

be introduced (Furnier and others 1999), causes 

perennial cankers on all above ground parts of the 

tree, even the buttress roots (Sinclair and others 

1987). The spores of the fungus are disseminated 

by rain splash and can travel in aerosols to adjacent 

trees where infection occurs on young branches in 

the upper crowns. Several insect species have been 

found associated with fungus spores on infected 

trees (Katovich and Ostry 1998). The fungus has 

also been found on the fruit of butternut and 

black walnut, causing lesions on the husks of both 

species (Innes 1998). The cankers cause the wood 

to turn dark brown to black in an elliptical pattern 

(Ostry and others 1996) that reduces the quality 

and marketability of the wood. The girdling effect 

of multiple coalescing cankers eventually kills the 

trees.

Butternut is propagated easily from seed (Brinkman 

1974), but the canker fungus is also seed-borne 

(Orchard and others 1981, Orchard 1984). 

Vegetative propagation will be required to produce 

clones of genotypes selected for resistance to 

butternut canker disease. Grafting of butternut to 

black walnut rootstock (Ostry and Pijut 2000) can be 

successful, but grafting is a time consuming process 

with variable success. There are few reports of 

vegetative propagation of butternut through cuttings 

(Pijut and Barker 1999, Pijut and Moore 2002). 

Rooting is reported for other Juglans species: J. 

hindsii (Lee and others 1977), J. microcarpa (Shreve 

1990), J. nigra (Farmer 1971, Shreve 1972, Shreve 

and Miles 1972, Farmer and Hall 1973, Carpenter 

1975), J. regia (Gautam and Chauhan 1990), J. 

sinensis (Kwon and others 1990), and hybrids 

(Serr 1964, Reil and others 1998). This paper 

describes the experimental conditions and results 

for successful cutting propagation of butternut, 

and growth and survival data of some of the rooted 

cuttings after 1- or 2-years in the field.

MATERIALS AND METHODS

Hardwood Cuttings

In 2000, 10 trees were selected from a 6-yearold butternut field plot located in Rosemount, 

MN. Hardwood cuttings were collected at specific 

growth stages: dormant (Mar. 29, 2000); budbreak 

(May 2, 2000); and branches flushed out (May 19, 

2000). Six to twelve cuttings (25 cm in length) were 

taken from each tree at each collection date. Two 

years of growth were pruned from the remaining 

branches (after last hardwood collection date) 

to encourage sprouting. Cuttings were placed in 

polyethylene bags, held on ice, and transported to 

the greenhouse, where the cuttings were processed 

the same day. Stems were recut to between 20 

and 23 cm in length. The basal 3 cm of cuttings 

were treated by dipping for 10 to 15 s in either 

0, 29, or 62 mM indole-3-butyric acid-potassium 

salt (K-IBA) dissolved in deionized water or 0, 34, 

or 74 mM indole-3-butyric acid (IBA) dissolved in 

70% ethanol. Cuttings were inserted vertically to a 

depth of 5 to 7 cm in DeepotsTM (D40) (Stuewe and 

Sons, Corvallis, OR) containing a moist medium of 

1 perlite : 1 peat (v/v). Cuttings in DeepotsTM were 

placed under intermittent mist (15 s every 18 min) 

on a greenhouse bench with bottom heat maintained 

at 27o C. Twelve hours of supplementary irradiance 

(from 0600 to 1800 HR) were provided by highpressure sodium lamps (60 μmol·m-2·s-1 at bench 

top), and greenhouse temperature was maintained 

at 22 + 2o C. After 6 to 8 weeks, cuttings were 

harvested and number of cuttings rooted, number 

of roots per cutting, and individual root lengths 

were recorded. Data were analyzed using statistical 

programs for categorical data (Rugg, unpublished 

data), log linear modeling (SPSS 1998), and logistic 

regression (Mehta and Patel 1996). The number of 

roots per cutting were square root transformed and 

the data analyzed using linear regression. The data 

for root lengths were log transformed and the data 

analyzed using linear regression.

Rooted cuttings were transplanted in TreepotsTM 

(Tall One) (Stuewe and Sons) containing a moist 

medium of Sunshine SB-40 bark mix (Sun Gro 

Horticulture, Bellevue, WA), with 14N–14P–14K 

Nutricote® timed-release fertilizer (Chisso-Asahi 

Fertilizer Co., Tokyo, Japan) and returned to 

intermittent mist for 1 week. Rooted cuttings were 

then acclimatized to greenhouse benches and 

allowed to initiate new shoot growth. In late October 

or early November, rooted cuttings were placed in 

a cooler environment (15o C or less) and lower light 

to induce dormancy. After a month, containerized 

cuttings were placed in polyethylene bags and 

moved to a controlled cold-storage environment 

(3 to 4o C in darkness) for 4 to 5 months. After 

overwintering, the pots were returned to the 

greenhouse (March/April, the following year), and 

allowed to acclimatize to this environment. Pots were 

hand-watered as needed until budbreak, after which 

the pots were placed on an automatic drip irrigation 

system until they were field-planted in July.
Propagation and Genetic Modification 39

Black Walnut in a New Century

Softwood Cuttings

In 2000, the same 10 trees as used for hardwood 

cuttings were used for softwood cuttings. Softwood 

cuttings were collected at specific growth stages: 

current season’s first flush of new growth (June 

12, 2000); softwood growth 40 cm or greater (June 

23, 2000); shoots beginning to become lignified 

(July 7, 2000); and shoots starting to set bud (July 

19, 2000). Six to twelve cuttings (40 to 45 cm in 

length) were taken from each tree at each collection 

date. All but two leaves were removed. Softwood 

cuttings were handled and processed as described 

for hardwood cuttings, except no bottom heat was 

used in the mist bed. After 5 to 6 weeks, cuttings 

were harvested and rooting data recorded. Data 

collection and statistical analyses were the same as 

described for hardwood cuttings. Rooted cuttings 

were maintained, allowed to initiate new growth, 

overwintered, and planted in the field as described 

for hardwood cuttings.

Field Plantings

Rooted cuttings were planted in the field at the 

Southeast Purdue Agricultural Center (SEPAC) 

(July 2001; 56 plants) located near North Vernon, 

IN, and at the Vallonia Nursery (July 2002; 81 

plants) located in Vallonia, IN. The butternut plants 

transported to the Vallonia nursery were kept in the 

TreepotsTM outdoors for one additional year until 

the site was available for planting in July 2002. 

Therefore, data on these plants are considered 

as 1-year-old field results. When feasible rooted 

cuttings were protected with mesh cages to prevent 

deer browse and rodent damage, and sprayed with 

herbicides to control weeds. Percent survival, stem 

heights (cm), and stem diameter (mm) at ground 

level (root collar) were recorded in 2003.

RESULTS

Hardwood Cuttings

Rooting success did not vary by growth stage 

(collection date) (P > 0.85). K-IBA or IBA were equally 

effective in promoting rooting (P = 0.38). There 

was a significant effect of the auxin concentration 

(P = 0.007). The controls [deionized water or 70% 

ethanol] were not different (P = 0.11) from the low 

concentration [29 mM K-IBA or 34 mM IBA], but 

did significantly differ (P = 0.005) from the high 

concentration [62 mM K-IBA or 74 mM IBA] (Table 

1). The low and high concentrations did not differ 

(P = 0.24) from each other in rooting success, but 

a logistic regression on these data showed that a 

linear response to concentration accounts for nearly 

all of the variability in the data. The greatest rooting 

success was with 62 mM K-IBA (22.2%) and 74 mM 

IBA (27.8%) when hardwood cuttings were collected 

when the branches had flushed out (mid-May). There 

was no difference in the number of roots regenerated 

per hardwood cutting (P > 0.1). No differences (P = 

0.02) were observed for root lengths. Six out of six 

rooted cuttings (100%) survived over-wintering in cold 

storage and acclimatization to the field.

Table 1.—Effects of time of collection and rooting 

treatment concentration on rooting percentage, 

root count, and root len gth of Juglans cinerea 

hardwood cuttings.1

Rooting 

Treatment

(mM)

Date of Collection2

2000

29 March 2 May 19 May

Rooting (percent) 

0 K-IBA 0 a3 0 a 11.1 a

29 K-IBA 9.1 a 6.3 ab 16.7 ab

62 K-IBA 0 a 12.5 b 22.2 b

0 IBA 0 a 0 a 11.1 a

34 IBA 0 a 6.3 ab 11.1 ab

74 IBA 0 a 6.3 b 27.8 b

Mean number of roots4

0 K-IBA - - - - 1.0 a

29 K-IBA 1.0 4.0 a 7.0 a

62 K-IBA - - 1.5 a 4.8 a

0 IBA - - - - 2.0 a

34 IBA - - 7.0 a 1.0 a

74 IBA - - 2.0 a 3.1 a

Mean root length (mm)w

0 K-IBA - - - - 8.0 a

29 K-IBA 22 21.5 a 23.4 a

62 K-IBA - - 7.3 a 9.4 a

0 IBA - - - - 13.7 a

34 IBA - - 17.9 a 8.0 a

74 IBA - - 5.0 a 11.2 a

1

Average sample size for each collection date and rooting 

treatment concentration, n =14.

2

Hardwood cutting growth stages: dormant (2000 March 

29); bud break (2000 May 2); and branches flushed out 

(2000 May 19).

3

Mean separation within a column by LSD, P < 0.05. Letters 

indicate significant differences among means within a 

column for a given variable. “- -“ indicates that no mean 

was calculated because no observations were available.

4

Means are per rooted cutting.
40 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

Softwood Cuttings

Sample development stages showed no change 

in rooting success (P > 0.40). Auxin type did not 

effect rooting success (P = 0.79). However, there 

was a significant additive effect on rooting success 

with the auxin concentration of rooting treatment 

used (P < 0.0001). Using K-IBA (29 or 62 mM) or 

IBA (34 or 74 mM) improved rooting success over 

the control, but there was no difference (P = 0.39) 

between the treatment levels (Table 2). The greatest 

rooting success of 76.9% (62 mM K-IBA) and 87.5% 

(74 mM IBA) was achieved when softwood cuttings 

were taken in June (Fig. 1A). Using K-IBA or IBA 

produced more roots than controls (P < 0.0001), 

but the auxin concentrations did not differ in 

number of roots produced (P = 0.43). There were no 

differences between the controls (water versus 70% 

ethanol) in number of roots produced (P = 0.69). In 

addition, the high concentrations of both K-IBA and 

IBA produced a similar number of roots (P = 0.11). 

However, the 34 mM IBA treatment resulted in 

significantly greater root numbers (P = 0.0001) than 

in the similar concentration of K-IBA. For K-IBA, 

the low concentration (29 mM) was not statistically 

different from either the control (P = 0.14) or the 

high concentration (P = 0.08). However, the high 

concentration produced more roots than the control 

(P = 0.006). Using K-IBA produced a linear doseresponse relationship for number of roots produced. 

Softwood cuttings have a uniformly higher number of 

roots produced than do hardwood cuttings. Cuttings, 

from the current season’s first flush of new growth 

(20.8 cm) and shoots beginning to become lignified 

(24.7 cm) development stages, had shorter roots than 

the softwood growth 40 cm or greater development 

stage (34.9 cm). There was no difference in root length 

from cuttings from the first, third, and fourth sample 

development stages. Mean root length was similar for 

all treatments and controls. One hundred and seventy 

three out of 186 rooted softwood cuttings (93%) 

survived over-wintering and acclimatization to the 

field (Fig. 1B).

Field Plantings

In the SEPAC planting, 51 out of 56 plants (91%) 

survived in the field after 2 years. These plants 

were protected with mesh cages to prevent deer 

browse and rodent damage, and routinely sprayed 

with herbicides to control weeds (Fig. 2). Average 

stem heights in centimeters (mean number + SE) 

ranged from 66.4 + 8.0 to 123.7 + 25.6, with a total 

average of 92.8 + 5.9 after 2 years (Table 3). Average 

stem diameters in millimeters (mean number + SE) 

ranged from 14.1 + 0.9 to 21.3 + 3.1, with a total 

average of 18.6 + 0.7 after 2 years.

In the Vallonia planting, 54 out of 81 plants (67%) 

survived in the field after 1 year. These plants 

were not protected with mesh cages to prevent 

deer browse. Herbicides were used at this site 

(prior to planting and once after planting), but 

several factors (remote site, unavailability of spray 

equipment, and wet weather) combined to render 

control of weeds ineffective. Deer browse, drought 

in the summer, and weed competition in this 

planting had the greatest overall effect on survival 

of the butternut plants. Average stem heights in 

centimeters ranged from 15.7 + 6.4 to 46.3 + 10.3 

Table 2.—Effects of time of collection and rooting 

treatment concentration on rooting percentage, 

root count, and root length of Juglans cinerea 

softwood cuttings.1

Rooting 

treatment

(mM)

Date of Collection2

2000

12 June 23 June 7 July 19 July

Rooting (percent)

0 K-IBA 37.5 a3 17.9 a 10.7 a 23.1 a

29 K-IBA 37.5 b 73.1 b 42.9 b 50.0 b

62 K-IBA 44.4 b 76.9 b 42.9 b 57.7 b

0 IBA 37.5 a 25.0 a 10.7 a 19.2 a

34 IBA 37.5 b 67.9 b 39.3 b 50.0 b

74 IBA 87.5 b 75.0 b 46.4 b 46.2 b

Mean number of roots4

0 K-IBA 3.0 a 3.5 a 1.7 a 3.8 a

29 K-IBA 7.0 b 9.7 b 3.9 b 8.0 b

62 K-IBA 13.8 c 7.3 c 13.3 c 11.2 c

0 IBA 3.0 a 3.1 a 1.7 a 3.0 a

34 IBA 9.7 c 20.6 c 10.7 c 12.2 c

74 IBA 18.7 c 21.0 c 7.4 c 6.0 c

Mean root length (mm)w

0 K-IBA 29.0 a 42.0 a 35.3 a 24.2 a

29 K-IBA 13.8 a 40.2 a 22.2 a 38.7 a

62 K-IBA 26.5 a 32.5 a 24.2 a 30.9 a

0 IBA 12.7 a 17.5 a 18.0 a 58.7 a

34 IBA 26.2 a 41.0 a 24.1 a 22.1 a

74 IBA 16.7 a 36.1 a 24.5 a 20.9 a

1

Average sample size for each collection date and rooting 

treatment concentration, n = 21.

2

Softwood cutting growth stages: current season’s first flush 

of new growth (2000 June 12); softwood growth 40 cm 

or greater (2000 June 23); shoots beginning to become 

lignified (2000 July 7); shoots starting to set bud (2000 

July 19).

3

Mean separation within a column by LSD, P < 0.05. Letters 

indicate significant differences among means within a 

column for a given variable. “- -“ indicates that no mean 

was calculated because no observations were available.

4

Means are per rooted cutting.
Propagation and Genetic Modification 41

Black Walnut in a New Century

Figure 1.—(A) Rooted softwood cuttings of Juglans 

cinerea after 4 wk under mist (from left to right: 34, 

74 mM IBA, and 29, 62 mM K-IBA). (B) Butternut plant 

(softwood cutting, June collection, 62 mM K-IBA) 

ready for transplanting to the field. [Reprinted with 

permission from HortScience 37(4): 697-700. 2002]

Figure 2.—Butternut plant (rooted cutting) after 2 years 

in the Southeast Purdue Agricultural Center field. For 

comparison, the woman in the photo is 160 cm tall.

J. regia) root with variable success (30 to 80%) after 

a quick dip in 20 mM to 39 mM IBA or a 24-hr soak 

in 1mM to 1.5 mM IBA (Serr 1964, Reil and others 

1998). Dormant cuttings taken from 4- to 5-year-old 

hedges of J. regia rooted (14.5%) when treated with 

74 mM IBA (Gautam and Chauhan 1990). Carpenter 

(1975) reported 60 to 70% rooting of hardwood 

cuttings taken from mature black walnut trees (J. 

nigra), when soaked in ethephon for 6 hr, but shoots 

did not elongate following this treatment and no 

survival was reported.

Softwood cuttings of butternut rooted better than 

hardwood cuttings. Softwood cuttings rooted 

(10.7 to 87.5%) at all collection dates and rooting 

treatment concentrations tested. Rooting success 

ranged from 17.9 to 87.5% when June cuttings 

(current season’s first flush of new growth or 

softwood growth 40 cm or greater) were treated with 

0 K-IBA or 74 mM IBA. The greatest rooting success 

(76.9% and 87.5%) was achieved when softwood 

cuttings collected in mid- to late-June 2000 were 

treated with 62 mM K-IBA or 74 mM IBA. Rooted 

softwood cuttings were successfully over-wintered 

and transplanted to the field. Propagation of 

other Juglans species by softwood cuttings is 

also successful. Cuttings taken from adventitious 

shoots of eastern black walnut, J. nigra, (from one 

to 135-year-old material) rooted 80 to 100% when 

with a total average of 28.6 + 1.9 after 1 year (Table 

4). Average stem diameters in millimeters ranged 

from 8.0 + 1.0 to 14.0 + 0.8, with a total average of 

12.4 + 0.5 after 1 year.

DISCUSSION

Propagation of 6-year-old J. cinerea from hardwood 

cuttings collected in mid-May (branches flushed 

out in the spring) was successful (11.1 to 27.8%), 

although at a low percentage. The greatest rooting 

success of 22.2% and 27.8% was achieved when 

hardwood cuttings were treated with 62 mM KIBA or 74 mM IBA. Rooted hardwood cuttings were 

successfully over-wintered in cold storage and planted 

to the field. Propagation of other Juglans species 

by hardwood cuttings has also been investigated. 

Hardwood cuttings of ‘Paradox’ walnut (J. hindsii x 
42 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

Table 3.—Growth parameters of butternut plants 

(rooted softwood cuttings) after 2 years in the 

Southeast Purdue Agricultural Center field.

Tree No. Height

(cm)

Stem Diameter

(mm)

103 126.0 24.3

103 142.0 28.5

103 158.0 23.8

103 105.0 18.6

103 39.0 11.7

103 93.0 18.3

103 98.0 21.5

Mean number + SE 108.7 + 14.7 21.0 + 2.0

104 51.0 12.4

104 95.0 19.3

104 96.0 20.5

104 89.0 22.1

104 167.0 29.5

104 55.0 14.1

Mean number + SE 92.2 + 17.0 19.7 + 2.5

105 45.0 10.2

105 79.0 16.0

105 70.0 21.2

105 51.0 10.8

105 87.0 14.1

Mean number + SE 66.4 + 8.0 14.5 + 2.0

108 102.0 15.1

108 94.0 15.9

108 31.0 12.1

108 56.0 13.1

Mean number + SE 70.8 + 16.6 14.1 + 0.9

110 47.0 13.0

110 132.0 27.6

110 100.0 16.3

110 153.0 28.6

110 145.0 21.0

Mean number + SE 115.4 + 19.3 21.3 + 3.1

112 80.0 18.9

112 96.0 21.9

112 50.0 14.9

112 90.0 24.9

112 19.0 9.9

Mean number + SE 67.0 + 14.4 18.1 + 2.6

113 69.0 16.2

113 148.0 24.0

113 102.0 20.1

113 32.0 12.5

113 124.0 21.4

113 99.0 14.1

Mean number + SE 95.7 + 16.7 18.1 + 1.8

114 153.0 19.9

114 42.0 11.0

114 148.0 21.4

114 166.0 24.3

114 47.0 23.1

114 186.0 23.9

Mean number + SE 123.7 + 25.6 20.6 + 2.0

115 96.0 17.9

115 70.0 17.3

115 46.0 13.8

115 149.0 26.4

Mean number + SE 90.3 + 22.1 18.9 + 2.7

117 87.0 20.3

117 57.0 14.8

117 72.0 16.3

Mean number + SE 72.0 + 8.7 17.1 + 1.6

TOTAL

Mean number + SE 92.8 + 5.9 18.6 + 0.7

Table 4.—Growth parameters of butternut plants 

(rooted softwood and hardwood cuttings) after 1 

year in the Vallonia field.

Tree No. Height

(cm)

Stem Diameter

(mm)

103 25.0 13.0

103 18.0 16.0

103 34.5 9.0

103 32.0 16.0

Mean number + SE 27.4 + 3.7 13.5 + 1.7

104 27.5 9.0

104 14.0 9.0

104 5.5 6.0

Mean number + SE 15.7 + 6.4 8.0 + 1.0

108 36.0 9.0

108 56.5 13.0

Mean number + SE 46.3 + 10.3 11.0 + 2.0

110 43.0 16.0

110 56.5 19.0

110 43.5 9.0

110 Hardwood 48.5 13.0

110 16.0 13.0

110 26.0 13.0

110 15.0 6.0

110 53.0 16.0

Mean number + SE 37.7 + 5.8 13.1 + 1.5

113 21.0 16.0

113 18.0 16.0

113 4.0 9.0

113 8.0 6.0

113 15.0 13.0

113 11.0 16.0

113 12.0 16.0

113 8.0 6.0

113 15.0 13.0

113 29.0 13.0

113 41.0 16.0

113 42.0 16.0

113 48.0 19.0

113 42.5 16.0

113 38.0 16.0

113 19.0 13.0

113 41.0 13.0

113 42.5 16.0

113 39.0 9.0

113 Hardwood 50.0 16.0

113 Hardwood 39.5 22.0

113 34.5 13.0

113 55.0 13.0

Mean number + SE 29.3 + 3.3 14.0 + 0.8

114 17.0 13.0

114 32.0 9.0

Mean number + SE 24.5 + 7.5 11.0 + 2.0

115 Hardwood 23.0 9.0

115 31.0 3.0

115 28.0 16.0

115 25.0 13.0

115 10.0 13.0

115 23.5 13.0

115 19.0 6.0

115 20.0 6.0

115 26.0 13.0

115 28.0 9.0

115 14.5 9.0

115 22.5 13.0

Mean number + SE 22.5 + 1.7 10.3 + 1.1

TOTAL

Mean number + SE 28.6 + 1.9 12.4 + 0.5
Propagation and Genetic Modification 43

Black Walnut in a New Century

treated with 25 to 49 mM IBA (Shreve 1972, Shreve 

and Miles 1972). Rooting (0 to 100%) of juvenile 

black walnut (2-year-old seedlings) occurs after 

extensive etiolation and auxin treatments (Farmer 

1971, Farmer and Hall 1973). Lee and others (1977) 

reported that cuttings taken from adventitious 

shoots of a 5-year-old J. hindsii tree had a 

significantly greater number of roots produced as a 

result of pretreatment with 2N sulfuric acid prior to 

a 10 sec dip in 15 mM IBA. Shreve (1990) reported 

rooting (percentage not stated) J. microcarpa (river 

walnut) softwood cuttings by treatment with 34 mM 

IBA and without the use of a mist bed.

This is the first report of field survival of Juglans

cinerea (butternut) plants vegetatively propagated 

from cuttings of 6-year-old trees.

CONCLUSIONS

Juglans species are, for all practical purposes, 

recalcitrant to routine, commercial-scale, vegetative 

propagation. However, successful propagation of J. 

cinerea on a commercial scale can be achieved if the 

type of cutting (softwood), date of collection (early 

season), auxin concentration (62 mM K-IBA or 74 

mM IBA), and greenhouse parameters (mist bed, 

supplemental lighting, etc.) are carefully considered. 

High field survival and good growth parameters can 

be achieved if plants are protected from deer browse 

and weeds are controlled.

ACKNOWLEDGMENT

The author gratefully acknowledges Daniel K. 

Struve and Brian K. Maynard for their constructive 

reviews and suggestions for improvement of this 

manuscript; John Seifert, Regional Extension 

Forester, Purdue University (SEPAC), and Phil 

O’Connor, Tree Improvement Specialist, Indiana 

Department of Natural Resources, Vallonia Nursery 

for outplanting, maintenance, and field data 

collection of own-rooted butternut plants.

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Propagation and Genetic Modification 45

Black Walnut in a New Century

GENETIC TRANSFORMATION OF BLACK WALNUT (JUGLANS NIGRA)

Michael J. Bosela, Gurpreet S. Smagh, and Charles H. Michler1

ABSTRACT—Disarmed Agrobacterium tumefaciens strains with binary vectors carrying 

transgenes for kanamycin resistance (npt II) and β-glucuronidase (GUS, uidA) were 

used for the genetic transformation of Eastern black walnut (Juglans nigra) somatic 

embryos. In total, explants from 16 embryo lines, representing 10 genotypes, were used 

and regeneration was compared both in the absence of kanamycin selection and with 

different levels of selection stringency (200-250 or 500 mg/L kanamycin). Histochemical 

GUS expression assays indicated that the rates of T-DNA transfer were high (75-100%), 

with gene transfer being demonstrated for 14/16 lines. Regeneration was observed for 

15% of the infected embryos, compared with frequencies of 50% for the non-infected 

controls. The regeneration frequencies were nearly two-fold higher when kanamycin 

was excluded from the culture medium, but most of the secondary embryos appeared 

non-transgenic. When kanamycin was present, most of the secondary embryos were 

transgenic, but chimeric, consisting of mixtures of transgenic and non-transgenic cells; 

and few exhibited de novo growth following harvest, even on non-selective medium. Less 

than 10% of the initial secondary embryos were wholly transgenic, as assessed by GUS 

assay; however, these embryos appeared essential for the initiation of stable transgenic 

lines. The establishment of transgenic lines was also facilitated by the use of lower 

kanamycin concentrations (200-250 mg/L) and by the precocious proliferation of the 

initial secondary embryos. 

INTRODUCTION

In contrast with conventional breeding, which 

primarily involves gene manipulation at the level of 

entire chromosomal regions, genetic engineering is 

more precise. Genetic transformation techniques 

can be used to alter the expression patterns of 

individual genes in a predictable manner, with 

silencing, over-expression, and ectopic expression 

all being possible. Genes can be transferred 

across species barriers and genetic transformation 

protocols can be used for gene discovery. 

Genetic transformation is especially attractive 

in light of the recalcitrance of most trees for 

conventional breeding. In contrast with agronomic 

crops, trees are characterized by long sexual 

generation times, high levels of heterozygosity 

(Hamrick 2000), and effective progeny screening 

is hindered by their large size and a frequent lack 

of juvenile:mature correlations, especially for 

vegetative characteristics (Ostry and Michler 1993, 

Tzfira and others 1998). For trees that are routinely 

grown in plantations, such as black walnut, the 

introduction genes for herbicide tolerance or pest 

resistance would be desirable. In addition, over a 

longer timeframe, strategies for the manipulation of 

wood quality traits (increased heartwood content, 

etc.) could be developed. 

We are not aware of any previous publications 

describing the genetic transformation of black 

walnut; however, the genetic engineering of 

English walnut (Juglans regia), using disarmed 

strains of Agrobacterium tumefaciens, has been 

widely reported. In fact, J. regia was the first plant 

species (woody or non-woody) to be transformed 

using somatic embryos (McGranahan and others 

1988, Dandekar and others 1989). Although the 

initial transformation frequencies were low (3%), 

the authors were able to establish that physical 

wounding was not necessary for gene transfer 

and that transformation competence was related 

to the size of the embryos. The efficacy of the 

1

Assistant Professor (MJB), Indiana University-Purdue University at Fort Wayne, Department of Biology, 2101 E. Coliseum Boulevard, 

Fort Wayne, IN 46805-1499; M.S. Candidate (GSS) and Director (CHM), Hardwood Tree Improvement and Regeneration Center, USDA 

Forest Service, North Central Research Station, Purdue University, 195 Marsteller Street, West Lafayette, IN 47907-2033. MJB is the 

corresponding author: to contact, call (260) 481-6038 or e-mail at boselam@ipfw.edu.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
46 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

protocol was improved via the use of selective media 

(McGranahan and others 1990); however, the initial 

secondary embryos regenerated were frequently 

chimeric (i.e., consisting of mixtures of transgenic 

and non-transgenic cells) or entirely nontransgenic, even when selective media was used 

(McGranahan and others 1990, Escobar and others 

2000). Derivative protocols have been developed in 

several additional labs (El Euch and others 1998, 

Tang and others 2000); however, in all cases, the 

development of efficient selection protocols has 

proved challenging. 

In this research, we used the basic J. regia

transformation protocol as a model for the 

development of initial J. nigra transformation 

protocols. However, our research differed from that 

of our predecessors in that media with multiple 

kanamycin concentrations were evaluated for 

selection. In addition, to increase the likelihood of 

concurrent secondary embryo induction and gene 

transfer, the embryos used for inoculation were 

larger (more mature) than those that have typically 

been used by previous experimenters.

MATERIALS AND METHODS

Plant Material

Somatic embryo cultures of J. nigra were induced 

from the cotyledon explants of immature zygotic 

embryos during the summer of 1999 (Bosela and 

Michler 2000). Walnut fruits were collected at 2-3 

week intervals from mid-July to the end of August 

for culture initiation. In total more than 40 embryo 

lines (differing in genotype or induction media) were 

obtained, including nearly 20 lines that have been 

maintained continuously in vitro to date on a 3-5 

week transfer cycle. The lines have been cultured 

primarily on basal (hormone-free) media; however 

the specific media types (MS or DKW) and sucrose 

concentrations (20 or 30 g/L) used have varied 

between the embryo lines.

Somatic Embryo Proliferation

Two mechanisms of proliferation were recognized 

for the lines, which are termed direct and indirect 

here, based on the degree of connectivity between 

the parent embryos and secondary embryos. Direct 

proliferation is characterized by the production of 

secondary embryos that are physically attached 

to the parent embryo (Fig. 1A). Most are produced 

from the cotyledons and are frequently arranged 

in rows that may correspond with the position of 

subtending vascular tissues (Fig. 1B). Cotyledon 

differentiation is typically observed shortly after 

embryo emergence (Fig. 1B), and continued 

proliferation is achieved via the subculture of 

the initial secondary embryos. However, since 

proliferation competence and embryo size are 

positively correlated, one to two culture periods of 

growth are typically required prior to the production 

of additional embryo generations.

Indirect proliferation is characterized by the 

production of secondary embryos that proliferate 

precociously (i.e., prior to cotyledon differentiation) 

to form masses of globular embryos, which 

lack obvious direct connections with the parent 

embryo (Figs. 1C, 1D). On basal medium, the 

degree of globular embryo proliferation is typically 

limited, with most enlarging and differentiating as 

cotyledonary embryos within 1-2 culture periods 

of their initiation. However, the continuous 

proliferation of globular embryos is generally 

possible upon transfer to MS medium with 2,4-D 

(0.2 – 2 μM). For the cumulative data set, indirect 

proliferation was only erratically observed; however, 

for some embryo lines, greater numbers of embryos 

were produced via indirect proliferation than direct 

proliferation. In addition, indirect proliferation 

appears to be facilitated by the use of MS media, 

elevated sucrose concentrations (30-50 g/L), and 

the inclusion of organic nitrogen sources (casein 

hydroslyate and glutamine).

Agrobacterium Strains

The GV3101/pMP90 strain of Agrobacterium 

tumefaciens (Koncz and Shell 1986) was used 

in conjunction with two binary vectors; pBI121 

(Jefferson and others 1987) and pBISN1 

(Narasimhulu and others 1996), depending upon 

the experiment. pBI121 and pBISN1 are nearly 

isogenic. Both are derived from pBIN19 (Bevan 

1984) and contain a kanamycin resistance gene 

(npt II) on the plasmid backbone. In addition, the 

T-DNA (transferred DNA) regions are equivalent 

aside from differences in β-glucuronidase (uidA) 

gene construction (Fig. 2). In pBI121, the uidA gene 

is regulated by the CaMV 35S promoter (Guilley 

and others 1982), while in pBISN1 it is combined 

with the “super promoter” of Dr. Stanton Gelvin’s 

lab (Purdue University, West Lafayette, IN, USA). 

The super promoter consists of a trimer from the 

octopine synthase promoter’s upstream activator 

sequence and a single copy of the mannopine 

synthase promoter and its upstream activator (Ni 

and others 1995, Fig. 2). Although both promoters 

have been generally observed to be “strong” and 

constitutive in their expression patterns, in tobacco 

leaves the levels of GUS expression achieved 

using the super promoter have been shown to be 

up to 156-fold greater than the equivalent gene 
Propagation and Genetic Modification 47

Black Walnut in a New Century

Figure 1.—Embryo proliferation mechanisms. (A) A larger embryo with direct secondary embryos 

from two of its three cotyledons. The epicotyl of the embryo is indicated with an arrow. (B) A 

cotyledon with rows of secondary embryos of direct origin. The largest secondary embryos 

shown are only 0.5-1 mm in size yet most have already differentiated cotyledons. (C) A 

cotyledonary embryo with a sheath of gelatinous callus and an associated mass of globular to 

polar embryos from its hypocotyl-root axis. (D) A larger embryo with masses of globular to polar 

embryos of indirect origin associated with its cotyledons.

expression levels observed for the CaMV 35S 

promoter (Ni and others 1995). The uidA gene of 

pBISN1 also differs from that of pBI121 in having 

an intron in its open-reading frame that precludes 

expression in Agrobacterium (Vancanneyt and 

others 1990, Liu and others 1992).

Transformation

Explants

In total, somatic embryos from the 16 distinct 

embryo lines, representing 10 distinct genotypes, 

were used. In general, the inoculated embryos 

were of direct origin, but embryos of indirect origin 

were also used. In either case, the embryos were 

generally isolated at an early cotyledonary stage 

of development and cultured individually for 3-4 

weeks on basal medium prior to inoculation. The 

infected embryos averaged 7-10 mm in size, but 

some were significantly larger (up to 20-30 mm 

in diameter), and were frequently broken either 

inadvertently during disinfection or intentionally 

(prior to co-cultivation) to facilitate contact between 

the explants and antibiotics in the media. 

Bacteria Cultures

The Agrobacterium cultures were grown in LuriaBertaini (LB) media [10% (w/v) tryptone, 5% 

(w/v) yeast extract, 10% (w/v) NaCl, pH 7.0] on 

an orbital shaker at 28° C at 200 rpm. The initial 

cultures were prepared in 3 mL volumes and 0.5-

1 mL aliquots from these cultures were used to 

seed larger cultures (25-50 mL) that were used for 

infection. Gentamycin (25 mg/L) and kanamycin 

(50 mg/L) were used to select for the disarmed 

Ti plasmid (pMP90) and binary vectors, pBI121 

and pBISN1, respectively. The LB media used for 

the initial (3 mL) cultures, was also spiked with 

rifampicin (40 mg/L) to preclude Escherichia coli

contamination. Once the cultures had grown to 

an absorbance at 600 nm (A600) of 0.3-0.9, they 

were pelletted by centrifugation (1000xg, 10-15 

min) and re-suspended in antibiotic-free media 

at a density of approximately ≈ 5 x 108 cells/mL 

(Leple and others 1992, Han and others 1996). 

A

D

B

C D
48 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

For the 2002 experiments (n = 3), LB medium was 

used for Agrobacterium re-suspension. In contrast, 

the cultures from the 2003 experiment were 

resuspended in liquid plant culture media (MS vs 

DKW). Acetosyringone (20 μM) was added and the 

cultures were grown for an additional 20-30 min for 

virulence gene induction. 

Infection, Co-cultivation, and Disinfection 

For infection, the somatic embryos were divided 

into sterile 50 mL centrifuge tubes with aliquots 

(3-5 mL) from the virulence-induced Agrobacterium 

cultures and placed on a rotary shaker at 

room temperature (23 ± 2° C). After 20-30 min 

the embryos were removed, blotted dry, and 

Figure 2.—Physical maps of the pBI121 and pBISN1 T-DNA regions. The T-DNA border sequences are indicated 

with arrowheads, and colored boxes are used to indicate the positions of promoters, open reading frames, 

and terminators. Green boxes indicate promoter elements; blue boxes indicate open reading frames; and 

red boxes indicate transcription terminator sequences. Map positions (± 10 bps) are indicted relative to the 

position of the right T-DNA border sequences. The cutting sites for several common restriction enzymes 

are also shown. Note: In pBISN1, the uidA promoter is chimeric, consisting of a trimer upstream activator 

sequences from the octopine synthase (ocs) promoter and a single copy of the mannopine synthase (mas) 

promoter and upstream activator (see Materials and Methods).

pBI121

nos npt II CaMV 35S uidA (GUS)

30-360 380-1180 2500-3300 3310-5130 Hind III (2500) Sph I Pst I Ba

m

HI (3300)

Sph I (920)

Pst I (570)

Eco RI (5450)

Right Border (0)

Left B

order (6120)

Sma I

Xba I

pBISN1 Intron

nos npt II (Aocs)3 Amas uidA (GUS)

30-360 380-1180 2500-3540 Pmas 3570-5560 Hin

d III (2500)

Eco RIBa

m HI

Eco RI

Sph I (920)

Pst I (570)

Eco

RI (5880)

Right Border (0)

Left B

order (6550)

Hin

d III

Ba

m HI

Ba

m

HI (3560)

Xba I

Pst I

Sma I

Ba

m HI

Xba I

Not I

Eco RI
Propagation and Genetic Modification 49

Black Walnut in a New Century

transferred to plates of basal medium with 100 

μM acetosyringone in the dark for co-cultivation. 

The embryos were removed from the co-cultivation 

plates after an additional 50-65 hr and “washed” 

five to six times, with the final three washes 

containing antibiotic (500 mg/L cefotaxime for the 

2002 experiments or 500 mg/L timentin for the 

2003 experiment) to prevent the growth of residual 

Agrobacterium cells. The explants were washed in 

sterile 50 mL centrifuge tubes on a rotary shaker 

with 15-20 min/wash. The medium used for 

washing (sterile water or liquid plant culture media) 

varied between experiments. 

Regeneration

Following washing, the embryos were blotted dry 

on sterile paper towels and transferred to plates 

of basal medium containing the same antibiotics 

as had been used for washing, (i.e., 500 mg/L 

cefotaxime or 500 mg/L timentin). For each 

embryo source, the embryos were divided between 

selective and non-selective media. In general, only 

4-6 embryos were assigned to the non-selective 

medium, and the rest were assigned to selective 

medium. For most embryo sources, only a single 

replicate of each treatment was prepared; however, 

when enough explants were present, multiple 

replicates of the kanamycin treatments were 

prepared (5-10 embryos per replication).

The explants were transferred to fresh media every 

3-4 weeks. Embryos assigned to the selective 

media were maintained for three culture periods, 

while those assigned to non-selective media (and 

the controls) were maintained for only two culture 

periods. Secondary embryos were harvested at 

the end of the same culture period when they first 

became apparent and sorted into transformation 

events based on their spacing (i.e., embryos found 

within 5 mm of one another were considered to 

be derived from the same transformation event, 

while embryos spaced further apart were presumed 

to represent separate transformation events). 

Depending on the number of transformation 

events and embryos per transformation event, the 

secondary embryos were subcultured (to one or 

more media types), stained for GUS activity, or 

both.

The labeling system of McGranahan and others 

(1988) was used to distinguish between embryo 

generations (i.e., the inoculated explants were 

designated as E0 embryos, the initial secondary 

embryos were termed E1 embryos, and the 

subsequent generations of embryos were assigned 

progressively higher order subscripts [E2, E3, etc.]). 

Proliferation was generally direct with the embryo 

generations being separated by 1-2 culture periods 

of growth. However, when the initial secondary 

embryos proliferated indirectly (at the globular 

stage), embryo generations were impossible to 

distinguish.

Selection

For the 2002 experiments, media with lower 

(250 mg/L) and higher (500 mg/L) kanamycin 

concentrations were tested in separate experiments. 

However, for the 2003 experiment, the effects 

of differences in kanamycin concentration were 

evaluated more directly. The basic selective medium 

contained 500 mg/L kanamycin; however, for 

lines with sufficient numbers of embryos for the 

preparation of three more replicates, the embryos 

were divided between media with 500 mg/L 

kanamycin (2 replicates) and 200 mg/L kanamycin 

(single replicates). The 200 and 500 mg/L 

kanamycin media were also compared using E1 and 

E2 embryos, especially when multiple embryos were 

available from the same transformation events. 

The sensitivity of non-transgenic walnut somatic 

embryos to kanamycin was evaluated in a series of 

preliminary, supplemental experiments. Kanamycin 

was tested at concentrations of 25, 50, 100, 250, 

500, and 1000 mg/L, using both small (2-5 mm) 

and large (10-20 mm) cotyledonary embryos. 

Embryos from multiple lines were evaluated; 

however, replicates of the treatments were not 

prepared on a per embryo line basis. After 4-6 

weeks, the embryos were evaluated for proliferation. 

The health of the embryos was also assessed and 

the small cotyledondary embryos were evaluated for 

de novo growth. 

GUS (uidA) Expression Assays

Somatic embryos were screened for GUS activity by 

histochemical assay (Jefferson and others 1987). 

The staining solution contained 1mM X-Gluc, 

10 mM EDTA, 0.5 mM potassium ferricyanide, 

0.5 mM potassium ferrocyanide, 0.1 M sodium 

phosphate buffer (pH 7.2), 0.1% Triton X-100, and 

2% dimethyl sulfoxide (DMSO), as recommended 

by Stomp (1992). Staining was carried out at 37° C 

for 16 to 20 h, followed by clearing in 70% ethanol. 

Samples with blue staining (indigo precipitate) were 

scored as positive for GUS gene activity. 

RESULTS AND DISCUSSION

Initial (E1) Regeneration

Secondary Embryo Formation

For the 2003 experiment, the regeneration 

competence of the E0 embryos was adversely 
50 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

affected by co-cultivation (Table 1), as has been 

previously observed for J. regia somatic embryos 

(Tang and others 2000). Similar differences in 

regeneration frequency between the infected and 

non-infected embryos were not apparent for the 

2002 experiment, presumably as a result of the 

recalcitrance of the embryos for regeneration in 

general, with secondary embryo formation being 

observed for only 15% of the control embryos 

(Table 1), compared with more typical regeneration 

frequencies of 40-75% as assessed during culture 

maintenance (data not shown). Several factors, 

including differences in binary vector type, 

antibiotic type, and embryo line representation may 

have contributed to the contrasting regeneration 

responses. However, the use of different media for 

Agrobacterium resuspension (i.e., LB medium for 

the 2002 experiments and plant medium for the 

2003 experiments) is thought to have played a 

critical role. 

For the infected embryos, the regeneration 

frequencies were approximately twice as high when 

non-selective media was used (Table 1), and the 

differences were statistically significant (∝ = 0.05) 

for both data sets (2002 and 2003) as evaluated by 

Chi-Square analysis. The regeneration frequencies 

were also highly dependent on the concentration of 

selective agent, with the frequencies of secondary 

embryogenesis being two- to three-fold greater 

when lower (200-250 mg/L) rather than higher 

kanamycin concentrations (500 mg/L) were used 

(Table 2). The number of transformation events and 

secondary embryo yields per responsive explant (E0

embryo) were also much higher for the 200 mg/L 

kanamycin treatment (Table 2). 

Table 1.—Regeneration frequency by experimental treatment and calendar year. For each treatment, 

the data is combined across embryo lines and experimental replicates. The direct, indirect, and 

cumulative (total) regeneration frequencies are indicated.

Regeneration (%)

Year1 Treatment2 N Total Direct Indirect

2002 Control (not infected) 20 15 15 0

No selection 54 14 10 4

Kanamycin selection 215 8 7 1

2003 Control (not infected) 101 57 54 3

No selection 85 30 24 6

Kanamycin selection 346 17 16 1 1

 Experimental conditions varied between years. For the 2002 experiments, the GV3101/pMP90/pBI121 A. 

tumefaciens strain was employed, the bacteria were resuspended in LB medium prior to infection, and 

cefotaxime was used for Agrobacterium elimination. For the 2003 experiment, a different binary vector (pBISN1) 

was employed, plant media (MS or DKW) was used for resuspension, and timentin was used for Agrobacterium 

elmination.

2

 The control embryos were subjected to the same infection, co-cultivation, and disinfection regimes as were 

typical for the experimental treatments; however, “blank” Agrobacterium resuspension medium was used for 

inoculation and antibiotic-free media were used for regeneration. 

Table 2.—Relationship between kanamycin (Kan) concentration and transformation efficacy. 

Year1

Kan 

(mg/L) % Regeneration

Transformation Events Per 

Responsive Embryo2

Secondary Embryo Yield Per 

Responsive Embryo3

2002 250 18 N.D. N.D.

500 5 N.D. N.D.

2003 200 33 2.9 5.7

500 13 1.4 1.4 1

 In 2002, the 250 and 500 mg/L kanamycin media were tested in separate experiments, employing different 

combinations of embryo lines. In 2003, the 200 and 500 mg/L kanamycin media, were tested in a single 

experiment using explants that were genotypically and physiologically equivalent. Note: The regeneration 

frequencies presented here are not directly comparable with those shown in Table 1, since the 2003 data 

presented here is only for the subset of embryo lines that were tested on both types of kanamycin media. 2

 The mean number of transformation events per embryo for the subset of E0 embryos with regeneration (i.e., 

secondary embryogenesis). Note: The E1

 embryos from the 2002 experiments were not sorted by transformation 

event. 

3

 Mean yield of secondary embryos per embryo (combined across transformation events, see above) for the 

subset of E0 embryos with regeneration, as calculated for the 2003 experiment.
Propagation and Genetic Modification 51

Black Walnut in a New Century

GUS Expression Assays

To assess the frequency of T-DNA transfer and 

integration, several E0 embryos that had remained 

sterile and relatively healthy, as assessed by their 

limited degree of discoloration, were stained for 

GUS activity. For the 2002 experiments nearly 

85% (21/25, data not shown) of the E0 samples 

tested positive. Since the uidA gene used for the 

2002 experiments did not contain an intron it is 

impossible to exclude the possibility that the blue 

staining may have resulted from T-DNA expression 

by residual, interstitial Agrobacterium cells. 

However, similarly high staining frequencies were 

obtained for the 2003 experiment (Table 3). 

The preferential staining of only the healthiest E0

embryos may have biased the staining results, 

especially if the rates of embryo discoloration and TDNA transfer were inversely related. However, since 

the E0 embryos evaluated in this study were stained 

primarily at the end of the second or third culture 

periods, their degree of browning (senescence) was 

frequently quite advanced, and selection of the 

least discolored embryos was thought necessary to 

ensure the visualization of cells exhibiting a positive 

staining response. In fact, it could be argued that 

T-DNA transfer rates reported are lower than the 

staining frequencies that would have been obtained 

if the embryos had been stained by the end of the 

first culture period. However, perhaps even more 

telling than the high frequencies of GUS-positive 

staining observed for the cumulative data set, is 

the consistency with which positive staining results 

were observed across embryo lines; i.e., stable 

uidA gene integration was demonstrated for 14 of 

16 embryo lines evaluated, representing 9 of 10 

genotypes. 

For most E0 embryos the staining observed was 

primarily distributed as discrete blue spots, the 

smallest of which were estimated to be only 20-

25 μM in size, and thought to represent single 

transgenic cells (Figs. 3A, 3B). However, for some 

of the E0 embryos, a subset of the blue spots were 

distinctly larger (0.2-0.5 mm in diameter) and were 

presumed to represent clones of daughter cells 

from the same transformation event (Fig. 3C). In 

other cases patches or bands of blue were apparent, 

especially in the vicinity of regions, such as the root 

axis or cotyledon bases, that would be expected to 

show rapid and directional growth during embryo 

development (Fig. 3D). The density of blue foci 

(spots and patches) was highly variable between 

embryos (Fig. 3) and appeared to be related to 

differences in embryo growth rate (i.e., the number 

of transformation events detected per embryo was 

greatest for embryo sources that exhibited the most 

vigorous growth as non-infected controls [data not 

shown]). 

Table 3.—Cumulative X-Gluc staining data for the 2003 experiment. Data are presented only for 

the subset of E1 and E2 embryos that were cultured on the same media type (selective vs. nonselective) as had been used for the E0 embryos.

Treatment Embryo Type1

GUS Positive Staining Distribution (%)2

N N %

Small 

spots

Larger 

spots Patches Entire

Kanamycin 

Selection

E0 44 44 100 89 18 25 0

E1 – Direct 39 37 87 82 12 3 16

E1 – Static 52 34 65 100 3 6 0

E1 – Growth 4 3 75 0 0 0 100

E2 – Direct 9 9 0 0 0 100

E2 – Growth 8 8 0 0 0 100

No E0 38 28 74 61 21 29 0

Selection E1

 – Direct 27 8 26 43 14 0 43

E1 – Static 9 3 33 33 0 33 33

E1 – Growth 32 0 0 - - - -

E2 – Direct 6 0 0 - - - -

E2 – Growth 0 - - - - - -

1The E1

 and E2 embryos are distinguished based upon whether they were stained directly or after one or more 

months of individual culture. Those that were subcultured are further distinguished based upon whether they 

exhibited a positive growth response.

2

 Four basic staining patterns were recognized for the subset of embryos that tested GUS-positive: 1) chimeric 

staining consisting of smaller blue spots (< 0.5 mm diameter), 2) chimeric staining consisting of larger spots (> 0.5 

mm tviameter), 3) chimeric staining consisting of discrete patches or sectors of blue (at least 2-3 mm in size), and 

4) continuous blue staining (wholly transgenic embryos). The percentage of embryos that were scored positive 

for each staining pattern is indicated. Note: Since some of the chimeric embryos scored positive for multiple 

categories, the percentages frequently add up to more than 100%. 

A All of the E2

 embryos evaluated were derived from the same two transformation events.
52 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

The frequency of GUS-positive E1 embryos varied 

widely between experimental treatments. When 

non-selective media was used for regeneration, 

only 16% (11/68) of the E1 embryos tested positive 

for GUS expression (Table 3; cumulative data, 

combined across embryo types). In addition, the 

degree of E1 embryo growth and the frequency of 

staining appeared to be inversely related (i.e., nearly 

30% of the embryos that were stained at an early 

cotyledonary stage, tested positive for GUS activity; 

however, none of the embryos assayed after one or 

more culture periods of growth exhibited definite 

staining (Table 3). When kanamycin was included in 

the regeneration medium, positive staining results 

were obtained for nearly all of the E1 embryos 

evaluated directly (87%, Table 3), irrespective of the 

specific kanamycin concentration used (data not 

shown). The percentage of GUS-positive embryos was 

lower for the subset of embryos that were stained after one 

or more months of culture (65-75%, Table 3). However, 

since most of the embryos had failed to exhibit de 

novo growth, the lower staining frequencies may have 

been a consequence of embryo quiescence rather than 

a lack of gene transfer. 

The effects of the presence or absence of selection 

agent on the frequency of transgenic embryo 

regeneration has also been evaluated in J. regia. 

For the study in question (McGranahan and 

others 1990), the EHA105/pBI 121 Agrobacterium

strain was used, 100 mg/L kanamycin was used 

for selection, and the E1 embryos were stained 

directly. However, only small samples from the 

E1 embryos were stained and the differences 

in staining frequency were even more dramatic 

than those observed here (i.e., only 0.05% of the 

embryos regenerated on non-selective media testing 

positive for GUS activity, compared with GUSpositive frequencies 31% for those regenerated on 

kanamycin medium). 

Figure 3.—T-DNA transfer and GUS expression patterns in the infected embryos (E0 generation). (A) An E0 explant 

stained at the end of the first regeneration culture period. Notice the variable size of the blue spots. The smallest 

spots, which were estimated to be 35-50 micrometers in diameter may have represented single transgenic cells. 

Two secondary embryos (E1) are visible on the left side of the explant (arrows). One of the embryos is entirely 

unstained, while the second (upper) contains a few blue spots and differs little in staining pattern from that of 

the E0 embryo. (B) An explant stained at the end of the second culture period. The number of blue spots is more 

limited here; however, a wholly transgenic E1 embryo (with a tubular morphology, see arrow) has developed 

from one of the spots of GUS-positive cells. (C) An E0

 embryo with three distinct cotyledons as viewed from 

below. Note the larger diameter of the blue spots for the cotyledon, which has grown in size since co-cultivation, 

compared with the size of the blue spots on the smaller (ungrown) cotyledons (labeled as 1 and 2). (D) An E0

embryo with spots of GUS positive cells on its cotyledon and a longitudinal band of GUS-positive cells along its 

hypocotyl-root axis, suggesting that most of the axis elongation has occurred since gene transfer (co-cultivation). 

Note: Asterisks indicate the root axes of the embryos (when visible).

A

C

B

*

1

2

3

D

*
Propagation and Genetic Modification 53

Black Walnut in a New Century

Despite the higher frequencies of transgenic embryo 

regeneration observed in the current study, most 

of the E1 embryos that tested positive for GUS 

staining were chimeric (Fig. 4D), irrespective of the 

experimental treatment (Table 3). Although some of 

the E1 embryos may have been present prior to cocultivation, in which case chimeric staining would 

have been expected, very few of the E0 embryos bore 

secondary embryos at the time of inoculation, as 

assessed by visual inspection (data not shown). For 

the subset of E1 embryos regenerated on kanamycin 

medium, only 6/39 (16%) of the embryos that were 

directly stained appeared fully transgenic (100% 

blue). However, three additional transformation 

events that had produced fully transgenic E1

embryos, but had been overlooked at the point 

of data collection, were identified in association 

with the evaluation of the stained E0 embryos. 

The percentages of GUS-positive E1 embryos that 

stained wholly blue was greater for the no-selection 

treatment (33-43%, Table 3); however, since the 

sample sizes of GUS-positive embryos were so 

small (n = 3-8), these percentages may not be 

representative of the population-level means.

For the direct selection treatment, the percentage 

of wholly transgenic E1 embryos was related to 

the mode of origin of the secondary embryos and 

Figure 4.—Secondary embryo regeneration and GUS staining phenotypes. (A) A mass of globular to polar-shaped 

embryos of indirect origin from its root pole of an E0 embryo on non-selective medium. (B) The X-Gluc staining 

patterns that were observed for a similar group of secondary embryos harvested from an explant on selective 

medium (500 mg/L kamanycin). Most of the embryos appeared either wholly transgenic or wholly nontransgenic. (C) A row of secondary embryos of direct origin from an E0

 embryo cultured on 200 mg/L kanamycin 

medium. (D) An E1 embryo of direct origin following X-Gluc staining. Notice the larger spots of blue, some of 

which appear to be associated with spots of localized tissue growth which may represent early secondary 

embryos. The asterisk indicates the position of the embryo’s root pole (facing upwards). (E) An E2 embryo 

that appears wholly transgenic as evidenced by its extensive blue coloration. (F) A larger, well-differentiated 

cotyledonary embryo from an established transgenic line. Note: Asterisks indicate the root axes of the embryos.

A B

C

F

D

*

*

A B

*

E

*
54 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

Table 4.—Relationships between secondary embryo origin, frequency of transformation events yielding wholly 

transgenic E1 embyros1, and the regeneration timecourse for the 2003 experiment direct selection treatment. 

Regeneration Time Course4

Origin2 N3

% Wholly GUS 

Positive

Culture

Period1

Culture

Period2

Culture

Period3

Direct-cotyledon 92 7 47 90 100

Direct-root 5 40 20 80 100

Indirect 4 75 0 25 100

1

 When regeneration was indirect, the initial harvested embryos presumably consisted of mixtures of E1 embryos and subsequent 

embryo generations (E2, E3, etc). Wholly transgenic E1 embryos were defined as those staining 100% blue when tested with XGluc (see Table 3)

2

 The transformation events were distinguished based on whether the E1 embryos were of direct or indirect origin, with those of 

direct origin being further distinguished based on whether the secondary embryos were derived from the cotyledons or root of 

the parent (E0) embryo.

3

 The number of transformation events. Each transformation event consisted of one or more E1 embryos that were located together 

on the surface of an E0 embryos (i.e., within 5 mm of one another)

4

 For each class of E1 embryos, the percentage of the total number of transformation events that were apparent at the end of each 

regeneration culture period is indicated. 

the timing of embryo formation (Table 4). For 

most of the transformation events evaluated, 

the initial secondary embryos produced were of 

direct cotyledonary origin. The rate of secondary 

embryo formation was rapid, with 90% of the 

transformation events being apparent by the 

end of the second culture period; however, fully 

transgenic E1 embryos were noted for only 7% of the 

transformation events. In contrast, for the subset of 

transformation events that involved the production 

of secondary embryos of either root or indirect 

origin, the frequencies of wholly GUS-positive E1

embryos were much higher (40-75%) and the rate of 

regeneration was slower (Table 4). 

Similar relationships between the frequency of 

GUS positive E1 embryos and the rate of embryo 

regeneration have been observed for J. regia

somatic embryos, as assessed via the staining of 

samples from the cotyledons of the E1 embryos 

(McGranahan and others 1990). For the study in 

question only 5% of the embryos harvested within 

6 weeks of co-cultivation tested positive for GUS 

activity, compared with GUS-positive frequencies of 

11-40% for the subset of embryos harvested within 

7-13 weeks of co-cultivation, and 55-65% for the 

subset harvested 14-16 weeks after co-cultivation. 

In addition, staining assays involving the evaluation 

of later generation embryos derived from the initial 

E1 regenerants, indicated most of the embryos that 

tested positive at the E1 stage had been wholly 

transgenic. 

The relationships between the rate of embryo 

regeneration and percentage wholly transgenic 

secondary embryos observed here and by 

McGranahan and others (1990) can be interpreted 

in terms of differences in the degree of embryo 

development at the time of gene transfer. In 

particular, secondary embryos transformed at a 

multicellular stage of development, which would 

exhibit chimeric staining, would be expected 

to appear prior to the emergence of embryos 

transformed at the unicellular stage, which would 

stain wholly blue. However, the higher frequencies 

of fully GUS-positive embryos that were observed for 

the transformation events involving the production 

of E1 embryos of root or indirect origin that were 

observed in this study cannot be fully accounted for 

by differences in the rate of embryo regeneration. In 

fact, the frequency of transformation events yielding 

fully transgenic E1 embryos was significantly 

lower for the transformation events involving 

direct embryo formation from the cotyledons of 

the infected embryos, even when the data was 

compared only for the subset of transformation 

events, where regeneration was not apparent until 

the end of the third culture period (data not shown). 

For the transformation events involving indirect 

embryo proliferation, the regeneration of stable 

transgenic lines may have been facilitated by the 

precocious proliferation of chimeric E1 embryos 

prior to the onset of a state of embryo quiescence 

(see below). 

E1 Proliferation and Establishment of 

Transgenic Lines

For the 2003 experiment, none of the E1 embryos 

that were regenerated on kanamycin-free media 

continued to grow or proliferate following transfer 

to kanamycin media for either of the concentrations 

tested. De novo growth and proliferation were 

observed for most of the embryos kept on nonselective medium (Table 5); however, none of the 

E2 embryos produced tested positive for GUS 

activity (Table 3). The number of embryos evaluated 
Propagation and Genetic Modification 55

Black Walnut in a New Century

Table 5.—Growth and proliferation responses of E1 embryos from the 2003 experiment. The data is 

sorted by experimental treatment and media type, but is combined across embryo lines and 

kanamycin (Kan) concentrations.

Treatment Media N % Growth1 % Prolif2

% Producing 

Stable Lines3

No Selection Kan 15 0 0 0

No Kan 35 63 55 0

Kanamycin Kan 114 7 5 5

Selection No Kan 34 9 6 0

1 Percentage with definite cotyledon enlargement (to at least 5 mm in maximal dimension).

2

 Percentage producing secondary embryos, typically observed after 2-3 culture periods.

3

 Percentage giving rise to stable kanamycin-resistant lines.

Table 6.—Kanamycin (Kan) sensitivity of non-transgenic walnut somatic embryos. The data 

presented is pooled from two experiments. For each experiment embryos from 4 to 8 lines 

were evaluated, with two to four embryos from each line being evaluated per media type. 

The cumulative data (collected after 5-6 wks of culture and pooled across embryo sources) 

are presented. Proliferation data are presented only for the subset of embryo lines that had 

produced secondary embryos on control medium (without kanamycin).

Media Small Cotyledonary1 Large Cotyledonary2

(mg/L Kan) N3 % Grown4 % Prolif5 N % Prolif5

0 29 (18) 79 75 16 44

25 21 (14) 81 64 16 50

50 28 (19) 75 16 16 31

100 29 (18) 72 0 16 0

250 29 (18) 21 17A 24 12A

500 29 (18) 3 0 24 0

1000 29 (18) 0 0 24 0

1

 The embryos were 2-5 mm in total length with cotyledons 1-3 mm long.

2

 The embryos were 10-20 mm in dimension with cotyledons 7-15 mm long.

3

 The sample size for the % proliferation data is indicated in parentheses.

4

 % embryos with cotyledon enlargement as visually assessed.

5

 % embryos producing secondary embryos. 

A For two-thirds of the embryos that exhibited proliferation on 250 mg/L kanamycin medium, the secondary 

embryos produced were of indirect, rather than direct, origin.

were kanamycin tolerant (0/12). In contrast, for 

five of the seven E1 embryos that were maintained 

on selective medium and exhibited de novo growth, 

stable transgenic lines (representing two distinct 

transformation events) were obtained. Although 

only two of the five E1 embryos that yielded 

transgenic lines were evaluated by X-Gluc assay, 

both stained fully blue. In contrast, none of the 

E1 embryos from the selective media plates that 

had failed to grow or proliferate following harvest 

stained 100% blue (Table 3).

For the combined data sets (2002, 2003) three 

distinct transgenic lines, representing separate 

embryo genotypes, were obtained. However, all 

three of the lines were derived from E0 embryos 

that had been cultured on regeneration media 

with 200-250 mg/L kanamcyin, suggesting that 

some or all of the kanamycin concentrations 

used in this study (200-500 mg/L) may have 

been supraoptimal for line establishment. In a 

was limited (6-35 depending on the parameter); 

however, similar results were obtained for J. regia

by Tang and others (2002) (i.e., when non-selective 

regeneration media was used, the frequency 

of GUS-positive E2 embryos ranged from 0-2% 

depending on the antibiotic used for Agrobacterium 

elimination). In contrast, when kanamycin (100 

mg/L) was used for selection, 14-22% of the 

embryos tested GUS positive. 

The recalcitrance of the E1 embryos regenerated 

on kanamycin medium for continued growth and 

proliferation was unexpected. Irrespective of the 

type of media used (selective or non-selective), less 

than 10% of the subcultured E1 embryos exhibited 

de novo growth (Table 5). De novo growth was 

observed for three of the embryos transferred to 

non-selective medium (3/34) and two of the three 

produced additional generations of secondary 

embryos; however, none of the E2/E3 embryos 

tested positive for GUS activity (0/14) and none 
56 Propagation and Genetic Modification

Proceedings of the 6th Walnut Council Research Symposium

series of preliminary kanamycin dose-response 

experiments, embryo proliferation was only 

erratically observed at kanamycin concentrations 

of 100 mg/L or greater (Table 6), and for several 

embryo lines, the secondary embryos produced 

in the presence of kanamycin exhibited retarded 

cotyledon differentiation and would not have been 

expected to produce additional generations of 

embryos of direct origin. Similarly, sub-lethal doses 

of kanamycin have been observed to antagonize 

shoot regeneration in several plant species (Yepes 

and Aldwinkle 1994, Tosca and others 1996, 

Peros and others 1998, Humara and Ordas 1999, 

Kapaun and Cheng 1999). In addition, in some 

studies, kanamycin has been found to interfere 

with plant regeneration even following NPT II gene 

incorporation (Mullins and others 1990, Robertson 

and others 1992, Ellis and others 1993, Aldemita 

and Hodges 1996, Azhakanandam and others 

2000).

For J. regia, 100 mg/L kanamycin have routinely 

been used for the selection of transgenic embryos 

(McGranahan and others 1990; Dandekar and 

others 1989, 1998; Escobar and others 2000; 

Tang and others 2000); however, this medium 

has been shown to be ineffective for the exclusion 

of non-transgenic secondary embryos in multiple 

labs (McGranahan and others 1990, Tang and 

others 2000). In addition, for certain J. regia x J. 

nigra lines the regeneration of non-transformed 

escapes has been observed even at kanamycin 

concentrations of up to 500 mg/L. However, in 

contrast with the results obtained in this study, 

when media with 100 mg/L kanamycin were used, 

the continued growth and proliferation of chimeric 

E1 regenerants was possible (McGranahan and 

others 1990, Tang and others 2000).

The delayed kanamycin sensitivity that was 

exhibited by the chimeric secondary embryos 

regenerated in this study suggests that the degree 

to which populations of transgenic black walnut 

cells are able to protect an embryo from the 

inhibitory effects of kanamycin exposure may be 

related to the size of the embryo and/or its level 

of tissue differentiation. During the initial stages 

of somatic embryogenesis, the transgenic sectors 

of the chimeric E1 regenerants were apparently 

able to cross-protect adjacent non-transgenic 

cells. However, with continued (symmetric) embryo 

growth, the ability of the transgenic cells to provide 

immunity for the entire E1 embryo decreased, 

presumably as a result of the increased spacing 

between the transgenic and non-transgenic cell 

groups and/or changes in NPT II diffusion/

transport characteristics, to such a degree that the 

competence of the embryos for continued growth 

and proliferation was ameliorated even prior to the 

point of embryo harvest. 

CONCLUSIONS AND FUTURE WORK

Despite the high rates of gene transfer and 

integration that were documented across multiple 

genotypes, robust protocols for the selection of 

transgenic J. nigra embryos and subsequent 

establishment of transgenic lines remain to be 

developed. When kanamycin (200-500 mg/L) 

was used for selection, nearly all of the initial 

regenerants were transgenic; however, most were 

chimeric and appeared incapable of continued 

growth and line establishment, at least via a direct 

proliferation pathway. Future investigations in our 

lab will focus on a more systematic evaluation of 

the kanamycin sensitivity of black walnut somatic 

embryos (both in the presence and absence of 

a heterologous NPT II gene), with the primary 

objective of identifying kanamycin concentrations 

that could be used to block non-transgenic 

regeneration without inhibiting the subsequent 

growth and proliferation of chimeric secondary 

embryos, which accounted for the majority of 

the regenerants in this and other studies. Other 

research possibilities include an evaluation of the 

effects of E0 embryo size on the transformation 

response. The use of smaller (younger) embryos for 

infection would presumably decrease the likelihood 

of secondary embryo formation prior to gene 

transfer, resulting in reductions in the frequency 

of chimeric embryo regenerantion. However, for 

high transformation rates to be achieved, the initial 

transgenic cells would need to be able to divide 

relatively freely, so as to produce populations 

of transgenic cells (i.e., entire blue sectors or 

patches) from which embryos were subsequently 

regenerated.

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Nursery Production and Plantation Establishment 59

Black Walnut in a New Century

PRODUCTION AND EARLY FIELD PERFORMANCE OF RPM® SEEDLINGS IN

MISSOURI FLOODPLAINS

Daniel C. Dey, Wayne Lovelace, John M. Kabrick, and Michael A. Gold1

ABSTRACT—A new nursery culture process has been developed to produce large 

container RPM® seedlings in an effort to improve the success in artificially regenerating 

hardwoods. Major features of the process include air root pruning of seedlings grown 

in a well aerated soil medium to encourage a dense, fibrous root system. Production 

has focused on native bottomland tree, shrub, and herbaceous species. Field plantings 

of RPM® seedlings have been done in agricultural floodplains throughout Missouri. In 

one such planting, the survival and growth of pin oak and swamp white oak RPM® and 

bareroot seedlings are evaluated when seedlings are established with soil mounding and 

a redtop grass cover crop. 

It is challenging to regenerate hardwoods such 

as oak (Quercus sp.), black walnut (Juglans nigra 

L.), and pecan (Carya illinoinensis [Wangenh.] K. 

Koch) on productive floodplains because competing 

vegetation, flooding, and animal damage retard 

seedling growth and decrease survival (Stanturf 

and others 1998). In addition, slow (shoot) growth 

rates are characteristic of oak, hickory (Carya sp.), 

and pecan reproduction, which further complicates 

efforts to regenerate these species, and usually 

necessitates controlling competing vegetation. 

Early growth of planted bareroot seedlings can be 

disappointing for black walnut (von Althen and 

Prince 1986, Van Sambeek and others 1987) and 

upland oaks (Johnson 1984). However, regeneration 

success can be improved by planting large, bigrooted seedlings and promoting early hardwood 

growth through such practices as fertilization, 

vegetation management, and protecting seedlings 

from wildlife herbivory.

Johnson and others (2002) stress the importance 

of planting large oak seedlings with well-developed 

root systems to promote regeneration success and 

increase dominance probabilities. Kormanik and 

others (1995, 1998) and Schultz and Thompson 

(1997) demonstrated that upland oak (i.e., northern 

red oak [Quercus rubra L.] and white oak [Q. alba

L.]) and black walnut 1+0 bareroot seedlings are 

more successful in open agricultural fields and 

forest clearcuts if they have a threshold number 

of first-order-lateral roots (FOLR) greater than 1 

mm in diameter. Schultz and Thompson (1997) 

recommended that northern red oak 1+0 bareroot 

seedlings should have a minimum of five FOLR and 

black walnut at least seven FOLR.

Nursery managers can increase the number of 

FOLR on bareroot seedlings by undercutting the 

taproot during the first or second year, or by 

transplanting 1+0 seedlings for a second year in 

the nursery to produce 1+1 transplant seedlings 

(Johnson 1988). Undercutting hardwood seedlings 

is now a common practice in the production of 

1+0 bareroot seedlings. Pre-plant treatments with 

auxins have also been used to promote enhanced 

root regeneration in black walnut (Van Sambeek 

and others 1982). Air pruning the roots of seedlings 

grown in open-bottomed containers is another way 

to promote lateral root growth and a dense fibrous 

root system. Based on 50 years of research in 

container production and hardwood regeneration, 

the Forrest Keeling Nursery in Elsberry, MO has 

developed a culture system, known as the Root 

Production Method (RPM®), to produce high quality 

hardwood seedlings that have large caliper and 

height, and a substantial fibrous, root system. 

For the past 5 to 7 years, private landowners, public 

land managers and scientists have been planting 

RPM® seedlings to regenerate primarily agricultural 

bottomlands. Bottomland oak species such as pin 

1

Research Forester (DCD and JMK), North Central Research Station, USDA Forest Service, 202 ABNR Bldg. Columbia, MO 65211; Vice 

President – General Manager (WL), Forrest Keeling Nursery, P.O. Box 135, Elsberry, MO 63343; Associate Director (MAG), Center for 

Agroforestry, University of Missouri, 203 ABNR Bldg. Columbia, MO 65211. DCD is corresponding author: to contact call (573) 875-

5341 ext. 225 or e-mail at ddey@fs.fed.us.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
60 Nursery Production and Plantation Establishment

Proceedings of the 6th Walnut Council Research Symposium

oak (Quercus palustris Muenchh.), swamp white 

oak (Q. bicolor Willd.) and bur oak (Q. macrocarpa 

Michx.) are often planted as RPM® seedlings in 

Missouri. To a lesser extent, black walnut, pecan, 

shellbark hickory (Carya laciniosa [Michx. f.] Loud.), 

sycamore (Platanus occidentalis L.) and other native 

bottomland species are planted as RPM® seedlings. 

Information on the growth and survival of RPM®

in floodplain plantings is lacking. In this paper, 

we summarize the RPM® nursery culture process 

and present early growth and survival of pin oak 

and swamp white oak RPM® seedlings in a study of 

regeneration methods for afforesting agricultural 

crop fields along the Missouri River.

PRODUCTION OF THE RPM® SEEDLING

The root production method, or RPM® (patent no. 

pending 60/312593) is a nursery culture process 

to grow seedlings with dense, fibrous root systems 

in large (e.g., 11- to 19-liter) containers (Lovelace 

1998). Using oak as an example, acorns are 

collected from trees growing in floodplains within 

161 km of the bottomland planting site. Acorns are 

graded and sized using an aspirator and gravity 

table. Only the largest and heaviest seeds are used 

to produce RPM® seedlings. Acorns are placed 

4 cm deep in mesh-bottomed trays filled with a 

composted rice hull, pine bark and sand medium 

(4:4:2 by volume). The shallow depth of media with 

35 to 40% air space promotes air pruning of the tap 

root near the root collar, production of first-order 

lateral roots near the soil surface and development 

of a fibrous root system. Slow release fertilizer, 

micronutrients and a wetting agent are added to 

the soil medium and trays are enclosed in plastic to 

maintain proper humidity during cold stratification 

at 1o C for the recommended period depending 

on species. Next, trays of acorns are moved into 

a heated greenhouse usually in early February to 

initiate germination. 

One to two months after emergence, when seedlings 

have completed their first shoot flush, they are 

transplanted into individual plastic, 10 cm deep 

bottomless band containers and placed on chicken 

wire and wood-framed benches in the greenhouse 

to permit continued air root pruning. Seedlings 

are graded based on height, stem caliper, and root 

development. Typically only the largest 50% of the 

seedlings continue through the RPM® process. In 

early May, seedlings are transplanted into 11 or 19 

liter plastic containers and placed outside under 

mist irrigation for 48 hours to acclimatize them to 

the outdoor environment. Shallow containers are 

used to concentrate root growth in the upper 15 to 

20 cm soil surface. The larger container is used for 

seedlings that will be grown a second year in the 

nursery.

After one to two growing seasons in the nursery, 

RPM® seedlings develop large root systems, have 

basal stem diameters exceeding 2.0 cm, and grow 

to over 1.5 m in height. Dey and others (2003) 

reported that root dry weight of pin oak and swamp 

white oak RPM® seedlings averaged 117 and 101 g 

for 11- and 19-liter container plants, respectively. 

In contrast, root dry weight of 1+0 bareroot oak 

seedlings averaged 18 g. Similarly, root volumes 

(by water displacement) of RPM® oak seedlings were 

substantially larger than 1+0 bareroot seedlings. 

Nineteen-liter swamp white oak and pin oak RPM®

seedlings averaged 252 and 222 ml root volume, 

respectively, whereas root volume of swamp white 

oak and pin oak bareroot seedlings averaged 33 and 

26 ml, respectively.

SMOKY WATERS AND PLOWBOY 

BEND RPM® PLANTING

In the fall of 1999 a study was established to 

evaluate methods for regenerating pin oak and 

swamp white oak on former agricultural crop fields 

in the Missouri River floodplain at Smoky Waters 

(Sec. 5, T 44 N, R 9 W and Sec. 1, T 44 N, R 10 

W; Cole County, MO) and Plowboy Bend (Secs. 24 

and 25, T 47 N, R 14 W; Moniteau County, MO) 

Conservation Areas. The study fields had been in 

crop production for years before this study.

Soils at the Plowboy Bend site were mapped 

as Sarpy Fine Sand (mixed, mesic, Typic 

Udipsamments). These soils are formed in sandy 

alluvium and consequently are excessively-drained. 

Soils at the Smoky Waters site were mapped as 

Haynie Silt Loam (coarse-silty, mixed, superacitve, 

calcarious, mesic Mollic Udifluvents), Leta Silty Clay 

(clayey over loamy, smectitic, mesic, Fluvaquentic 

Hapludolls), and Waldron Silty Clay Loam (fine, 

smectitic, calcareous, mesic Aeric Fluvaquents). 

These soils are formed in silty or clayey alluvium 

and range from somewhat-poorly drained (Leta and 

Waldron) to moderately well drained (Haynie). The 

Plowboy Bend site is also protected by a levee and 

the Smoky Waters is not protected by levees. 

Dey and others (2003) provided a detailed 

explanation of experimental design and study 

establishment. In general, both 1+0 bareroot and 

11- and 19-liter RPM® seedlings were planted to 

evaluate the effect of seedling size and nursery 

stock type on the survival and growth of pin oak 

and swamp white oak seedlings. Seedlings were 

planted in soil mounds created with a rice plow or 

in unmounded soil; and with either a cover crop of 
Nursery Production and Plantation Establishment 61

Black Walnut in a New Century

redtop grass (Agrostis gigantea L.) or with natural 

vegetation that normally colonizes abandoned 

bottomland crop fields. 

Natural Disturbances

In 2 of the 4 four years (2001, 2002) the study site 

at Smoky Waters was flooded for up to 3 weeks 

in June. Also, every winter after the first year, 

cottontail rabbits (Sylvilagus floridanus Allen) have 

girdled and shoot clipped oak seedlings and oak 

sprouts, which profoundly affected oak survival and 

growth. The amount and severity of rabbit damage 

to planted oaks varied greatly between the cover 

crop treatments (i.e., redtop grass versus natural 

vegetation fields). 

Rabbit Herbivory

In the natural vegetation fields, the composition 

and structure of winter cover provided by forbs 

promoted higher rabbit densities (7.4 rabbits per 

ha) than in the redtop grass fields (2.5 rabbits per 

ha) (Dugger and others 2003). In the winter, the 

dead tops of forbs and clumps of Johnsongrass 

(Sorghum halepense [L.] Pers.) remained somewhat 

erect providing cover that was 1.0 m tall. However, 

redtop grass matted down to 0.20 m and provided 

rabbits little hiding cover from predators. Thus, 

rabbits were able to move freely across the natural 

vegetation fields causing damage to nearly all of 

the seedlings each winter. Rabbits clipped the 

shoots of all bareroot seedlings and severely girdled 

(more than half of the circumference of the stem) 

90% or more of the RPM® seedlings in the natural 

vegetation fields by the end of the second winter. 

In comparison, only 8% of the bareroot seedlings 

and 26% of RPM® seedlings in the redtop grass 

field at Plowboy Bend Conservation Area had 

herbivory damage from rabbits. Similarly, but to 

a lesser extent, 12% of the bareroot seedlings and 

23% of the RPM® seedlings in the redtop grass 

field at Smoky Waters Conservation Area were 

damage free. Moreover, the severity of damage to 

RPM® trees in redtop grass fields was less than in 

the natural vegetation fields. These differences in 

winter habitat between the cover crop treatments 

affected rabbit densities and movements, which in 

turn, contributed to the significant differences in 

oak seedling survival, growth, and acorn production 

between the cover crop treatments.

Survival

Survival of oak RPM® seedlings remained high 

(i.e., > 94%) during the first 3 years (Fig. 1), while 

survival of bareroot seedlings continued to decline 

for both swamp white oak and pin oak. After 3 

years in the field, survival of swamp white oak 

bareroot seedlings was significantly higher (P < 

0.01) than pin oak bareroot seedlings, based on 

an analysis of variance of treatment effects on 

survival (Dey and others 2003). Swamp white oak 

bareroot survival averaged 76%, while survival for 

pin oak bareroot seedlings was 54%. There was no 

significant difference (P = 0.24) between 11- and 19-

liter RPM® seedlings, nor between swamp white oak 

and pin oak RPM® seedlings (P = 0.87). Survival of 

oak seedlings was not significantly affected by soil 

mounding or cover crop treatments. However, an 

assessment of the survival of individual trees that 

were not damaged by rabbits indicated that redtop 

grass cover crop did significantly increase survival 

over that of trees grown with natural vegetation 

(Fig. 2) based on logistic regression analysis 

conducted by Dey and others (2003). There were no 

significant differences in survival among trees on 

mounded and unmounded soils.

Basal Diameter Growth

Basal diameter increment after 3 years was 

significantly greater (P < 0.01) for RPM® seedlings 

than bareroot stock, regardless of species (Fig. 3), 

based on an analysis of variance (Dey and others 

2003). The average basal diameter of all RPM® oak 

seedlings increased 0.8 cm in the first 3 years, 

whereas bareroot seedlings increased only 0.3 cm. 

There was no significant difference (P = 0.34) in 

basal diameter increment between the 11- and the 

Figure 1.—Annual survival of oak seedlings by species 

and nursery stock type.

Year 1 Year 2 Year 3

Survival (%)

50

60

70

80

90

100

Pin oak bareroot

Pin oak RPM 11 liter

Pin oak RPM 19 liter

Swamp white oak bareroot

Swamp white oak RPM 11 liter

Swamp white oak RPM 19 liter
62 Nursery Production and Plantation Establishment

Proceedings of the 6th Walnut Council Research Symposium

19-liter RPM® seedlings. The basal diameter of pin 

oak 19-liter RPM® seedlings increased the most 

during the first 3 years, averaging 1.0 cm of new 

growth. Basal diameter increment was least (0.1 cm 

in 3 years) for pin oak bareroot seedlings. The above 

analysis includes rabbit damaged and undamaged 

trees. By removing the rabbit damaged trees, 

average basal diameter increment was 1.6 cm for 

RPM® seedlings and 0.2 cm for bareroot trees.

Although soil mounds functioned as anticipated by 

improving drainage and aeration, diameter growth 

was not significantly affected by soil mounding. 

Basal diameter increment of all trees combined 

was substantially larger in redtop grass fields (1.4 

cm) compared to natural vegetation fields (0.2 

cm); however, no significant differences can be 

reported as yet (P = 0.08). For undamaged trees, 

average basal diameter increment was 1.6 cm for 

RPM® seedlings in redtop grass fields and 0.4 cm in 

natural vegetation fields, while the basal diameter 

of bareroot seedlings increased by 0.3 cm in redtop 

grass but decreased by 0.2 cm in natural vegetation 

fields.

Height Growth

Average height increment after 3 years was negative 

for most species and nursery stock types because 

cottontail rabbits caused extensive damage by 

girdling the stems of RPM® seedlings, or by clipping 

the shoots of bareroot seedlings at ground-level, 

which caused shoot dieback and loss of height (Fig. 

4). Three year height increment was significantly 

less (P < 0.01) for RPM® seedlings than bareroot, 

based on an analysis of variance (Dey and others 

2003). For bareroot seedlings that had been shootclipped by rabbits, annual sprout growth came 

close to, or slightly exceeded the initial height, 

resulting in small negative or positive increments 

in height. In contrast, net height increment was 

much lower in RPM® trees because rabbit girdling, 

which occurred in the lower 0.30 m of the stem, 

caused shoot dieback to near ground-level, and 

annual sprout growth was not enough to recover 

the original height. In addition, trees were often 

repeatedly damaged by rabbits each winter. Three 

year height increment averaged -0.50 m for the 

RPM® seedlings. Despite rabbit browsing, RPM®

trees remained taller than bareroot seedlings three 

years after planting. Undamaged trees in the redtop 

grass fields had slightly positive average height 

growth (0.10 m for bareroot and RPM® seedlings), 

but net growth was negative in natural vegetation 

fields, averaging -0.53 m for RPM® and -0.13 m for 

bareroot seedlings. 

Height growth of undamaged RPM® seedlings may 

be low because these trees were planted on a 

Initial Year 1 Year 2 Year 3

Basal diameter (cm)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Pin oak bareroot

Pin oak RPM 11 liter

Pin oak RPM 19 liter

Swamp white oak bareroot

Swamp white oak RPM 11 liter

Swamp white oak RPM 19 liter

Figure 2.—The probability of mortality after three 

years in relation to initial seedling height of pin 

oak seedlings planted with a cover crop of redtop 

grass or in natural vegetation that develops in 

abandoned bottomland crop fields at Smoky 

Waters Conservation Area. In this illustration, 

mortality is estimated for oak seedlings with initial 

basal diameter = 0.5 cm (representative of the 

average pin oak bareroot seedling in our study); 

and for seedlings with initial basal diameter = 2.0 

cm (representative of the average RPM® pin oak in 

our study).

Figure 3.—Average initial and annual basal diameter 

(measured 2.5 cm above the ground) of oak 

seedlings by species and nursery stock type.

Initial height (m)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Probability of mortality after 3 years 0.0

0.2

0.4

0.6

0.8

1.0

Initial basal diameter = 0.5 cm, Bareroot Seedlings

Initial basal diameter = 2.0, RPM Seedlings

No Redtop Grass

Redtop Grass
Nursery Production and Plantation Establishment 63

Black Walnut in a New Century

9 x 9 m spacing; and widely spaced, open-grown 

trees often experience reductions in height growth, 

especially trees with weak epinastic control such 

as the oaks (Oliver and Larson 1996). Also, height 

growth of oak reproduction is slow at first because 

seedlings characteristically allocate photosynthates 

to root growth often at the expense of shoot growth 

(Johnson and others 2002). Height growth of 

RPM® and bareroot seedlings may also be limited 

by low levels of foliar nitrogen, which averaged 

2.05% at Smoky Waters and 1.71% at Plowboy 

Bend Conservation Areas (J.W. Van Sambeek and 

N. Sullivan, Research Physiologist and Research 

Forester, U.S. Forest Service, North Central 

Research Station, personal communication).

An analysis of all trees by cover crop treatment 

showed that 3 year height increment was 

significantly higher (P = 0.02) for oak seedlings 

growing in the redtop grass fields than those trees 

competing with natural vegetation. There may be 

less light competition in the redtop grass fields 

during the growing season than in the natural 

vegetation fields. Redtop grass typically grows to a 

height of 0.4 to 0.6 m whereas herbaceous ground 

cover in the natural vegetation fields grew to over 2 

m in height, overtopping many of the oak seedlings. 

Also, rabbit densities were less in the redtop grass 

fields than in the natural vegetation fields. There 

was no significant difference in height growth 

among trees on mounded and unmounded soils.

Acorn Production

Swamp white oak RPM® seedlings that were 18 to 

24 months old at time of planting produced acorns 

in each of the first 4 years following outplanting 

(Table 1). Acorn production occurred in a small 

proportion (3.5%) of the 2,522 swamp white oak 

RPM® seedlings their first year in the field. Most of 

the production (60%) occurred in oaks from 19-liter 

containers, but larger 11-liter container RPM® trees 

also produced acorns. During the first 4 years in 

the field, average RPM® acorn production increased 

from 4.3 to 12.5 acorns per tree. Individual trees 

were able to produce as many as 125 acorns. A 

single pin oak RPM® seedling produced acorns for 

the first time in year 4.

The probability of a RPM® swamp white oak seedling 

producing at least one sound acorn in the first 

year after planting was significantly (P < 0.001) 

related to initial basal diameter and height of the 

seedling, based on logistic regression analysis by 

Grossman and others (2003). Acorn production 

was more likely to occur in the first year for large 

diameter (> 1.3 cm), tall (> 1.5 m) RPM® seedlings. 

For example, the probability of producing a sound 

acorn after 1 year is 2% for a 1.5 m RPM® seedling 

with a basal diameter of 1.8 cm, but increases to 

15% for a 2.5 cm basal-diameter tree of similar 

height. Consistent, early production of acorns was 

surprising considering that open-grown oaks do not 

begin producing seed until they are 20 to 30 years 

old (Burns and Honkala 1990). In contrast, no 

bareroot oak seedlings have produced acorns after 

four growing seasons.

Initial Year 1 Year 2 Year 3

Height (

m) 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Pin oak bareroot

Pin oak RPM 11 liter

Pin oak RPM 19 liter

Swamp white oak bareroot

Swamp white oak RPM 11 liter

Swamp white oak RPM 19 liter

Figure 4.—Average initial and annual height of oak 

seedlings by species and nursery stock type.

Table 1.—Acorn production by swamp white oak RPM® seedlings planted at 

Plowboy Bend and Smoky Waters Conservation Areas.

Years After 

Planting

Number of Trees 

With Acorns

Mean Number of 

Acorns Per Tree

Standard

Deviation Range

1 86 4.3 4.4 1-21

2 29 5.2 9.2 1-45

3 70 6.3 7.5 1-42

4 151 12.5 17.0 1-125
64 Nursery Production and Plantation Establishment

Proceedings of the 6th Walnut Council Research Symposium

SUMMARY

Large container RPM® seedlings had significantly 

greater survival and basal diameter growth than 

bareroot seedlings after three years. Three-year 

height increment for RPM® seedlings was negative 

and significantly less than that of bareroot 

seedlings largely due to the loss of initial height 

from rabbit herbivory on oak seedlings. There was 

no difference in growth or survival to-date between 

the 11- and 19-liter RPM® trees. 

A redtop grass cover crop benefited oak 

regeneration by controlling competing vegetation 

and reducing the amount and severity of rabbit 

damage to oak seedlings. Redtop grass was 

effective in preventing the development of much 

of the forb and woody growth that normally forms 

on abandoned bottomland crop fields, thereby 

reducing plant competition for light and the quality 

of winter habitat for rabbits. Oak seedlings growing 

in redtop grass fields had significantly greater 

height increment and substantially larger basal 

diameter growth after 3 years than trees competing 

with natural vegetation. 

After 3 years, and two June floods at Smoky Waters 

Conservation Area, soil mounding did not improve 

height and diameter growth, or survival of oak 

seedlings at either site. Use of soil mounding to 

improve drainage, reduce flooding effects on trees 

and improve soil environments for root growth may 

be worthwhile on soils that are more poorly drained, 

and clayey than those of our study site but this 

remains to be tested.

One- to two-year-old swamp white oak RPM®

seedlings produced acorns in the first year after 

planting in Missouri River floodplain crop fields. 

The number of acorn bearing trees and production 

per tree has increased during the first four years. 

Acorn production did drop in year two because 

rabbit girdling caused shoot dieback in RPM® trees 

in the natural vegetation field, taking them out of 

production. But production rose in years 3 and 4 

as more trees came into production. Most acorn 

bearing trees are located in the redtop grass fields. 

Pin oak RPM® have begun producing acorns in year 

4. Larger RPM® seedlings are more likely to produce 

acorns within the first few years of plantation 

establishment.

Under controlled conditions in the nursery, the 

RPM® process produces seedlings with very large 

root systems that have numerous large diameter 

lateral roots (or multiple taproots), and a high 

density of fine roots. These are desirable root 

characteristics that many have recognized as 

being essential for successful regeneration of 

species such as the oaks. Early results from this 

study indicate that planting RPM® seedlings of 

large size in a redtop grass ground cover appears 

to be a successful formula for regenerating oaks 

and restoring acorn production in agricultural 

floodplains. It is critical to successful oak 

regeneration on productive bottomlands that high 

quality nursery stock be planted in combination 

with vegetation management practices that 

control competing vegetation and animal damage. 

Further research is needed to evaluate the how 

RPM® seedlings, soil mounding, and cover crops 

may enhance black walnut establishment in 

bottomlands.

ACKNOWLEDGMENT

This work was funded, in part, through the 

University of Missouri Center for Agroforestry under 

cooperative agreements 58-6227-1-004 with the 

ARS and C R 826704-01-2 with the US EPA. The 

results presented are the sole responsibility of the 

P.I. and/or MU and may not represent the policies 

or positions of the ARS or EPA.

LITERATURE CITED

Burns, R.M.; Honkala, B.H., eds. 1990. Silvics 

of North America. Vol. 2, Hardwoods. Agric. 

Handb. 654. Washington, DC: U.S. Department 

of Agriculture, Forest Service. 877 p.

Dey, D.C.; Kabrick, J.M.; Gold, M.A. 2003. Tree 

establishment in floodplain agroforestry 

practices. In: Proceedings, 8th North American 

agroforestry conference; 2003 June 23-25; 

Corvallis, OR (in press).

Dugger, S.; Dey, D.C.; Millspaugh, J.J. 2003. 

Vegetation cover effects mammal herbivory 

on planted oaks and success of reforesting 

Missouri River bottomland fields. In: 

Proceedings, 12th biennial southern silvicultural 

conference; 2003 February 25-27; Biloxi, MS. 

Gen. Tech. Rep. SRS-. Asheville, NC: U.S. 

Department of Agriculture, Forest Service, 

Southern Research Station: (in press).

Grossman, B.C.; Gold, M.A.; Dey, D.C. 2003. 

Restoration of hard mast species for wildlife in 

Missouri using precocious flowering oak in the 

Missouri River floodplain, USA. Agroforestry 

Systems. 59: 3-10.

Johnson, P.S. 1984. Responses of planted northern 

red oak to three overstory treatments. Canadian 

Journal Forest Research. 14: 536-542.
Nursery Production and Plantation Establishment 65

Black Walnut in a New Century

Johnson, P.S. 1988. Undercutting alters root 

morphology and improves field performance of 

northern red oak. In: Worrall, J.; Loo-Dinkins, 

J.; Lester, D.P., eds. Proceedings, 10th North 

American forest biology workshop; Vancouver, 

BC: University of British Columbia, Department 

of Forest Science: 316-323.

Johnson, P.S.; Shifley, S.R.; Rogers, R. 2002. The 

ecology and silviculture of oaks. New York, NY: 

CABI Publishing. 503 p.

Kormanik, P.P.; Sung, S.S.; Kormanik, T.L.; 

Zarnock, S.J. 1995. Oak regeneration – why 

big is better. In: Landis, T.D.; Cregg, B., tech. 

coords. National proceedings, Forest and 

Conservation Nursery Assocations. Gen. Tech. 

Rep. PNW-GTR-365. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, 

Pacific Northwest Research Station: 117-123.

Kormanik, P.P.; Sung, S.S.; Kass, D.J.; 

Schlarbaum, S. 1998. Effect of seedling size and 

first-order-lateral roots on early development of 

northern red oak on mesic sites. In: Waldrop, 

T.A., ed. Proceedings, 9th biennial southern 

silvicultural research conference; 1997 

February 25-27; Clemson, SC. Gen. Tech. Rep. 

SRS-20. Asheville, NC: U.S. Department of 

Agriculture, Forest Service, Southern Research 

Station: 247-252.

Lovelace, W. 1998. The root production method 

(RPM) system for producing container trees. 

Combined proceedings of the International 

Plant Propagators Society. 48: 556-557.

Oliver, C.D.; Larson, B.C. 1996. Forest stand 

dynamics. New York, NY: John Wiley and Sons, 

Inc. 520 p.

Schultz, R.C.; Thompson, J.R. 1997. Effect of 

density control and undercutting on root 

morphology of 1+0 bareroot hardwood 

seedlings: five-year field performance of rootgraded stock in the central USA. New Forests. 

13: 301-314.

Stanturf, J.A.; Schweitzer, C.J.; Gardiner, E.S. 

1998. Afforestation of marginal agricultural 

land in the Lower Mississippi River Alluvial 

Valley, U.S.A. Silva Fennica. 32(3): 281-287.

Van Sambeek, J.W.; Rietveld, W.J. 1982. Preplant 

inolebutyric acid treatment of outplanted black 

walnut seedlings. In: Proceedings, 7th North 

American forest biology workshop; 1982 July 

26-28; Lexington, KY: University of Kentucky 

Press: 272-280.

Van Sambeek, J.W.; Williams, R.D.; Hanover, J.W. 

1987. Comparison of planting methods for 

nursery- and container-grown black walnut 

seedlings. In: Hay, R.L.; Woods, F.W.; DeSelm, 

H.R., eds. Proceedings, 6th Central Hardwood 

forest conference; 1987 February 24-26; 

Knoxville, TN: The University of Tennessee: 

69-73.

von Althen, F.W.; Prince, F.A. 1986. Black walnut 

(Juglans nigra L.) establishment, six-year 

survival and growth of containerized and 1+0 

seedlings. Tree Planters’ Notes. 37: 11-14.
66 Nursery Production and Plantation Establishment

Proceedings of the 6th Walnut Council Research Symposium

FACILITATING NUTRIENT ACQUISITION OF BLACK WALNUT AND

OTHER HARDWOODS AT PLANTATION ESTABLISHMENT

Douglass F. Jacobs and John R. Seifert1

ABSTRACT—Bareroot hardwood seedlings typically undergo transplant shock 

immediately following planting before root systems are established. Fertilization at 

planting may act to minimize transplant shock by reducing nutrient stresses. However, 

previous work with fertilization of hardwoods at planting has generally relied on 

fertilizers with nutrient forms immediately available. These fertilizers have relatively 

low rates of fertilizer use efficiency, as the vast majority of applied nutrients are not 

available for plant uptake. Thus, response of hardwood seedlings to fertilization at 

planting has traditionally been minimal and is rarely recommended operationally. 

Controlled-release fertilizer (CRF) is a relatively new technology available for reforestation 

in which nutrients are slowly released over the course of one to two growing seasons. 

We tested the use of Osmocote® Exact 15N-9P-10K plus minors (16 to 18 month release) 

polymer-coated CRF applied to the root zone at planting with three hardwood species: 

black walnut, white ash, and yellow-poplar in southern Indiana. Application of 60 g 

per seedling accelerated first-year height growth by 52% and diameter growth by 37% 

compared to controls (averaged over species). These preliminary results indicate that 

CRF may provide a previously untested means to improve hardwood afforestation 

planting success.

Bareroot seedlings typically grow slowly during 

the first 1 or 2 years following planting until root 

systems can establish and proliferate through the 

soil to exploit site resources (Rietveld 1989). While 

water limitations are often noted as a major barrier 

to transplanting success (Burdett and others 1984), 

nutrient limitations may also slow initial seedling 

growth. Fertilization at the time of planting has 

been recommended for conifer seedlings as a means 

to minimize transplant shock and facilitate rapid 

growth above competing vegetation and the level of 

deer browse (Carlson and Preisig 1981). 

Fertilization at planting is rarely recommended 

(Ponder 1996) and often discouraged (Beineke 

1986) for hardwood seedlings, likely due to 

previous studies reporting neutral or negative 

effects from the practice. Williams (1974) reported 

reduced survival and growth of planted black 

walnut (Juglans nigra L.) seedlings that received 

fertilization. Black walnut fertilized at 1, 2, and 

6 years after planting on a good site exhibited 

no differences in height or diameter at breast 

height after 12 years (Braun and Byrnes 1982). 

Previous studies, however, have generally relied 

upon surface application of traditional agronomic 

fertilizers (e.g., urea or ammonium nitrate), which 

release nutrients immediately upon application. 

These fertilizers typically have relatively low rates of 

nutrient recovery by crop species, termed fertilizer 

use efficiency. Additionally, surface application of 

fertilizer may act to stimulate growth of competing 

vegetation which is a predominant reason that 

fertilization at planting has been discouraged 

(Ponder 1996). 

Controlled-release fertilizer (CRF) offers an 

alternative to standard agronomic fertilizers. With 

a single application, CRF may provide plants with 

enhanced nutrition for extended periods, typically 

ranging from 3 to 18 months. This acts to provide 

a more consistent and sustained nutrient supplies 

that may better coincide with the developmental 

needs of tree species (Donald 1991). The gradual 

1Assistant Professor of Forest Regeneration (DFJ), Hardwood Tree Improvement and Regeneration Center, Department of Forestry 

and Natural Resources, Purdue University, West Lafayette, IN 47907-2033; and Extension and Research Forester (JRS), Hardwood Tree 

Improvement and Regeneration Center, Department of Forestry and Natural Resources, Purdue University, PO Box 216, Butlerville, IN 

47223. DFJ is corresponding author: to contact, call (765) 494-3608 or e-mail at djacobs@fnr.purdue.edu.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
Nursery Production and Plantation Establishment 67

Black Walnut in a New Century

release pattern of CRF may also minimize nutrient 

leaching, reduce plant damage, and improve overall 

fertilizer use efficiency. Hangs and others (2003) 

reported that < 1% of nitrogen from Polyon®, a 

polymer-coated CRF, applied to conifer seedlings 

at planting was lost to leaching after two growing 

seasons. CRF may be particularly effective 

when applied to the seedling root zone, as CRF 

applications of 5 to 7 cm from roots resulted in 

substantially greater fertilization use efficiency 

than surface application (Hangs and others 2003). 

Many different types of CRF are available, primarily 

differing in terms of nutrient formulations, 

estimated product longevities, and mechanisms of 

nutrient release (Jacobs and others 2003b).

Although use of CRF has traditionally been limited 

to the horticultural industry, interest in using 

these fertilizers for tree plantings has increased 

in the past decade (Haase and Rose 1997). Some 

impressive results have been reported when 

incorporating CRF into tree plantings with conifer 

species (Carlson 1981, Carlson and Preisig 1981, 

Nursery Technology Cooperative 2001). However, 

we know of no reports examining the use of CRF 

in hardwood afforestation plantings in the eastern 

United States. Thus, the purpose of this trial was 

to study the response of three hardwood species 

to CRF applied to the root zone at planting. In 

this paper, we present preliminary, first-year 

observations indicating that CRF accelerated initial 

plantation establishment of these three hardwood 

species.

MATERIALS AND METHODS

Bareroot (1+0) seedlings of three species [black 

walnut, white ash (Fraxinus americana L.), and 

yellow-poplar (Liriodendron tulipifera L.)] were grown 

under standard nursery cultural regimes at the 

Indiana Department of Natural Resources’ Vallonia 

Nursery during 2001. Following lifting, seedlings 

were cold-stored at 2o C until delivered to the field 

planting site. The outplanting site for this study was 

located at the Southeastern Purdue Agricultural 

Center (SEPAC) in southeastern Indiana (39°01’N, 

85°35’W). The site was formerly in agricultural 

production and was tilled prior to planting. 

The CRF examined in this study was Osmocote®

Exact Lo-Start 15N-9P-10K plus minors. Nutrients 

in this CRF are encapsulated within multiple 

layers of a polymeric resin, which acts to slow the 

dissolution rate of nutrients into the soil solution. 

This CRF was designed by the manufacturer to 

release over 90% of its nutrients within 16 to 

18 months following application, assuming a 

soil temperature of 21o C at position of fertilizer 

placement. At this longevity rating, we assumed 

that seedlings would receive enhanced nutrition for 

two growing seasons since fertilizer release during 

the winter months should be slowed. To estimate 

rates of nutrient release, we placed 75 g CRF within 

PVC rings covered in nylon. These were buried 

at the approximate depth of fertilizer placement 

(30 cm) and excavated periodically during the 

experiment (8 sampling intervals × 5 replications). 

The excavated fertilizer was dried at 70o C for 48 hr 

and re-weighed. 

Seedlings were planted on either 2002 April 10 

(three blocks) or 2002 April 24 (remaining three 

blocks) using a tractor-hauled coulter with trencher 

and packing wheels. Fertilizer was applied at six 

rates (0, 15, 30, 45, 60, and 75 g per seedling) 

using a modified funnel system installed onto the 

machine planter (Fig. 1). For ease of application, 

rates were converted to volume estimates from 

lab trials prior to planting. For each seedling, the 

appropriate CRF rate treatment was measured 

by volume and applied into the planting trench 

(approximately 30 cm depth) through the funnel 

system. The 75 g per seedling rate encompassed a 

length of about 15 cm along the trench. Seedlings 

were planted directly over fertilizer with roots 

extending to just above the fertilizer layer. Herbicide 

applications were made prior to and following 

planting, and weed competition was minimal 

throughout the first growing season.

Seedlings were measured for initial height and root 

collar diameter on 2002 April 29 and 30. In early 

May 2002, an electric deer fence was installed 

around the perimeter of the study and maintained 

for the duration of the experiment. Seedlings 

were re-measured in November 2002 for height, 

diameter, and incidence of shoot dieback when 

terminal bud died with a lateral bud becoming the 

new leading shoot. 

Figure 1.—Modified funnel system on machine planter 

for fertilizer application. 
68 Nursery Production and Plantation Establishment

Proceedings of the 6th Walnut Council Research Symposium

The study was established as a randomized 

complete block design with six blocks and factorial 

treatments (3 species × 6 CRF rates). Environmental 

conditions differed among blocks but conditions 

were relatively homogenous within blocks. 

Within a block, each of the 18 treatments was 

randomly assigned to a row in which 20 seedlings 

representing a treatment combination were planted. 

The sampling unit was each individual seedling 

and the experimental unit used for data analysis 

was the mean value of the 20 seedlings within a 

row for each treatment. Data were analyzed using 

analysis of variance (ANOVA). When significant (P ≤ 

0.05) effects were detected in the ANOVA, Fisher’s 

Protected Least Significant Difference test was used 

to identify differences among treatments ( = 0.05). 

RESULTS

Fertilization (averaged over species) affected both 

seedling height (P = 0.0019) and diameter (P = 

0.0002) growth during the first growing season. 

Fertilizer effects on first-year growth did not 

vary among species, as indicated by the lack of a 

significant species × fertilizer rate interaction for 

both height (P = 0.3358) and diameter (P = 0.6188) 

growth. Height growth was increased by 52% as 

compared to the control at the 60 g per seedling 

fertilizer rate (Fig. 2). The 60 g per seedling rate was 

also the most effective rate for diameter growth, 

with a 37% increase as compared to the control 

(Fig. 3). For both height and diameter growth, mean 

values increased along the continuum in fertilizer 

rate from 0 to 60 g, until decreasing at 75 g (Figs. 

2 and 3). Fertilization had no significant effect on 

survival (P = 0.3594) with all treatments resulting in 

≥ 91% survival or on the incidence of terminal bud 

dieback (P = 0.3387).

When examining species independently of fertilizer 

treatment (i.e., averaged over fertilizer rates), there 

were significant differences among species for both 

first-year seedling height (P < 0.0001) and diameter 

(P < 0.0001) growth. For height growth the ranking 

was white ash > yellow-poplar > black walnut 

(Fig. 2), while for diameter growth the ranking 

was yellow-poplar > white ash > black walnut 

(Fig. 3). There were differences in survival (P < 

0.0001) among species, with yellow-poplar having 

significantly lower survival (85%) than either white 

ash (100%) or black walnut (97%). There were also 

differences in incidence of terminal bud dieback 

among species (P < 0.0001), with black walnut 

having a significantly greater percentage of trees 

with dieback (22%) than either white ash (10%) or 

yellow-poplar (13%). 

Figure 2.—First-year height growth for species (P < 

0.0001) and fertilizer rate (P = 0.0019). Species × rate 

was non-significant (P = 0.3358). For either species 

or fertilizer rate treatments, similar letter groupings 

did not differ significantly at  = 0.05. 

Analysis of changes in residual fertilizer weight 

indicated that fertilizer weight decreased to 68% 

of the original weight by the end of the first 

growing season. Approximately 33% of fertilizer 

weight (primarily comprised of residual polymer 

prill materials) typically remains at the end of the 

designated release period with polymer-coated 

CRF (Jacobs and others 2003a). Thus, we expect 

that additional fertilizer nutrients should still 

be available for plant uptake during the second 

growing season. 

DISCUSSION

Application of CRF at plantation establishment 

clearly accelerated first-year seedling growth of 

Species

White ash Black walnut Yellow-poplar

Height Growth (cm)

0

10

20

30

40

50 a

c

b

Fertilizer Rate (g)

0 15 30 45 60 75

Height Growth (cm)

0

10

20

30

40

a

c

bc

ab b b
Nursery Production and Plantation Establishment 69

Black Walnut in a New Century

seedlings at 30 kg per m3 resulted in a doubling 

of stem volume compared to controls after three 

growing seasons (Nursery Technology Cooperative 

2001). 

These results suggest that CRF may offer a 

means to enhance initial plantation growth of 

hardwood seedlings, which may act to improve the 

establishment and early productivity of hardwood 

afforestation plantings. Hardwood plantations have 

historically been relatively difficult to establish, 

with a high degree of variability in success. A 

random survey of 88 recently-established (ages 1 

to 5) plantations throughout Indiana indicated that 

seedling survival averaged approximately 65%, with 

the majority of mortality occurring the first year 

following planting (Jacobs and others in press). 

This survival rate contrasts dramatically with 

operational conifer tree plantations in the Pacific 

Northwest and the southeastern United Steas where 

plantation survival is routinely above 90%. 

The majority of hardwood plantation failures 

in the eastern United States are attributed to 

vegetative competition and damage from animal 

browse incurred during the first 1 or 2 years after 

planting (see citations in Jacobs and others in 

press). Application of CRF at planting may provide 

hardwood seedlings with a means to rapidly 

accelerate over the level of browse and competing 

vegetation, allowing seedlings to reach a free-togrow status in a shorter time frame. It should be 

noted, however, that weed control on this site was 

excellent and is likely a necessary prerequisite 

for attaining positive seedling response from any 

fertilizer application. Although this study was 

not intended to provide a financial assessment 

of incorporating CRF technology into forest tree 

plantation establishment, we estimate the cost of 60 

g (the most effective rate per seedling in this study) 

of this CRF at approximately $0.20 - $0.30 per 

seedling. 

Estimates of fertilizer release in our study indicated 

that approximately 50% of nutrients had actually 

released from CRF prills during the first growing 

season, suggesting that additional nutrients may 

be available for plant uptake during the second 

growing season. Additionally, growth of seedlings is 

largely dependent on the re-translocation of nutrients 

from stored reserves (Salifu and Timmer 2003). The 

increased growth of fertilized seedlings compared 

to seedlings in the control treatment was likely 

associated with greater nutrient uptake, and implies 

that enhanced levels of nutrients were also stored 

by fertilized seedlings which may act to fuel growth 

during the second season. We expect to conduct longterm monitoring of this plantation to assess whether 

the growth responses observed during the first year 

continue to be maintained over time. 

Figure 3.—First-year diameter growth for species 

(P < 0.0001) and fertilizer rate (P = 0.0002). Species 

× rate was non-significant (P = 0.6188). For either 

species or fertilizer rate treatments, similar letter 

groupings did not differ significantly at  = 0.05. 

each of these three hardwood species. Regardless of 

species, fertilization at 60 g per seedling increased 

height growth by 52% and diameter growth by 

37% compared to controls without CRF fertilizer 

applications (Figs. 2 and 3). Although we know of 

no published observations examining use of CRF at 

the time of planting with hardwood species, similar 

results have been reported for conifer species. After 

two growing seasons, total height was increased by 

42% in Douglas-fir (Pesudotsuga menzeiseii (Mirb.) 

Franco) (Carlson and Preisig 1981) and 27% in 

western hemlock (Tsuga heterophylla Raf. Sarg.) 

seedlings with application of 21 g of Osmocote 

CRF at time of planting. Incorporation of Osmocote 

18N-5P-12K into the growing media of container 

Species

White ash Black walnut Yellow-poplar

Diameter Growth (mm)

0

2

4

6

8

10

12

a

c

b

Fertilizer Rate (g)

0 15 30 45 60 75

Diameter Growth (mm)

0

2

4

6

8

10

a

c

bc

ab ab ab

Species

White ash Black walnut Yellow-poplar

Diameter Growth (mm)

0

2

4

6

8

10

12

a

c

b

Fertilizer Rate (g)

0 15 30 45 60 75

Diameter Growth (mm)

0

2

4

6

8

10

a

c

bc

ab ab ab
70 Nursery Production and Plantation Establishment

Proceedings of the 6th Walnut Council Research Symposium

ACKNOWLEDGMENTS

Indiana DNR Vallonia Nursery donated seedlings, 

and O.M. Scotts Co. donated fertilizer for this 

experiment. The following assisted with field and 

lab work: Patricio Alzugaray, Anthony Davis, Bill 

Maschino, Amy Ross-Davis, Barrett Wilson, and 

Jewell Yeager. 

LITERATURE CITED

Beineke, W.F. 1986. Black walnut plantation 

management. FNR-119. West Lafayette, IN: 

Purdue University, Cooperative Extension 

Service. 11 p.

Braun, J.M.; Byrnes, W.R. 1982. Growth of black 

walnut in a fertilized plantation. In: Black 

walnut for the future: 3rd walnut symposium; 

1981 August 10-14; West Lafayette, IN. 

Gen. Tech. Rep. NC-74. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, 

North Central Forest Experiment Station: 74-

104.

Burdett, A.N.; Herring, L.J.; Thompson, C.F. 1984. 

Early growth of planted spruce. Canadian 

Journal of Forest Research. 14: 644-651.

Carlson, W.C. 1981. Effects of controlled-release 

fertilizers on the shoot and root development of 

outplanted western hemlock (Tsuga heterophylla 

Raf. Sarg.) seedlings. Canadian Journal of 

Forest Research. 11: 752-757.

Carlson, W.C.; Preisig, C.L. 1981. Effects of 

controlled-release fertilizers on the shoot and 

root development of Douglas-fir seedlings. 

Canadian Journal of Forest Research. 11: 230-

242.

Donald, D.G.M. 1991. Nursery fertilization of 

conifer planting stock. In: Van Den Driessche, 

R., ed. Mineral nutrition of conifer seedlings. 

Boca Raton, FL: CRC Press: 135-167.

Haase, D.L.; Rose, R., eds. 1997. Forest seedling 

nutrition from the nursery to the field 

(Symposium proceedings). Corvallis, OR: 

Oregon State University, Nursery Technology 

Cooperative. 161 p.

Hangs, R.D.; Knight, J.D.; van Rees, K.C.J. 2003. 

Nitrogen accumulation by conifer seedlings 

and competitor species from 15nitrogen-labeled 

controlled-release fertilizer. Soil Science Society 

of America Journal. 67: 300-308.

Jacobs, D.F.; Rose, R.; Haase, D.L. 2003a. 

Development of Douglas-fir seedling root 

architecture in response to localized nutrient 

supply. Canadian Journal of Forest Research. 

33: 118-125.

Jacobs, D.F.; Rose, R.; Haase, D.L. 2003b. 

Incorporating controlled-release fertilizer 

technology into outplantings. In: Riley, L.E.; 

Dumroese, R.K.; Landis, T.D., eds. National 

proceedings: Forest and Conservation Nursery 

Associations-2002. Proceedings RMS-P-28. 

Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research 

Station: 37-42.

Jacobs, D.F.; Ross-Davis, A.; Davis, A.S. 2004. 

Establishment success of conservation tree 

plantations in relation to silvicultural practices 

in Indiana, USA. New Forests. In press.

Nursery Technology Cooperative. 2001. Field 

performance of Douglas-fir container stock 

grown with fertilizer-amended media. In: 

Nursery Technology Cooperative 2000-2001 

Annual Report. Corvallis, OR: Oregon State 

University, Nursery Technology Cooperative: 

7-11.

Ponder, F., Jr. 1996. Walnut fertilization and 

recommendations for wood and nut production. 

In: Van Sambeek, J.W., ed. Knowledge for 

the future of black walnut: 5th Black walnut 

symposium; 1996 July 28-31; Springfield, MO. 

Gen. Tech. Rep. NC-191. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, 

North Central Forest Experiment Station: 128-

137. 

Rietveld, R. 1989. Transplanting stress in bareroot 

conifer seedlings: Its development and 

progression to establishment. Northern Journal 

of Applied Forestry. 6: 99-107.

Salifu, K.F.; Timmer, V.R. 2003. Nitrogen 

retranslocation response of young Picea 

mariana to nitrogen-15. Soil Science Society of 

America Journal. 67: 309-317.

Williams, R.D. 1974. Planting methods and 

treatments for black walnut seedlings. Res. 

Pap. NC-107. St. Paul, MN: U.S. Department 

of Agriculture, Forest Service, North Central 

Forest Experiment Station. 5 p.
Nursery Production and Plantation Establishment 71

Black Walnut in a New Century

SOILS AND NUTRITION MANAGEMENT FOR BLACK WALNUT

Felix Ponder, Jr.1

ABSTRACT—Although walnut may survive when planted in unsuitable soils, most likely 

growth will be slow and the trees will be of poor quality. Sufficient time should be spent 

locating quality sites for this valuable and high-site demanding species. Undoubtedly, 

only ideal soil conditions will allow for adequate root expansion and for soil levels of 

nutrients and water to be sufficient for consistent growth and nut production. Nitrogen 

(N) is often the limiting nutrient to tree growth. Fertilization of young walnuts for growth 

is usually offset by an increase in vegetation competition from weeds. Benefits from 

fertilization are more likely before crown closure, after thinning, and for active growing 

large-diameter trees a few years before harvesting. Greater payoffs from fertilization can 

likely be expected from nut production, especially for trees of nut varieties with larger 

and more consistent nut crops.

Black walnut (Juglans nigra L.) is perhaps the most 

valuable hardwood tree species in North America. It 

is prized for manufacturing high quality furniture 

and for carvings because of its hard wood, dark 

luminous color, and ease of processing. Its nutmeat 

is used for human consumption and is important 

for wildlife. In addition, the shells are ground for 

use in many products. Black walnut occurs as a 

minor component in mixed deciduous forests in 

eastern United States and in the deciduous forest 

region of southern Ontario (Burns and Honkala 

1990). Quality black walnut logs for veneer and 

lumber are declining faster than they can be 

grown as harvesting continues and fewer quality 

sites are being regenerated to the species. In some 

plantings, the management objective is production 

of wood and nuts or just nuts. Many of the acres 

regenerated to black walnut and other high value 

species lack sufficient cultural treatments required 

for sustained good growth and high quality logs. 

In most cases, except for correcting soil pH, early 

fertilization is not required or recommended. 

Several years of weed control is needed. If nut 

production is the management goal, even on good 

sites, bearing trees will require fertilization.

The poor performance of black walnut on marginal 

sites can often be explained by reduced potential 

for rooting and consequent water stress impacts 

on photosynthesis and leaf abscission (Pallardy 

and Parker 1989). Walnut seedlings had poor root 

growth in soils that had high clay content and 

poor internal drainage (Dey and others 1987). In 

this paper I will briefly summarize the available 

literature on soil and nutrient relations for the 

species. Readers wanting more specific information 

on experimental results with fertilization should see 

Ponder (1997) and Pallardy and Parker (1989). 

SOILS ON THE PLANTING SITE

Most of the minerals used by plants are derived 

from the soil; therefore, because black walnut is a 

high site-demanding tree, considerable attention 

should be placed on soil selection for the species, 

and on management practices that increase 

the opportunity for its roots to be in a healthy 

environment. Particular attention should be given 

to soil drainage (both surface and internal) and soil 

depth. Walnut grows best on land of good quality. 

Throughout its range, walnut generally reaches its 

greatest size and value along streams and on the 

lower portion of north or east facing slopes (Table 

1). It needs a fertile soil. Black walnut needs a 

soil that provides an adequate supply of moisture 

throughout the growing season while at the same 

time provides adequate aeration for roots. Soils 

with medium to fine textures such as loam, sandy 

clay loam, silt loam or clay loam with good internal 

drainage are ideal for the species. The combined 

1Research Soil Scientist (FPJr), USDA Forest Service, North Central Research Station, 208 Foster Hall, Lincoln University, Jefferson 

City, MO 65102. To contact, call 573-681-5575 or 573-875-5341, ext. 234 or e-mail at fponder@fs.fed.us.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
72 Nursery Production and Plantation Establishment

Proceedings of the 6th Walnut Council Research Symposium

effect of soil texture and depth determines, to a 

large degree, available soil moisture, which can 

become critically low during some parts of the 

growing season.

The effective soil rooting depth should be 3 to 5 

feet or more and not restricted by a sand or gravel 

layer, compacted till, massive clay layer or bedrock. 

Soils that are poorly drained, droughty, and sandy 

should be avoided. Suitable soils may be found on 

bottomlands that are not prone to excessive flooding 

(floods lasting more than 3 days) and low uplands 

including terraces. 

With an increase in the development of plantations 

of grafted, high-producing nut cultivars, comes 

the increased need for supplemental fertilization. 

Research is being done to improve the benefits 

of nutrient fertilization by studying nutrient 

application rates for mineral and organic fertilizer 

amendments based on soil test results. Fertilization 

recommendations for black walnut do exist (Parker 

and others 1992). Soil fertility levels should be in 

the range of pH 6.5 to 7.2, 2.0 to 3.5% organic 

matter, 0.25 to 0.3% nitrogen (N), 60 to 80 lbs/ac 

phosphorus (P), 225 to 275 lbs/ac potassium (K), 

2,000 to 3,000 lbs/ac calcium (Ca), and 250 to 300 

lbs/ac magnesium (Mg). However, in most cases, 

the correlation between soil nutrient supply and 

tree growth and nut production is poor.

It is recommended that a soil test for nutrient levels 

be done as part of the site selection process and 

as part of the maintenance program to maintain 

growth and nut production. Care must be used 

in the interpretation of the results and laboratory 

recommendations because most soil tests have been 

calibrated to agronomic crop needs rather than for 

trees or horticultural crops. However, pH is a good 

indication of lime needs and the availability of some 

nutrients. For example, in soils with pH of 7.0 or 

higher, zinc (Zn) is held very tightly by the soil and 

may be unavailable for plant uptake.

Leaf analysis is a much better indicator of N, P, 

K and Zn deficiencies than soil test. Leaf nutrient 

concentrations give a much clearer indication of 

the tree’s actual nutrient status. Black walnut leaf 

samples should be collected from several trees 

between the last 2 weeks of June and first 2 weeks 

of July from current year’s growth. Leaves that 

are dirty or that have been damaged by insects or 

disease should be discarded. Sampling should be 

delayed for several days after heavy rains. Leaflets 

from the mid portion of the leaf omitting the first 

two to three leaflets at the bottom and at the top of 

the leaf should be collected with sufficient leaflets 

(75 to 100) to yield a 5- to 10-gram sample after 

leaflets are processed (dried and ground). 

The nutrient-testing laboratory selected should 

be suitable for your needs. Most leading state 

universities that have agriculture programs 

maintain and operate soil and plant testing 

laboratories. If the laboratory does not make 

fertilizer recommendations for black walnut, you 

may need someone to interpret the results. The 

Soil and Plant Analysis Laboratory in Madison, WI 

(University of Wisconsin, Soil and Plant Testing 

Laboratory, 5711 Mineral Point Road, Madison, 

WI 53705, phone 606/262-4364) can analyze soil 

and plant samples and make recommendations 

Table 1.—Significant soil /site characteristics for black walnut growth.

Soil/site

Characteristics Suitable Questionable Unsuitable 

Slope exposure North or east South or west

Position on slope Middle, or lower with 

good drainage

Upper, or lower with 

poor drainage

Slope 0 to 15 % 15 to 30 % >30 %

Depth to bedrock, gravel or 

clay layer

> 3 feet 2 to 3 feet < 3 feet

Drainage class Well to moderately well 

drained

Somewhat poorly 

drained

Excessively, poorly, or 

very poorly drained

Duration of flooding Standing water up to 4 

days in early spring

Standing water more 

than 4 days

Soil texture Loam, silt loam, silty 

clay loam, silt, clay loam, 

sandy loam, sandy clay, 

fine sandy loam

Silty clay Clay, sand, loamy sand, 

loamy fine sand
Nursery Production and Plantation Establishment 73

Black Walnut in a New Century

for walnut based on the soil’s pH, organic matter, 

and nutrient content (Parker and others 1992). 

Visit with personnel at your local Natural Resource 

Conservation office, County Extension office, or 

District Forester office for help in locating plant 

tissue testing laboratories and for other helpful 

suggestions.

Once a leaf analysis program has been established, 

leaves should be collected from the same trees and 

branches during the period as in previous years. 

Although, the grower will decide how often trees will 

be tested, it is best to test trees annually, especially 

if trees are being fertilized for nut production. The 

benefits of fertilization are limited by the tree’s 

ability to convert nutrients into wood and nuts. 

The grower must remember that factors other than 

nutrition, such as inadequate moisture during the 

growing season and other weather related problems 

as well as inherent soil/site problems could limit 

good annual growth and nut production.

While the leaf analysis is a representation of the 

tree’s actual nutrient condition, the combination of 

soil test and leaf analysis gives better information 

on what may be limiting the tree’s productivity. 

The two tests provide additional information for 

making management decisions. Table 2 contains 

leaf nutrient values from walnut trees growing on 

a good soil for wood and nut production. Also, the 

change in nutrient levels during the growing season 

demonstrates the importance of collecting samples 

the same time as in previous years for comparing 

nutrient changes.

FERTILIZATION

Nitrogen can be supplied either in an organic form 

from legumes growing on the site (plantation) or by 

applying inorganic nitrogenous fertilizers. Legumes 

can supply in excess of 150 pounds of N per acre 

in one growing season. Nitrogen fixed by legumes 

is released to the soil and is free to be taken up by 

the trees or other vegetation. Because N is released 

from the legume over the growing season, it may be 

more beneficial than annual fertilizer applications 

for tree growth (Ponder 1994). However, an 

application of commercial N fertilizer can be done 

without the complications that may be associated 

with establishing and growing legumes. Diameter 

growth of 11-year-old walnut trees treated with 

slow-release fertilizer increased by 33 to 87% the 

first year and had more diameter growth 5 years 

later than tree fertilized with ammonium nitrate.

Phosphorus and K can be applied in combination 

with N or applied individually when inadequate 

levels are indicated by leaf analysis. Diammonium 

phosphate contains both N and P in concentrations 

of 18 and 46%, respectively. Nutrient sources such 

as triple superphosphate and muriate of potash can 

be used to supply P and K when applied separate or 

in combination with each other. 

Table 2.—Mean concentrations of N, P, K, Ca, and Mg in leaves from 8-year-old black walnut trees at 

different times of the growing season for 2 years and suggested sufficiency range.1

Date Nutrient Element

Sampled N P K Ca Mg

Percent

First year

June 11 3.11 0.32 1.07 1.96 0.51

July 6 2.91 0.25 0.97 1.97 0.42

August 11 2.84 0.24 1.02 2.36 0.44

Second year

June 11 3.52 0.25 0.98 1.28 0.37

July 6 3.02 0.20 0.80 1.61 0.36

August 11 2.73 0.21 0.83 2.15 0.41

Suggested 

sufficiency range2 2.47 – 2.98 0.16 – 0.24 1.32 – 1.47 1.90 – 2.01 0.51 –0.64

1

Ponder and others 1979.

2

Mills and Jones 1996, sampled in summer.
74 Nursery Production and Plantation Establishment

Proceedings of the 6th Walnut Council Research Symposium

Fertilization of newly established walnut plantings 

has not proven very beneficial, especially if ideal 

soils were on the site. In most cases fertilizers 

depress the growth of seedlings and young trees 

by stimulating the growth of competing weeds. 

Controlling competing weed can increase walnut 

growth. In natural stands, Clark (1967) and Phares 

(1973) determined that only minor improvements 

in diameter growth could be obtained by reducing 

competition (release) and fertilization of walnut 

trees larger than poles. Ponder (1998) and Ponder 

and Schlesinger (1986) reported that while 

there was some benefit from fertilization, release 

increased diameter growth more. The diameter 

growth of 18-year-old plantation walnut trees was 

better when P and K were combined with N than 

when N was applied alone or P and K together.

Nutrient sufficiency ranges for various nutrients 

have been developed for black walnut to help 

determine nutrient needs based on leaf analyses 

(Mills and Jones 1996). As a guide, nutrients are 

added as leaf tissue levels approach the lower end 

of the range or below and nutrients are removed 

from the fertilizer program as the leaf tissue level of 

a given nutrient approaches the upper end of the 

range or higher. The nutrient sufficiency range for 

N, P, K, Ca, and Mg are presented in Table 2. The 

sufficiency range for some other nutrients in parts 

per million are: manganese (Mn), 207 to 274; iron 

(Fe), 69 to 129; Zn, 33 to 55; boron (B), 66 to 81; 

copper (Cu), 10 to 12; molybdenum (Mb), 0.10 to 

0.30; and sulfur (S), 1500 to 1600. In general, to 

satisfy N needs for nut production, an application 

of 90 to 100 lbs of actual N per acre should be 

applied in March to early April and again in early 

August depending on leaf analysis. Both P and K 

should be applied at a rate of 60 to 100 pounds of 

actual ingredient in the spring. In cases of severe 

deficiency larger quantities can be applied.

Jones and others (1993) provided evidence that 

nut production was increased with an application 

of 6.4 oz of N, P, and K (13-13-13)/cm of tree 

diameter late in the summer compared with spring 

a spring application (Table 3). In another study, nut 

production was significantly increased following 

annual applications of N, P, and K fertilizers 

combined (Ponder 1998). 

When legumes are used in the management of 

trees, soil test should be used to indicate the need 

for P and K rather than leaf analysis. Excessive 

P and K applied for legume nutrition will not 

adversely affect trees. By effectively taking care of P 

and K requirements for legumes, the trees need for 

these nutrients are also met.

While many black walnut growers are interested 

in producing wood and nuts, some are mostly 

interested in producing nuts. Nut producing 

cultivars are being grafted onto rootstocks 

of plantation trees that lack many of the nut 

producing characteristics of the cultivars (Hanson 

1999). Several walnut plantations in Missouri 

are being managed primarily for nut production. 

Grafted trees of improved varieties are producing 

good nut crops annually. One reason for these 

growers success is likely due to the attention they 

give their trees and not to the soils the trees are 

planted on which are typically not suited or would 

not be recommended for growing black walnut for 

wood production.

For example, the Knaust planting, which is 

approximately one-half acre in size with 35 trees, 

is being managed for nut production (Table 4). The 

deep silt loam soil has a fragipan at 18 to 27 inches 

of the soil surface, which severely limits available 

water during some parts of the growing season. 

The grower spreads 21 to 28 pickup loads of cow 

manure around trees annually in early spring. In 

mid April, the grower applies 450 pounds of 13-

13-13 fertilizer. The orchard floor is covered with 

“cheat”, a grass that dies by late June to form a 

mulch mat that conserves moisture and stops 

the invasion of other plants. Although restricted 

by water pressure and only with a garden hose 

available for use, some trees are watered. For this 

site, the difference in nut production in dry years 

may mean few or no nuts if trees were not watered. 

Nutritionally, under good management, the soil 

quality of the orchard exceeds that of a soil on a 

good site (Table 4). The grower summed up nut 

production, as “I am kept busy from fall into the 

winter cracking nuts, which I sell for by mail-order 

at $8.50 a pound.”

The Gardner planting is another profitable nut 

operation (Table 4). Trees were, also, planted on 

soils typically not recommended for walnut wood 

production because the silt loam soil is poorly 

drained. The trees were later severed and stumps 

Table 3.—Mean 5-year nut production of black walnut 

trees fertilized at different times of the year1.

Treatment

Time

Fertilizer

Applied

Nut

Production

#/year/tree

Spring Yes 127.5

Spring No 125.7

Late summer Yes 187.0

Late summer No 122.1

1

Jones and others 1993.
Nursery Production and Plantation Establishment 75

Black Walnut in a New Century

were grafted with several nut-producing cultivars. 

However, the grower maintains cheat grass for 

weed suppression and soil fertility, including the 

addition of minor nutrients, for good nut production 

(Table 4). The grower said that in some years he 

has all the nuts he can process without completely 

harvesting the nut crop. 

While these two success stories are practical and 

not scientifically replicated, they do indicate that 

taking care of the nutritional needs of black walnut 

can increase nut production. These examples also 

suggest that providing good nutrition on marginal 

soils can make a difference for nut production. 

In summary, planting black walnut on a good site 

is the first step to having a successful plantation. 

Soils that are 3 to 5 feet deep, well-drained, and 

have good moisture availability are important 

site criteria for the species. Sites with good soil 

moisture are candidates for fertilization. However, 

some sites with limiting site factors such as less 

than ideal soil depth, can respond to fertilization. 

The largest fertilization response will occur where 

nothing other than nutrient deficiencies limit the 

site’s productivity. Annual testing of soil and leaves 

will serve as the best guide for determining the 

frequency of fertilizer application.

Table 4.—Soil chemistry for the upper six inches of soil at Marcules, Knaust, and Gardner black walnut 

sites1

.

Depth (in) Depth (in) Depth (in)

Sites 0-2 2-4 4-6 0-2 2-4 4-6 0-2 2-4 4-6

pH EC (mmhos cm-1)

2 % Carbon (OM)3

Marcules 7.2 6.7 6.6 127 23 14 2.1 1.0 0.9

Knaust 7.5 7.5 7.4 259 173 109 5.2 2.6 1.1

Gardner 7.5 7.5 7.4 152 144 97 3.5 3.0 2.0

P (lbs/A) K (lbs/A) NO3-N (lbs/A)

Marcules 195 118 119 648 339 242 12.7 6.3 4.2

Knaust 427 259 114 1339 907 755 25.8 14.7 9.7

Gardner 186 154 148 644 523 426 9.3 8.8 4.7

Ca (lbs/A) Mg (lbs/A) Na (lbs/A)

Marcules 3627 2565 2571 271 108 89 17.8 13.8 17.3

Knaust 7691 5105 2949 639 373 220 33.3 23.3 23.6

Gardner 4889 4392 3893 536 432 386 18.3 17.5 16.0

S (lbs/A) Fe (lbs/A) Mn (lbs/A)4

Marcules 19.5 12.6 10.7 124 129 126 183 165 123

Knaust 67.7 23.7 14.0 99 88 69 163 170 1855

Gardner 22.0 20.8 16.3 73 69 59 233 216 198

Zn (lbs/A)4 Cu (lbs/A) B (lbs/A)

Marcules 62.3 46.0 39.7 7.2 6.2 7.2 1.4 0.6 0.5

Knaust 42.3 25.7 13.3 4.8 4.6 5.7 2.7 1.4 0.6

Gardner 93.8 90.5 118 6.4 5.7 5.5 2.0 1.6 1.1

1

Mean of four random samples. The Marcules plantation is on a bottomland site with deep, nearly level, well drained 

Huntington silt loam soil. The Knaust plantation is on a bottomland site with deep, moderately well drained silt loam 

soil with a firm and brittle 15 inch thick fragipan less than 28 inches from the soil’s surface. The Gardner plantation is on 

a bottomland site with deep, somewhat poorly drained silt loam soil.

2

Electrical conductivity used as an index of the total concentration of dissolved salts.

3

OM=organic matter, P=phosphorus, K=potassium, NO3-N=nitrate nitrogen, Ca=calcium, Mg=magnesium, Na=sodium, 

S=sulfur, Fe=iron, Mn=manganese, Zn=zinc, Cu=copper, B=boron.

4 High Mn and Zn levels are due to their application to the soil.

5 Samples are in the correct order.
76 Nursery Production and Plantation Establishment

Proceedings of the 6th Walnut Council Research Symposium

LITERATURE CITED

Burns, R.M.; Honkala, B.H., tech. coords. 

1990. Silvics of North America. Volume 2, 

Hardwoods. Agric. Handb. 654. Washington, 

DC: U.S. Department of Agriculture, Forest 

Service. 877 p.

Clark, F.B. 1967. Pole-size black walnut responds 

quickly to crown release. Journal of Forestry. 

65: 406-408.

Dey, D.; Conway, M.R.; Garrett, H.E.; Hinckley, T.M.; 

Cox, G.S. 1987. Plant-water relationships and 

growth of black walnut in a walnut-forage multicropping regime. Forest Science. 33: 70-80.

Hanson, B.L. 1999. Commercial black walnut 

orchard in Iowa (Ben’s black walnuts). 

Annual Report of the Northern Nut Growers 

Association. 90: 119-126.

Jones, J.E.; Garrett, H.E.; Haines, J.; Lowenstein, 

E.F. 1993. Genetic selection and fertilization 

provide increased nut production under walnutagroforestry management. Annual Report of the 

Northern Nut Growers Association. 86: 28-34.

Mills, H.A.; Jones, J.B. 1996. Plant analysis 

handbook II. Athens, GA: MicroMacro 

Publishing. 442 p.

Pallardy, S.G.; Parker, W.C. 1989. Water and 

nutrient relations of black walnut. In: Phelps, 

J.E., ed. The continuing quest for quality: 4th 

Black walnut symposium; 1989 July 30-August 

2; Carbondale, IL. Indianapolis, IN: Walnut 

Council Inc.: 46-66.

Parker, D.; Bockheim, J.; Meyer, D. 1992. Soil 

and site selection. Fact Sheet 68, Walnut Tips. 

Wisconsin Walnut Council.

Phares, R.E. 1973. Managing immature trees for 

more high-quality logs and released products. 

In: Black walnut as a crop: 2nd black walnut 

symposium; 1973 August 14-15; Carbondale, 

IL. Gen. Tech. Rep. NC-4. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, 

North Central Forest Experiment Station: 49-

54.

Ponder, F., Jr. 1997. Walnut fertilization and 

recommendations for wood and nut production. 

In: Van Sambeek, J.W., ed. Knowledge for 

the future of black walnut: 5th Black walnut 

symposium; 1996 July 28-31; Springfield, MO. 

Gen. Tech. Rep. NC-191. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, 

North Central Forest Experiment Station: 

128-137. 

Ponder, F., Jr. 1998. Fertilizer combinations benefit 

diameter growth of plantation black walnut. 

Journal of Plant Nutrition. 21: 1329-1337.

Ponder, F., Jr. 1994. Comparison of slow- and 

quick-release fertilizers on black walnut growth. 

Annual Report of the Northern Nut Growers 

Association. 85: 40-43.

Ponder, F., Jr.; Schlesinger, R.C. 1986. Release and 

fertilization of natural black walnut. Northern 

Journal of Applied Forestry. 3: 153-155.
Natural Stand Management 77

Black Walnut in a New Century

SITE RELATIONSHIPS AND BLACK WALNUT HEIGHT GROWTH IN

NATURAL STANDS IN EASTERN KANSAS

Wayne A. Geyer and Felix Ponder, Jr.1

ABSTRACT—Prediction of forestland productivity is needed for proper species selection in 

tree planting. By relating site quality to site and soil characteristics, potential productivity 

can be estimated for non-forested areas. Our study measured the growth potential of 

black walnut in natural stands in southeastern Kansas. We looked at over 200 stands on 

unglaciated soils. Numerous environmental factors were evaluated within the following 

broad categories: site, soil chemical, and soil physical. These environmental factors were 

related to tree height at the age of 50 years. Simple correlation and multiple regression 

analyses were run against site index using over 60 variables of soil and topographic 

characteristics. Depth to a restrictive layer explained 74% of the variation in height. 

The natural range of black walnut (Juglans nigra

L.) extends westward into central Kansas (Williams 

1990). Walnut is found as a major component of the 

prairie-forest fringe environment in eastern Kansas. 

Although nearly pure stands of black walnut 

are uncommon, it is found on a wide variety of 

ecological sites. Most often, black walnut occurs as 

small patches along the river valleys, side drainages 

and adjacent slopes, and is often bordered by 

abandoned cropland and abused pasture-land 

(Grey and Naughton 1971). 

Generally black walnut is found on deep, moist, 

well-drained areas having good soil structure 

(Auten 1945, Carmean 1970) and near neutral in 

reaction (Spurway 1941, Wilde 1934). Soil reaction 

and nutrient levels were related to walnut growth 

in 15-year-old walnut plantations in southeastern 

Iowa (Thompson and McComb 1962). Prediction 

equations, using important soil and topographic 

variables, have been generated in an earlier study 

by Geyer and others (1980) for northeastern 

Kansas. 

Our study attempts to relate tree growth differences 

as indicated by site index at 50 years to many 

physical and chemical site factors. Generally, 

factors relating to moisture availability are of 

utmost importance to tree growth.

METHODS

Description of the Study Area

The study area is located within the Central 

Lowlands physiographic province (Keys and 

others 1995). Elevations range from 800 to 1200 

feet. The surface soils are mostly of silt loam 

to clay loam texture, often with limestone and 

shale outcroppings on the slopes. Most are of 

Mollisol origin (soils that developed under prairie 

vegetation). The climate is typical continental with 

most of the precipitation in the warm season. The 

study range covered over 135 miles north and south 

and 115 miles east to west with moisture patterns 

of 28 inches annual precipitation in the northeast 

to 45 inches in the southeast portions of the study 

area (Fig. 1).

Field Plots

Our investigation evaluated over 210 plots in the 

unglaciated area of southeastern Kansas on all 

topographic positions (upland, bottomland, lower 

slopes, and intermittent streams) and generally well 

distributed on all four-direction quadrants. About 

two-thirds of the study plots were given detailed 

1Professor of Forestry (WAG), Division of Forestry, Throckmorton Hall, Kansas State University, Manhattan KS and Research Soil 

Scientist (FPJr), North Central Research Station, USDA Forest Service, 208 Foster Hall, Lincoln University, Jefferson City, MO 66506. 

WAG is the corresponding author: to contact, call 785-532-1409 or e-mail at wgeyer@oznet.ksu.edu.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
78 Natural Stand Management

Proceedings of the 6th Walnut Council Research Symposium

analysis. The remaining plots were discarded 

because of discrepancies in tree numbers, tree 

quality, and missing data. All plots were located 

in natural hardwood timber stands containing 

significant amounts of black walnut.

Trees selected for analysis were in dominant and/or 

codominant crown classes and had no visible signs 

of stagnation or suppression. We measured two to 

four walnut trees in each plot. Total height of each 

tree was measured with a Blume-Leiss altimeter. 

Tree age was determined from core samples of wood 

taken at about 4 inches above the ground. Using 

Kellogg’s site index curves for walnut plantations 

(Kellogg 1939) and plot averages of height and age, 

we evaluated site index. Site index is the average 

height of dominant and codominant trees in a stand 

at age 50 years. Taller trees at a given age indicate 

a site of higher productivity.

Site Variables

We classified each site by topographic 

characteristics and each soil profile by soil 

parent material, either alluvial (bottomland) or 

residual (upland), and assigned values of +1 

or –1, respectively. Landform was classified as 

bottomland, intermittent stream, lower one-third 

slope or upland, and assigned values of 1, 2, 3, 

or 4. Azimuth and slope were determined with a 

compass pointed down slope through plot center 

and an altimeter, respectively. Assuming the 

northeast aspect as the most favorable for tree 

growth and southwest as the least favorable, we 

tested two transformations of aspect. First, the 

cosine (azimuth – 45 deg.) +1 as described by 

Beers and others (1967) and second, the corrected 

azimuth from the southwest as described by Munn 

(1974) along with true azimuth to see if they would 

correlate with walnut growth. Slope steepness was 

expressed as slope percentage, less than 10%, less 

than 5% or equal to or exceeding 10%. 

Other variables tested were depth to a restrictive 

layer (defined as textural B horizon—a massive rock 

layer, gravel layers of at least 75% rock, a water 

table, mottling, or 60 inches, whichever occurred at 

the least depth), and effective depth to a restriction 

(defined as rock percentage in inches removed from 

depth to restriction). Also total soil depth, effective 

soil depth (defined as rock percentage in inches 

removed from total soil depth), (Steinbrenner 1965) 

and water holding capacity of total soil depth or 

water-holding capacity to the restrictive layer. 

Both are sums of available water-holding capacity 

calculated by textural horizons.

Soil Analysis

Soil pits were located in between the trees and 

were dug to a maximum of five feet and the 

morphological characteristics of each soil profile 

described. One soil sample from each horizon 

was analyzed in the laboratory for both chemical 

and physical characteristics. Soil textures were 

determined by the Bouyoucos method (1951), 

except sodium hexametaphosphate was used 

instead of Calgon as a dispersing agent. Chemical 

tests were run for both the surface and subsurface 

horizons. Lime (ECC/ac) pounds needed for crop 

production, pH, Bray 1 phosphorus, potassium, 

calcium, magnesium, sodium, and organic matter 

were done by the Kansas State University Soil 

Testing Laboratory using standard agricultural 

testing procedures. The relationship between site 

index and soil factors was evaluated using simple 

correlations. Correlation coefficients for the 60 

independent variables were calculated with the 

independent variable, site index, to find the best for 

generating a multiple regression equation using the 

stepwise backward elimination procedure (Barr and 

others 1976). 

RESULTS AND DISCUSSION

Analyses of the data showed the largest simple 

correlation of site index was with soil depth to a 

restrictive layer (r = +0.85). Other highly correlated 

factors were rooting depths, expressions of water 

holding capacity, and soil parent material. Three 

of the four factors (soil depth, rooting depths, and 

expressions of water holding capacity) are very 

closely related. Deep soils can hold more water 

and have more space for root development. Soil 

Figure 1.—Study area for location of black walnut 

field plots.
Natural Stand Management 79

Black Walnut in a New Century

moisture is likely the underlying factor associated 

with the high correlation of soil depth and site 

index. Even though black walnut is deep rooted, 

almost 80% of its root system occupies the upper 

24 inches of the soil (Pham and others 1978). 

Water needs for black walnut are greatest during 

the growing season at a time when soil water 

content within the rooting zone decreases due to 

plant uptake and evapotranspiration. Carpenter 

and Hanover (1974) reported that for black walnut

seedling height growth slows after mid June and 

ceases by late July, suggesting that seasonal height 

growth is completed early for the species. These 

authors also suggested that black walnut is typical 

of those deciduous species with preformed seasonal 

shoot growth. 

Black walnut is believed to function as drought 

avoidance species as opposed to drought tolerant 

species (Lucier and Hinckley 1982). As a drought 

avoidance species, black walnut is able to maintain 

plant water content near optimum levels during 

periods of low soil moisture availability and high 

atmospheric evaporative demands by initiating 

stomatal closure restricting photosynthesis 

and depending on the duration of unfavorable 

conditions, shedding its leaves. An investigation 

of black walnut site quality in relation to soil 

characteristics in northeastern Kansas indicated 

that site index increased as effective soil depth 

increased (Geyer and others 1980).

Surprisingly, neither aspect (r = -0.07, p = 0.484), 

nor the amount of sand in either the surface or 

subsurface horizons was correlated with site index 

(surface r = 0.059, p = 0.563; subsurface r = 0.05, p 

= 0.618) or silt + clay (surface r = -0.07, 

p = 0.487, subsurface r = -0.010, p = 0.329). 

Neither was organic matter (surface r = 0.47, p = 

0.001, subsurface r = 0.36, p = 0.0004). Aspect 

and silt plus clay were important to black walnut 

site quality in northeastern Kansas (Geyer and 

others 1980). Soil texture was a major determining 

factor in the growth of planted walnuts in Kentucky 

(Kalisz and others 1989). The growth rates of young 

black walnut plantations were significantly greater 

on suitable compared to questionable soils. In the 

Kentucky study, approximately 75% of the soils 

judged to be questionable for walnut management 

were limited by the presence of clay subsoil, 10% by 

shallowness to bedrock, 10% by imperfect drainage, 

and 5% by the presence of a fragipan.

Soil organic matter had a negative relationship 

when greater than 4.3%, but thickness of the A 

horizon had an “r” value of 0.66 and p = 0.0001 

(Table 1). However, it is important to take soil 

texture into account in this layer because 

texture affects soil water storage and availability. 

Another study (Geyer and others 1980) found 

that coefficients for soil texture in the A horizon 

decreased with an increase in clay and increased 

when the silt content of the clay increased. 

Soil nutrient concentrations were not positively 

correlated with site index for any of the tested 

nutrients (data not presented). However, optimum 

walnut growth has been suggested to occur when 

soil pH values range from 6.0 to 8.0 (Spurway 

1941). In the present study, calcium and potassium 

seem to be the overriding factors. We ran a range 

of surface pH values and found pH was negatively 

related to site index when greater than 7.4. 

Table 1.—Simple correlations with site index (20 thru 80 years age).

Variable Mean Range Simple Correlation (r)

Depth to restrictive layer (DRL) 31.9” 2.0-60.0 + 0.85

Estimated soil depth to DRL 31.4 3.0-60.0 + 0.82

Water holding capacity to DRL 6.2 0.4-14.9 + 0.82

Soil origin (upland/bottomland) -- +1, -1 + 0.67

Landform (bottomland/intermittent/

 lower 1/3rd, upland) -- 1,2,3,4 - 0.67

Thickness “A” horizon 20.0 5.3-60.0 + 0.66

Total soil depth 44.1 6.0-60.0 + 0.66

Estimated soil depth 41.0 5.3-60.0 + 0.64

Water holding capacity all horizons 8.1 1.0-14.9 + 0.63
80 Natural Stand Management

Proceedings of the 6th Walnut Council Research Symposium

CONCLUSION

Depth to restrictive layer was a good predictor of 

height growth for black walnut stands in eastern 

Kansas. The positive coefficient in the equation for 

this variable indicates that site index increased as 

the effective soil depth increased. Many factors used 

in this study were related to each other and we 

selected those most apparent and easy to recognize 

for developing our multiple regression equations. 

Our best equation, site index = 43.84 + 0.443 depth 

to restrictive layer + 1.98 soil type, explained 74% of 

the variation in height. Adding the other soil factors 

improved predictability by only 5%. Avoiding sites 

with shallow soils and other properties that reduce 

water availability and concentrating management 

on good sites will save resources and increase the 

growth and value of the trees. Although, data for 

this study was collected in southeastern Kansas, 

these results should be useful and applicable for 

areas in surrounding states. 

LITERATURE CITED

Auten, J.T. 1945. Some soil factors associated with 

site quality for planted black locust and black 

walnut. Journal of Forestry. 43: 592-598. 

Barr, A.J.; Goodnight, J.H.; Sall, J.P.; Helwig, J.T. 

1976. A user’s guide to SAS. Raleigh, NC: SAS 

Institute, Inc. 329 p.

Beers, T.W.; Dress, P.E.; Wensel, L.C. 1967. Aspect 

transformation in site productivity research. 

Journal of Forestry. 65: 22-24.

Bouyoucos, G.J. 1951. A recalibration of the 

hydrometer method for making mechanical 

analysis of soils. Agronomy Journal. 43: 432-437.

Carmean, W.H. 1970. Tree height–growth patterns 

in relation to soil and site. In: Tree. Corvallis, 

OR: Oregon State University Press: 499-512.

Carpenter, S.B.; Hanover, J.W. 1974. Comparative 

growth and photosynthesis of black walnut and 

honey locust seedlings. Forest Science. 20: 317-324. 

Geyer, W.A.; Marquard, R.D.; Barber, J.F. 

1980. Black walnut site quality in relation 

to soil and topographic characteristics in 

northeastern Kansas. Journal of Soil and Water 

Conservation. 35: 135-137.

Grey, G.W.; Naughton, G. 1971. Ecological 

observations on the abundance of black walnut 

in Kansas. Journal of Forestry. 69: 741-743.

Kalisz, P.J.; Stringer, J.W.; Hill, D.B. 1989. Growth 

of young black walnut plantations in Kentucky. 

Northern Journal of Applied Forestry. 6: 17-20.

Kellogg, L.F. 1939. Site index curves for plantation 

black walnut in the Central States Region. 

Note 35. Columbus, OH: U.S. Department of 

Agriculture, Forest Service, Central States 

Forest Experiment Station. 3 p.

Keys, J., Jr.; Carpenter, C.; Hooks, S.; Koenig, F.; 

McNab, W.H.; Russell, O.; Smith, M.L. 1995. 

Ecological units of the eastern United States—

first approximation (map and booklet of map 

unit tables). Atlanta, GA: U.S. Department of 

Agriculture, Forest Service. 

Lucier, A.A.; Hinckley, T.M. 1982. Phenology, 

growth, and water relations of irrigated and 

non-irrigated black walnut. Forest Ecology and 

Management. 4: 127-142.

Munn, L.C. 1974. Predicting height growth of 

yellow-poplar from soils and topography on a 

Muskinggum-Berks-Neotoma soils complex in 

southeastern Ohio. Columbus, OH: Ohio State 

University. 61 p. M.S. thesis.

Pham, C.H.; Yen, C.P.; Cox, G.S.; Garrett, H.E. 

1977. Slope position, soil water storage 

capacity, and black walnut root development. 

In: Balmer, W.E., ed. Soil moisture-site 

productivity symposium; Myrtle Beach, SC. 

Asheville, NC: U.S. Department of Agriculture, 

Forest Service, Southern Forest Experiment 

Station: 326-335.

Spurway, G.W. 1941. Soil reaction (pH) preferences 

for plants. Bull. 306. East Lansing, MI: 

Michigan Agricultural Experiment Station.

Steinbrenner, E.C. 1965. The influence of individual 

soil and physiographic factors on the site index 

of Douglas-fir in western Washington. In: 

Youngberg, C.T., ed. Forest soils relationships 

in North America. Corvallis, OR: Oregon State 

University Press: 261-270.

Thompson, G.W.; McComb, A.L. 1962. Growth of 

plantation black walnut in relation to pH and 

certain chemical factors of the soil. Forest 

Science. 8: 322-333. 

Wilde, S.A. 1934. Soil relation to forestry, and 

its determination by simple tests. Journal of 

Forestry. 32: 415.

Williams, R.D. 1990. Juglans nigra L. black walnut. 

In: Burns, R.M.; Honkala, B.H., tech. coords. 

Silvics of North America: 2. Hardwoods. Agric. 

Handb. 654. Washington, DC: U.S. Department 

of Agriculture, Forest Service: 391-399.
Natural Stand Management 81

Black Walnut in a New Century

MANAGING BLACK WALNUT IN NATURAL STANDS: 

THE HUMAN DIMENSION

H.E. “Hank” Stelzer1

ABSTRACT— In managing black walnut, or any forest tree species, the human 

dimension is often overlooked. As a result, both the number of landowners managing 

their land and the number of forested acres under management has not significantly 

increased over the past 30 years. Elements of the human landscape are explored and a 

roadmap for engaging landowners is proposed.

When asked to write on the subject of managing 

black walnut (Juglans nigra L.) in natural stands, 

my first response was, “Why?” What had changed 

since the last overview was presented at the Fifth 

Black Walnut Symposium (Slusher 1997)? Other 

than a few new herbicides to control competing 

vegetation, the answer was “Not much”. During 

my search for new information, I realized just how 

much information was available to the landowner, 

especially with respect to managing hardwood 

stands. Much of this information is contained 

within the published proceedings from past Central 

Hardwood Forest Conferences. All papers from the 

first twelve conferences are now available on-line 

through the table of contents viewed at http://

www.ncrs.fs.fed.us/pubs/viewpub.asp?key=1065

(Fralish 2002). 

But, how extensively has this wealth of knowledge 

been applied? Forest statistics yield a sobering 

answer. Within the naturally occurring range of 

black walnut non-industrial private forest (NIPF) 

landowners own 86% of the timberland area with 

black walnut present; 13.2 million acres (Schmidt 

and Kingsley 1997). But, only an estimated 5% of 

NIPF landowners have a written management plan 

(Reed 1998). While many forestland owners indicate 

that they have a plan, it often is not documented 

in a way that would allow its easy sharing with 

others. What is most distressing is that the number 

of managed acres has not changed in the past 30 

years!

Why hasn’t the acreage of managed forests 

increased? Among the most striking feature of 

NIPF land is its concentration in small tract sizes. 

More than 90% of these parcels are less than 100 

acres and of those more than 70% less than 10 

acres (Reed 1998). Current information additionally 

suggests that the number and proportion of 

NIPF landowners who own small parcels will only 

increase in the coming years.

While some appreciation for the economic value of 

fine hardwoods in natural stands has been passed 

from one generation to the next, it is painfully 

obvious that there is a growing lack of appreciation 

for this economic value among the new landed 

gentry. One just has to look around and observe 

the indiscriminant clearing of small-diameter 

black walnut and other fine hardwood species as 

drainages and fencerows are “cleaned out” or in 

other cases, no management activity occurs as the 

land sits in a descendant’s trust. 

THE HUMAN LANDSCAPE

So, today’s challenge is actually the same challenge 

from days past; but, with a twist. The time-honored 

challenge is how to interest private landowners with 

walnut in their woodlots to manage this resource. 

The twist is that the target audience is growing 

larger each day as land ownership continues to 

fragment, but the number of professional foresters 

to generate interest in forest management has 

remained relatively flat over the past 20 years.

To gain the most effect from limited resources 

invested in NIPF assistance, identification of 

submarkets for focus may help accomplish more 

1

Extension Forester (HES), Department of Forestry, University of Missouri, 203 ABNR, Columbia, MO 65211. HES is corresponding 

author; to contact, call (573) 882-4444 or e-mail at stelzerh@missouri.edu.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
82 Natural Stand Management

Proceedings of the 6th Walnut Council Research Symposium

immediate results. Black walnut growing in 

natural stands, for example, is an appropriate 

model submarket. Stands that are nearing harvest 

age could focus landowner attention on the 

economic value of fine hardwoods for lumber and 

veneer markets. Immature stands could provide 

opportunity for showcasing even larger economic 

return through simple management activities such 

as thinning and pruning. 

By determining who owns or manages land that 

could contain natural stands of black walnut, 

foresters could concentrate their outreach efforts on 

selected audiences. Three distinct groups come to 

mind: long-time woodland owners, new woodland 

owners and trust officers. Long-time landowners 

are older, have more practical experience and rely 

upon personal relationships in learning new skills. 

They usually reside on property that has been in 

the family for generations. Of these three groups, 

the long-time landowner stands the best chance 

of recognizing the value of timber, since in all 

likelihood they have sold timber from their land at 

some point in time. These individuals are also more 

likely to know, or at least recognize, named and 

branded organizations like the Walnut Council or 

the American Tree Farm System. 

New landowners tend to be younger, better 

educated, and use technology to access and 

manage information. They also tend to be absentee 

landowners. Trust officers comprise a new segment 

of forest “ownership”. While they do not own the 

land, they serve the interests of their clients. With 

the need to serve many clients simultaneously, 

these individuals tend to have many of same 

attributes of new landowners. Individuals belonging 

to both of these groups have a much more casual 

relationship with trees and as such do not fully 

appreciate the many benefits of a managed forest, 

let alone see the forest’s economic potential. 

Mention of the name “Walnut Council” would in all 

likelihood generate a blank stare. 

Even within these narrow subgroups, one size does 

not fit all when it comes to learning. While there 

are many learning styles, behavioral scientists 

have divided these styles into two types: analytical 

and rational. Individuals possessing an analytical 

learning style have long attention spans and can 

resist distractions. They tend to be highly reflective 

and can concentrate on the activity at hand. They 

prefer complexity. These individuals also prefer 

a more formal learning environment and see the 

instructor as an information giver. They tend to 

view the learning experience as a nonsocial event.

On the other hand, individuals having a rational 

learning style have relatively short attention 

spans and are easily distracted. Simple tasks and 

concepts appeal to them. They value learning 

centered in the self that has personal relevance. 

People in this group prefer the informal learning 

setting and see the instructor first as a person.

Up to now, the focus has been on target audiences. 

What about instructors and their message? 

While long a foundation of forestry outreach and 

education programs, foresters need to shift away 

from one-dimensional prescriptions of basically 

timber management information and advice. Due in 

part to the huge and increasing diversity of owners, 

broader management approaches are needed. 

Natural resource disciplines can work together. 

Wildlife biologists and foresters can deliver an 

integrated, value-added package to landowners so 

that they see multiple benefits, both in the shortterm (improved wildlife habitat) and long-term 

(higher quality timber). These benefits in turn make 

their land more valuable in real dollars.

A ROADMAP FOR ENGAGING LANDOWNERS

What do submarkets, learning styles and multidimensional delivery programs have to do with 

managing black walnut in natural stands? Plenty! 

Foresters and Walnut Council members alike must 

use their limited resources to speak to the right 

audiences. If they do not, then as older landowners 

die and land fragmentation increases, the number 

of acres of natural black walnut and other fine 

hardwood stands under management will not 

increase, but decrease.

How can the number of natural black walnut 

stands that are actively managed be increased? 

There are undoubtedly many ways to reach this 

goal. From my perspective I offer the following four 

recommendations to motivate the collective “us” to 

work smarter AND harder. The old cliché, “work 

smarter, not harder” is a misnomer—it has allowed 

our society to take the easy way out. We will have to 

do both if we want to be successful.

First, identify landowners who own natural 

hardwood stands that might include black walnut. 

Geographical information systems are becoming 

more widely used by state and county governments 

for tracking a wide array of information, including 

property and timber sale transactions. In many 

cases, this information can be gathered free of 

charge; in other cases, data analysts provide this 

information for a nominal fee. Outreach activities 

can target these landowners with the primary goal 

of raising their level of awareness as to the total 

value of the natural resources on their property. 

The aforementioned trust officers must not be 

forgotten. Efforts need to be made to network with 
Natural Stand Management 83

Black Walnut in a New Century

property appraisers and financial institutions 

and identify those officers who are responsible for 

managing these land trust accounts.

Second, refine both low- and high-tech information 

vehicles and make sure they are accessible and 

relevant to individuals managing hardwood stands. 

Newsletters need to deliver information in a timely 

manner that is practical and easily digested so 

landowners can apply this new knowledge to their 

land. An emphasis on access to information will 

offer opportunities to reach a greater number 

of motivated owners in contrast to the more 

typical delivery of structured programs. This 

will be especially helpful for those new, younger 

landowners and trust officers.

Web sites need to be easily navigable with 

sophisticated graphics kept to a minimum in order 

for rapid downloading of information from the 

site. An excellent example of an easy-to-navigate 

Web site is Walnut Notes at http://www.ncrs.

fs.fed.us/pubs/viewpub.asp?key=103. A visitor 

can quickly download a series of short PDF files 

on a wide variety of black walnut management 

topics. However, if net surfers have to wait for 

dazzling images to appear on their screens or the 

information they seek is contained in exceedingly 

large document files, chances are very good that 

they will not re-visit the site. Case in point, the last 

proceedings from the 1996 Walnut Symposium 

is available on-line at http://www.ncrs.fs.fed.

us/pubs/viewpub.asp?key=255. If one wants 

information on, for example, top-working black 

walnut then they would have to download the entire 

proceedings—all 59.3 MB. This might not appear 

to be a big issue to residents of metropolitan areas 

with high-speed internet access. But, to individuals 

living in rural areas who are constrained by 56K 

modems, downloading web pages with large 

embedded files is a big and very real frustration. 

Providing technical assistance to NIPF owners has 

clear results—more active forest management. 

However, important questions remain about the 

origin, quality and cost of such assistance. With 

the number of landowners increasing and the 

number of resource professionals holding constant 

at best, the days of multiple one-on-one on-site 

consultations are clearly numbered. 

The third recommendation is shift the paradigm 

where both landowners and foresters see the role of 

the local forester less as a “county agent” and more 

of a “coach”. Among the most innovative technical 

assistance tools being extensively tried these days 

is coached planning. This outreach effort redefines 

technical assistance to utilize professionals as 

advisers of work that is actually done by the 

landowners themselves. However, to implement this 

team approach, each state will have to come to its 

own set of terms about the appropriate roles and 

mix of services by the private consulting sector and 

public service foresters, as well as the prospective 

role played by NIPF owners themselves.

The role of NIPF owners as teachers is increasingly 

recognized. A growing number of successful 

volunteer programs, such as Master Gardener, have 

only begun to tap the potential of using private 

citizens to advance information in their own groups 

of influence, as well as teach professionals about 

emerging interests or challenges. Walnut Council 

members are instrumental in this type of outreach 

effort. The learning and knowledge based that 

is accumulated and shared among landowners 

empowers them to become even more active in the 

management of their land. It also gives them the 

confidence to teach other landowners. 

The fourth, and final, recommendation is think 

outside the box when “recruiting” either clients 

or fellow landowners. Most natural resource 

professionals have been trained to approach “the 

big picture” from their own, sometimes rather 

myopic, perspective. Sure, most professionals 

endured some cross-training in their collegiate 

years and they might even interact with others 

outside their discipline. But, for the most part they 

still think the best way to approach the problem is 

through the “enlightenment” that comes through 

their discipline.

My epiphany came when a consulting forester 

and avid walnut grower was asked to speak about 

managing timber for wildlife at a local Quality 

Deer Management meeting in central Missouri. 

Over 200 people from the local area came out to 

learn how they could harvest bigger deer. Wildlife 

management was the hook, the motivator to get 

them managing their woods. At the meeting there 

were large land plat maps posted along one wall. 

One by one, landowners approached the maps, 

took the highlighter and colored in their property 

indicating their willingness to participate in the 

program. This simple exercise accomplished three 

things. First, it gave each landowner a view of the 

bigger picture that managing his or her land would 

have some impact on neighboring property. Second, 

it gave them a sense of ownership as they took pen 

in hand and colored in their parcels. Third, it put 

subtle pressure on those landowners who were not 

participating to manage their woods.

The Walnut Council was formed in 1970 by a small 

group of landowners, foresters, forest scientists 

and wood processors to promote the proper 

management and utilization of black walnut. 

Over the years the Council has grown into an 

international organization that reaches private 
84 Natural Stand Management

Proceedings of the 6th Walnut Council Research Symposium

landowners through newsletters, and chapter, state 

and national meetings. In that time, professional 

foresters and forest scientists delivered information 

in a timely manner as it became available from 

Federal and State agencies, and universities. 

Yet, after 33 years there are still less than 1,000 

members; of which even fewer are landowners if you 

subtract professional members.

So, the background information and strategy 

presented here is extremely relevant if we are 

serious about increasing both the number of 

individuals actively managing their woodlands 

and in turn the number of acres under good 

forest management. Of all the dimensions of 

cultivating black walnut that have been researched 

and published over the years, it is time we turn 

our attention to a key piece of the puzzle often 

overlooked: the human dimension.

LITERATURE CITED

Fralish, J.S., ed., comp. 2002. The Central 

Hardwood Forest conferences 1976-1999: Title, 

subject, author, and species indexes for the 

proceedings. Gen. Tech. Rep. NC-225. St. Paul, 

MN: U.S. Department of Agriculture, Forest 

Service, North Central Research Station. 227 p.

Reed, A.S. 1998. Targeting extension programs 

to nonindustrial private woodland owners. 

In: Johnson, J.E., ed. Proceedings, Extension 

forestry: bridging the gap between research 

and application. Blacksburg, VA: Virginia 

Polytechnic Institute and State University, 

College of Forestry and Wildlife Resources: 213-

220. 

Schmidt, T.L.; Kingsley, N.P. 1997. Status of 

black walnut in the United States. In: Van 

Sambeek, J.W., ed. Knowledge for the future 

of black walnut: 5th Black walnut symposium; 

1996 July 28-31; Springfield, MO. Gen. Tech. 

Rep. NC-191. St. Paul, MN: U.S. Department 

of Agriculture, Forest Service, North Central 

Forest Experiment Station: 9-22. 

Slusher, J.P. 1997. Principles of managing for black 

walnut in mixed hardwood stands. In: Van 

Sambeek, J.W., ed. Knowledge for the future 

of black walnut: 5th Black walnut symposium; 

1996 July 28-31; Springfield, MO. Gen. Tech. 

Rep. NC-191. St. Paul, MN: U.S. Department of 

Agriculture, Forest Service, North Central Forest 

Experiment Station: 148-154.
Agroforestry and Nut Production 85

Black Walnut in a New Century

GROUND COVER MANAGEMENT IN WALNUT AND

OTHER HARDWOOD PLANTINGS

J.W. Van Sambeek and H.E. Garrett1

ABSTRACT—Ground cover management in walnut plantings and established stands 

can include (1) manipulating the resident vegetation, (2) mechanical control, (3) chemical 

control, (4) mulching, (5) planting cover crops, or (6) interplanting woody nurse crops. 

Data from over 110 reports were used to compile a database that compared growth of 

black walnut and other hardwoods under different ground cover treatments as either a 

percentage of tree growth in the absence of ground cover vegetation or with little or no 

management of the resident vegetation. Overall, ground cover treatments with the best 

tree growth were application of organic mulches and annual cultivation. Ground covers 

associated with the slowest tree growth were grass sods and unmanaged or mowed 

resident vegetation. Black walnut tended to be slightly more sensitive to ground cover 

management practices than other hardwoods. The choice of a vegetation management 

system is largely controlled by management objectives, site characteristics, equipment 

costs, and, most importantly, labor availability. 

Vegetation management, especially weed control, 

is second in importance only to site selection in 

establishing plantations of black walnut (Juglans 

nigra L.) as no silvicultural practice can rescue 

a black walnut plantation on an unsuitable site 

(Burde 1988, Burke and Pennington 1989). Unlike 

many other hardwoods, walnut in managed 

plantings will not dominate a site sufficiently to 

exclude a ground layer of semi-shade tolerant 

vegetation. Because they have thin, open crowns, 

walnut trees absorb less than 60% of the incoming 

solar radiation in fully stocked stands (Smith 1942). 

In addition, walnut is one of the last tree species 

to leaf-out in the spring and one of the first to 

defoliate in the fall. This makes it possible to grow 

a wide variety of shade-tolerant forbs and grasses 

within plantings and agroforestry practices that use 

walnut. The questions addressed in this paper are 

(1) does the type of ground cover management affect 

the growth of hardwood saplings and pole-sized 

trees and (2) is the response of black walnut to 

ground cover management similar to that for most 

other hardwoods? If the latter is true, we can use 

the research information from other hardwoods to 

recommend alternative management scenarios for 

walnut without actually testing them.

The wide variation in growth rates both within and 

among hardwood species such as that reported 

by Hansen and McComb (1955) for Iowa makes 

it difficult to use measurements such as annual 

height, diameter, or volume growth to directly 

compare tree response to different ground cover 

treatments. One alternative is to calculate the 

growth under different ground cover treatments 

as a percentage of the growth to a treatment 

commonly used in most ground cover studies. A 

quick survey of the literature suggests there are two 

commonly used treatments that have this potential: 

unmanaged plots and vegetation-free plots. For 

example, Schlesinger and Van Sambeek (1986) 

reported that walnut saplings in a tall fescue sod 

grew 0.56 inches in DBH over a 5-year period while 

walnut saplings in cultivated plots grew 1.85 inches 

over the same period. In this case, walnut diameter 

growth in tall fescue was only 30% of that in the 

vegetation-free control. As another example, Van 

Sambeek and others (1986) reported that walnut 

saplings planted with hairy vetch on a bottomland 

site averaged 4.7 feet in height after 3 years 

compared to only 3.7 feet for walnut saplings in 

unmanaged plots growing with a normal succession 

of weeds. Similarly, height of walnut with hairy 

1

Research Plant Physiologist (JWVS), USDA Forest Service, North Central Research Station, 202 Natural Resource Bldg.-UMC, 

Columbia, MO 65211-7260 and Director (HEG), Center for Agroforestry, University of Missouri, 103 Natural Resource Bldg.-UMC, 

Columbia, MO 65211-7270. JWVS is corresponding author: to contact, call (573) 875-5341 ext 233 or e-mail at jvansambeek@fs.fed.us.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
86 Agroforestry and Nut Production

Proceedings of the 6th Walnut Council Research Symposium

vetch on an upland site averaged 6.4 feet compared 

to that of 5.3 feet in the unmanaged control. In 

this case, height growth of walnut planted with 

hairy vetch was 127% and 121% of that in the 

unmanaged control for the upland and bottomland 

plantings, respectively. 

For this paper, we examined the methods and 

results from over 110 reports (52% of which included 

walnut) resulting in over 2,500 comparisons that 

examined effects of different ground cover treatments 

on growth of hardwood saplings and pole-sized 

trees. To create the ground cover management 

database, we compiled the following information into 

a spreadsheet: literature source, planting location 

by state, the year experiment was initiated, tree 

species, growing space and percent of growing space 

in experimental treatment, initial and final tree ages, 

control treatment (resident vegetation or vegetation 

free), experimental treatment(s), initial and final 

measurements (height, diameter, volume, fruit yield, 

or foliage nitrogen) for both control and treatment 

trees, and whether or not the authors reported 

statistical differences (P < 0.05%). (The spreadsheet 

titled “BW_GCM_Database_Dec2003.xls” is available 

electronically from the FS-NC-4154-02-03 study files 

of the Ecology and Management of Central Hardwoods 

Project of the U.S.Department of Agriculture, Forest 

Service, North Central Research Station.) 

Criteria for being included in the database were 

that (1) one or more hardwoods or broad-leaved 

tree species were included in the study, (2) at 

least one treatment was left unmanaged or one 

or more treatments were maintained relatively 

free of competing vegetation, and (3) reported 

measurements spanned two or more years to 

minimize responses to transplant shock or atypical 

climatic conditions.

We calculated tree growth response to each ground 

cover treatment as a percentage of either the 

unmanaged or vegetation-free control for each 

hardwood species included in each paper. We 

assume the vegetation-free control is an estimate 

of maximum tree growth in the planting and is the 

average of one or more treatments with mulches, 

mechanical and/or chemical weed control. 

Preliminary analysis of the resultant percentages 

showed responses were normally distributed with 

only a few outliers. Outliers primarily occurred in 

studies where trees in the experimental treatment 

showed excellent growth with little or no growth in 

an unmanaged control (resident vegetation). For 

this paper, treatment responses as a percent of 

the control exceeding two standard deviations of 

the group mean were treated as missing values. 

Because of unequal variances and degrees of 

freedom, adjusted variances and tabulated t-values 

(t’) were calculated according to Steel and Torrie 

(1960) to determine appropriate degrees of freedom 

and probability of statistical differences.

Ground cover management treatments can be 

divided into six broad categories that include (1) 

manipulation of the resident vegetation (mowing, 

nitrogen fertilization, irrigation, or applying 

selective herbicides), (2) bare ground maintained 

by mechanical methods (scalping, disking, or 

rototilling), (3) bare ground established using 

various chemicals, (4) mulching (organic or 

inorganic), (5) cover crops (legumes or grass sods), 

and (6) woody nurse crops. Average percentage + 

standard deviation (the range within which two 

thirds of all results are expected to occur) were 

computed for each category for walnut and for all 

other hardwoods. We used analyses of variance 

to test for differences among the categories and 

for differences between black walnut and other 

hardwoods within the six broad categories and 

several subcategories (SAS Institute, Cary, NC). 

The following sections report the average response 

of black walnut and of other hardwoods to each 

ground cover approach. The advantages and 

disadvantages of using each approach in hardwood 

plantings are also discussed in the following 

sections.

RESIDENT VEGETATION

Resident vegetation is the forbs and grasses that 

naturally emerge following site preparation or 

disturbance and are left to grow with minimal 

management (Ingels and others 1998). On 

previously cropped lands or following intensive 

site preparation, resident vegetation will consist 

of a succession of plant species dominated first 

by annual forbs and grasses, giving way to, 

presumably, more competitive perennial forbs, and 

then eventually perennial grasses. To minimize 

competition in new hardwood plantings, perennial 

vegetation should be minimized. The best time to 

do this is during site preparation before the trees 

are planted. During site preparation, landowners 

can choose from a wider range of post-emergent 

herbicides and tillage equipment that will be easier 

to implement and be more effective than after trees 

are planted.

There are disadvantages to using resident 

vegetation as a ground cover in hardwood plantings. 

When hardwood plantings are established using 

resident vegetation as living mulch (without 

spot or strip weed control), seedling and sapling 

growth is typically as little as 60% of the growth 

found in plantings maintained free of vegetation 

through cultivation or use of herbicides (Fig. 1). 

Growth response of walnut to resident vegetation is 
Agroforestry and Nut Production 87

Black Walnut in a New Century

statistically similar to that of most hardwoods (56 

and 63% of vegetation-free control, respectively; t’

= -1.22, 37 df, p < 0.30ns). With resident vegetation 

there is little control on what the succession of 

species will be or whether it will include noxious 

weeds. In addition to competing for light, nutrients, 

and water, several weeds including tall fescue 

(Festuca arundinacea Schreb.), quackgrass 

(Agropyron repens (L.) Beauv.), yellow nutsedge 

(Cyperus esculentus), and goldenrod (Solidago spp.) 

have been shown to secrete toxins into the soil 

which stress and slow the growth of trees (Larson 

and Schwarz 1980; Rink and Van Sambeek 1985, 

1987; Rice 2001).

There are some advantages to growing unmanaged 

ground covers in hardwood plantings. It is the least 

expensive and least labor intensive method for 

establishing a ground cover. During the summer, 

resident vegetation provides shade that can reduce 

soil temperature maximums and daily fluctuations. 

A vegetative cover also slows surface run-off and 

results in greater infiltration of water.

On excellent walnut sites where vegetation is 

controlled, we should expect annual height growth 

around 2.5 feet per year for which there would 

be an expected diameter growth of 0.5 inches per 

year. If this rate of diameter growth is sustained 

throughout the rotation, we should expect a 

rotation length of 45 years to produce an 8-foot 

veneer log with a 20-inch top diameter or 55 years 

for a similar length veneer log with 20-inch core 

of heartwood. In contrast, walnut trees competing 

with resident vegetation would have rotation 

lengths of 75 and 85 years or more, respectively. 

Because of poor site selection, growth rates for 

walnut plantings are typically 0.2 to 0.3 inches 

DBH per year which effectively doubles rotation 

lengths. Although the ground cover approach using 

resident vegetation involves the least cash outlays, 

it is unlikely that the high initial costs to establish 

the planting can profitably be carried over rotation 

lengths of 75 or more years. 

MANIPULATION OF RESIDENT VEGETATION

Presumably, reduced growth of walnut in resident 

vegetation compared to vegetation-free plantings 

is due to competition for soil nutrients, especially 

nitrogen, and competition for limited available 

soil moisture during the summer. A number of 

approaches can be used to reduce competition 

when using resident vegetation as a cover crop. 

The most common approaches to manipulating 

the resident vegetation include mowing to reduce 

transpiring biomass, irrigation to increase available 

soil moisture, fertilization to increase available 

nitrogen, and grass-selective herbicides to eliminate 

grass competition.

Figure 1.—Average hardwood growth (dark band) for nine ground 

cover management approaches as a percentage of growth for 

black walnut and other hardwoods in unmanaged controls or 

resident vegetation. Approximately two-thirds of all reported 

values occur within one standard deviation as shown within 

the shaded area.

0

50

100

150

200

250

Cultivated

Mulched

Herbicides

Barriers

Nurse crop

Fertilized

Legumes

Mowed

Grasses

Growth (

% of r

e

sident v

egetation)
88 Agroforestry and Nut Production

Proceedings of the 6th Walnut Council Research Symposium

Mowing

Mowing of the resident vegetation does not 

significantly improve tree growth over that of trees 

growing in unmanaged resident vegetation (Fig. 

1). Walnut growth in frequently mowed plantings 

averages 16% less (84% of the unmanaged control) 

than that of walnut with unmanaged resident 

vegetation. In contrast, growth of other hardwoods 

in mowed plantings is increased by 10% (110% 

of unmanaged control) over that of hardwoods in 

resident vegetation. Tree responses to mowing are 

statistically different between walnut and other 

hardwoods (t’ = -3.60, 20 df, p <0.01**). Similar 

reductions in growth are also found for studies 

with vegetation-free controls where walnut growth 

is 43% of the controls compared to 61% for other 

hardwoods (t’ = -2.58, 23 df, p <0.02*). Rice 

(2001) indicated the practice of mowing shifts the 

competitive advantage away from mostly forbs in 

which growing points are sheared off with each 

mowing to grasses which have their growing 

points near the soil line. Davies (1985) and Ponder 

(1991) also suggest that reduced tree growth is a 

consequence of shifting competition to grasses and 

the renewal of vegetative growth following mowing. 

For the small growth gains achieved with other 

hardwoods, mowing is an expensive, labor intensive 

management practice that must be completed 2 

to 5 times a year for 3 to 5 years before hardwood 

saplings will dominate the site. In addition to 

little improvement in tree growth, mowing leads 

to “mower blight” or the inadvertent damaging 

of tree stems when accidentally hit by tractor or 

mowing equipment. Also, repeated mowing can lead 

to compaction of upper soil layers where walnut 

feeder roots are located. Mowing, however, has the 

advantages of preventing noxious weeds from going 

to seed, exposing rabbits and mice to predators, 

maintaining a ground cover to protect the soil from 

wind and water erosion, aesthetically creating a 

more pleasing appearance to the planting, and 

facilitates the harvesting of nuts. To reduce risks 

of wildfires and to slow plant succession from forbs 

to grasses, we recommend mowing only in the fall 

after a hard killing frost.

Irrigation

Too few reports were found to indicate if irrigation 

could be used to increase available soil moisture 

sufficiently to increase tree growth. Dey and others 

(1987) and Van Sambeek and McBride (1991) found 

little or no increased growth of walnut in response to 

irrigation on their sites with grass ground covers. It is 

possible that the resident vegetation and grass sods 

may benefit more from irrigation than will the trees.

Fertilization

On most sites, especially old field sites, available 

soil nitrogen tends to be the nutrient limiting 

hardwood growth. The published literature 

shows a slight increase in tree growth with the 

addition of nitrogen fertilizers (Fig. 1). Walnut 

may not compete as successfully with ground 

covers for added nitrogen as other hardwoods 

because growth increases for walnut average 

24% compared to 52% for other hardwoods (124 

and 152% of unmanaged control, respectively; t’

= -2.37, 27 df, p < 0.05 *). Similar patterns exist 

when nitrogen fertilizers are applied to plantings 

where ground covers have been eliminated. In this 

case, growth increases for walnut averaged 14% 

compared to 31% for other hardwoods (114% and 

131% of vegetation-free control, respectively; t’ = 

-2.68, 41 df, p < 0.02*). Ponder (1997) previously 

did an in depth review on walnut fertilization and 

reported responses to added nitrogen can be quite 

variable and concluded that it is not uncommon 

to see little or no response to fertilization, 

especially on good walnut sites. 

Selective Herbicides

Grasses tend to be more competitive than broadleaved legumes or other forbs (Fig. 1); thus, 

selective removal of resident grasses should 

improve tree growth. Selective control can be 

done with over-the-top applications of postemergent herbicides such as Fusilade (fluazifopbutyl) or Poast (sethoxydim). These herbicides 

are selective for grasses, although we found 

no reports in the literature documenting their 

effectiveness in hardwood plantings. Multiple 

applications may be needed as they are most 

effective if applied when grasses are 6 to 12 

inches tall. Fusilade is more effective than Poast 

on perennial grasses; conversely, Poast is more 

effective than Fusilade on annual grasses such 

as foxtail and crabgrass. Selective control of 

annual grasses can also be achieved with Surflan 

(oryzalin) and Stomp (pendimethalin), two preemergent herbicides that require 1 to 2 inches 

of rainfall within several weeks of application for 

activation. These herbicides are more effective 

on germinating grasses than broad-leaved forbs. 

After tree seedlings are established, pre-emergent 

herbicides that are selective for germinating seeds 

are unlikely to harm deeper-rooted perennial 

plants or established tree seedlings. Selective 

control of the tallest and usually most competitive 

weeds can also be achieved using wipe-on 

applications of broad spectrum herbicides if care 

is taken to keep the herbicide from coming in 

contact with the walnut foliage. 
Agroforestry and Nut Production 89

Black Walnut in a New Century

MECHANICAL VEGETATION CONTROL

Mechanical vegetation control involves the cutting 

off at or below ground line, i.e., scalping, or the 

uprooting and burial of competing plants, i.e., 

cultivation. Mechanical control of competing 

ground covers tends to nearly double hardwood 

growth over that of trees with a ground cover of 

resident vegetation (Fig. 1). Growth of walnut with 

mechanical weed control increases on average by 

117% compared to 79% for other hardwoods (217% 

and 179% of unmanaged control, respectively; t’ = 

1.40, 9 df, p < 0.20ns). The result of no statistical 

difference should be considered preliminary 

because too few studies were found that compared 

growth of walnut in plantings with unmanaged 

control and cultivated plots.

It is generally assumed that mechanical control 

of competing vegetation will allow hardwood 

seedlings and saplings to grow at their full 

biological potential for the site. It is one of the 

more effective methods of controlling competing 

ground cover vegetation, especially if done when 

weeds are small and perennial weeds are not yet 

established. Exposed soils usually have higher soil 

temperatures which increases decomposition of 

organic matter and nitrogen mineralization, making 

more nutrients available to the trees. Cultivation 

also buries diseased walnut leaves that disrupt the 

normal dispersal of disease spores (Kessler 1988). 

Vegetation-free plantings also allow for better air 

drainage reducing the risk of spring frost injury and 

destruction of female flowers. Finally, bare ground 

treatments provide little winter cover for rabbits and 

mice that chew on tree seedlings and saplings. 

Besides the increased potential for soil erosion, 

cultivation has a number of other potential 

disadvantages. Cultivation is relatively labor 

intensive because it must be repeated two to five 

times a year depending on the weed seeds present 

in the seed bank and rainfall. In years with heavy 

spring rainfall, cultivation on a timely basis may 

not be possible. Traffic from heavy equipment 

needed for cultivation can lead to soil compaction 

and impede water infiltration. Frequent and close 

cultivation means a higher proportion of tree stems 

are likely to be damaged than with other ground 

cover management treatments, except maybe 

mowing. Deep tillage also results in destruction 

of shallow feeder roots in the uppermost fertile 

layers of the topsoil. It is not uncommon to find 

walnut roots extending out more than twice the 

width of the crown. Repeated cultivation for several 

years decreases the amount of organic matter in 

the soil reducing stable soil aggregates needed for 

the retention of soil water and nutrients. While 

accelerating nitrogen mineralization, high soil 

temperatures in exposed soil also accelerates 

nitrogen loss through volatilization. Unless cleaned 

frequently, tillage equipment may aggravate future 

weed problems by spreading seed or chopped plant 

parts that can become established in other parts of 

the planting. Finally, it is unclear whether the costs 

incurred to cultivate a planting three or four times 

a year can be offset by a mere doubling of the tree 

growth expected in unmanaged plantings. 

It is generally assumed that bare soil treatments 

retain significantly more soil moisture than 

treatments with a vegetative cover. Unless the trees 

are large enough to shade the planting, this may 

not be true. Lull and Fletcher (1962) report soil 

moisture depletion rates of 0.35 inches/day from 

surface evaporation for bare ground compared 

to 0.37 inches/day from evapotranspiration by 

herbaceous ground covers. By comparison, soil 

moisture depletion from woodland soils was deeper 

and averaged 0.42 inches/day. 

There are several things that can be done to 

minimize the detrimental impact of cultivation. 

Cultivating when the ground is dry and hard 

and when the weather is hot is more effective 

and will cause less compaction than when the 

soil is moist. Use tractors with floatation tires to 

reduce compaction. Use cultivators with V-shaped 

sweeps or weed knives that sever weeds off near 

the soil line rather than deep penetrating disks 

or rototillers that tend to bring new weed seeds 

to the surface and damage tree feeder roots (Rice 

2001). Specialized equipment such as the Weed 

Badger can be used for precision, within row 

tillage to reduce damage to saplings or pole-sized 

trees (Garrett and others 1989). Although not 

experimentally tested, tilling one half the planting 

(between alternate rows) may be as effective as 

annually tilling the entire tree planting (Schlesinger, 

personal communication).

CHEMICAL VEGETATION CONTROL

The growth response of hardwoods to herbicide 

applications to control competing vegetation is 

very similar to that for mechanical control and 

nearly twice that for trees in unmanaged controls 

(Fig. 1). Increased growth of walnut averages 95% 

for trees with chemical weed control compared to 

that in unmanaged controls and is not statistically 

different from increased growth of other hardwoods 

that averages 72% (195 and 172% of unmanaged 

control, respectively; t’ = 1.33, 33 df, p < 0.20ns). 

Tree response to chemical weed control has a large 

standard deviation partially because all herbicides 

are not equally effective and some herbicides may 

be slightly phytotoxic to the trees at reported 

experimental application rates. Hardwood plantings 

with repeat applications of RoundupTM (glyphosate) 
90 Agroforestry and Nut Production

Proceedings of the 6th Walnut Council Research Symposium

tend to have slightly higher growth rates than trees 

in treatments using pre-emergent herbicides (131% 

and 102% of vegetation-free control, respectively). 

Seifert and Woeste (2002) reported that walnut 

growth is greater with tillage than with springapplied pre-emergent herbicides, especially when 

using Oust (sulfometuron) alone or in mixes. 

Chemical vegetation control has several distinct 

advantages over other methods of ground cover 

management. It may be the least costly and least 

labor intensive method for reducing or eliminating 

ground cover competition. The application of 

chemicals requires fewer workers and equipment 

costs are less than equipment for mechanical weed 

control. Post-emergent herbicides are more effective 

for eliminating grass sod and perennial weeds than 

is cultivation. In many cases, a single application of 

a pre- and post-emergent herbicide mix will control 

ground cover competition for the entire growing 

season. Timing of herbicide applications are less 

critical than with cultivation. Chemical treatments 

usually result in higher available soil moisture 

and less erosion during the growing season than 

mechanical vegetation control. 

The most significant problem associated with 

chemical control of ground cover vegetation is the 

limited and declining choice of registered herbicides 

that can be used (Seifert 1993). Other problems 

include the lack of selectivity among the currently 

registered herbicides and the tree damage they can 

cause if not applied properly (Seifert and Woeste 

2002). Improper calibration, herbicide drift, or 

accidental spraying of saplings can result in either 

mortality or slowed tree growth. Wet field conditions 

can prevent entry into the planting when vegetation 

is at the appropriate stage. Conversely, lack of 

rainfall can keep some pre-emergent herbicides 

from becoming soil activated. Post-emergent 

herbicides generally require multiple applications 

per year for effective control of competing 

vegetation. Because soils remain relatively 

undisturbed, the impact of rain droplets on bare 

soil can lead to surface compaction thus reducing 

surface infiltration and increasing surface run-off 

and soil erosion compared to that in unmanaged 

plantings.

There are several approaches that can be used to 

minimize negative impacts of using herbicides. 

Resident vegetation can be allowed to develop in the 

spring before application of post- and pre-emergent 

herbicides creating a layer of dead vegetation that 

will shade the soil reducing surface evaporation and 

decreasing impact of rain drops. Because herbicides 

can effectively control perennial plants, spraying 

is not required every year assuming that annuals 

are less competitive than perennial vegetation. 

Alternatively, the area between every other tree 

row could be treated in alternate years retaining 

resident vegetation on half the planting. When more 

than one herbicide will control the target vegetation, 

the herbicide(s) that is least toxic to earthworms 

and other soil macrofauna and macroflora should 

be selected. Earthworms alone can move more 

soil/year than is moved with one plowing using 

a tractor-pulled moldboard plow (Minnich 1977). 

When managing for nut crops, use strip chemical 

control within the tree row during the growing 

season followed by fall mowing between rows to 

facilitate harvesting of nuts.

MULCHES

Ground cover management using mulches falls 

into two broad categories: organic carbon-based 

mulches and inorganic or barrier mulches. The 

ideal mulch will (1) be opaque or block sunlight 

discouraging germination and growth of weeds, (2) 

be porous enough to allow water to infiltrate, (3) 

protect the soil from sun and wind to conserve soil 

moisture by reducing evaporation, (4) moderate 

daily soil temperature fluctuations and extremes, 

(5) be made of biodegradable materials but still 

have the strength and durability to last until trees 

are established, (6) be easily transported to the 

tree planting, (7) blend into the landscape, and (8) 

be relatively easy to install (Windell and Haywood 

1996). Other benefits of mulching include making 

trees more visible thereby reducing the chances 

for mechanical damage from mowers and other 

maintenance equipment. 

Organic Mulches

Applying organic mulches around the base of 

hardwood trees is about as effective as mechanical 

or chemical weed control (Fig. 1). Organic 

mulches increase walnut growth on average by 

89% compared to walnut growth in unmanaged 

plantings. Growth increases are similar for other 

hardwoods (78%) when mulched and are not 

statistically different from walnut (189% and 178% 

of unmanaged control, respectively; t’ = 0.41, 12 df, 

p < 0.50ns). In studies that have compared organic 

mulches to other vegetation-free treatments, 

mulching usually results in better tree growth than 

other bare ground treatments (Fig. 2). Based on 

very few studies, organic mulches may increase 

walnut growth by 29% over other vegetationfree treatments with similar increases for other 

hardwoods (26%) when mulched compared to that 

of other bare ground treatments (129% and 126% of 

vegetation-free control, respectively; t’ = 0.17, 7 df, 

p < 0.50ns).
Agroforestry and Nut Production 91

Black Walnut in a New Century

Duryea and others (1999) indicate the physical 

effects of mulching are much greater than the 

fertilizer value associated with the release of 

nutrients as mulches decompose. Decomposition 

of organic mulches increases the organic matter 

content of the soil leading to better soil aggregation 

and increased retention of soil water and nutrients. 

Organic mulches also reduce the impact of 

raindrops, slow the flow of water over the soil 

surface, reduce soil erosion, and increase rates 

of water infiltration. Organic mulches also can 

increase the activity of worms, fungi, and other soil 

organisms resulting in increased diffusion of soil 

oxygen, organic matter, nutrient availability, and 

root growth.

Although somewhat dependent on the density and 

texture of the materials used, a 4-inch thick layer 

of an organic mulch is a good compromise—not 

so deep as to inhibit soil aeration but still thick 

enough to prevent emergence of germinating 

seedlings. Mulches with a good balance of carbon to 

nitrogen (30C:1N) are likely to increase both organic 

matter and mineral nitrogen in the soil. Examples 

of balanced C:N mulches include rotted manures 

and compost (20C:1N), grass clippings (20C:1N), 

pine needles, shredded leaves (60C:1N), chipped 

landscape and utility right-of-way trimmings, baled 

hay, and straw (80C:1N). During decomposition, 

mulches with high carbon to nitrogen ratios such 

as shredded waste paper, sawdust (500C:1N) and 

wood shavings (700C:1N) will compete with the 

trees for available soil nitrogen. When using high 

carbon mulches, a nitrogen fertilizer should be 

incorporated into the mulch or broadcast under the 

tree before applying the mulch. 

A major problem with using organic mulches 

is subsidence, especially mulches with a high 

proportion of leaves, and the need to reapply 

additional mulch annually to remain effective. 

Without specialized equipment, mulching trees 

is a labor intensive operation. A cubic yard of 

mulch will cover approximately 20 square feet 

and it may require 150 cubic yards per acre to 

mulch hardwood plantings when applied as twenty 

4-foot wide strips. Other concerns include the 

introduction of new weed seeds or pathogens into a 

planting, especially with mulches such as manure, 

compost, or straw. Since mulches provide cover for 

mice and voles, they need to be pulled away from 

trees during the winter when these animals do the 

most damage. 

Inorganic or Barrier Mulches

Examples of inorganic or barrier mulches include 

gravel, crushed rock, or manufactured products 

such as plastic films and landscape fabrics. 

Inorganic mulches used in hardwood plantings 

are primarily opaque polyethylene (plastic) films 

and woven polypropylene (landscape) fabrics. 

These materials are usually placed under gravel 

and crushed rock to keep them from sinking into 

the soil. Barrier mulches can be very effective for 

increasing early growth in hardwood plantings (Fig. 

2). When compared to other methods of maintaining 

Figure 2.—Average hardwood growth for nine ground cover 

management approaches as a percentage of growth for black 

walnut and other hardwoods planted in vegetation-free control 

plots. Approximately two-thirds of all reported values occur 

within one standard deviation as shown within the shaded area.

0

25

50

75

100

125

150

175

200

Mulched

Fertilized

Barriers

Cultivated

Herbicides

Legumes

Grasses

Resident

Mowed

Gro

wth (% of v

e

g

etati

o

n-free)
92 Agroforestry and Nut Production

Proceedings of the 6th Walnut Council Research Symposium

vegetation-free growing space, barrier mulches 

increased growth of black walnut by 35% compared 

to a 5% increased growth of other hardwoods (135% 

and 105% of vegetation-free control, respectively; t’

= 1.99, 12 df, p < 0.10ns). Too few studies have been 

done with walnut to make statistical comparisons; 

however, increased growth of other hardwoods 

established with barrier mulches may average 

66% more than for hardwoods growing in resident 

vegetation (Fig. 1).

Major advantages of inorganic or barrier mulches 

are one-time installation with minimal annual 

maintenance to achieve a vegetation-free zone 

around tree seedlings and saplings for 2 or more 

years. By incorporating carbon black into the 

polyethylene or polypropylene, the harmful effects 

of the sun’s ultraviolet radiation can be slowed 

so that materials will last for 3 or more years. 

Woven polypropylene or landscape fabrics readily 

permit water infiltration; whereas, polyethylene 

films must be punched or fabricated with holes to 

permit infiltration of water. Clear and dark-colored 

mulches absorb and trap more solar radiation 

than mulched or bare soil resulting in warmer soils 

(3 to 10o F) and earlier root growth in the spring. 

Consequently, these films can result in summer soil 

temperatures lethal to roots and tender stems. It 

has lead to development of promising colored and 

light-reflective films that block sunlight but absorb 

less solar radiation (Ham and others 1993). 

There are significant problems associated with 

use of barrier mulches besides the high cost for 

materials and labor. The main difficulty with plastic 

films and landscape fabrics is holding the material 

in place. Both labor costs and anchorage problems 

can be reduced by using tractor-drawn machines 

that will lay and bury edges of polyethylene films 

or landscape fabric on cultivated fields. Because 

woven polypropylene tends to be thicker, they 

are both more durable and more expensive than 

polyethylene films. Although permeable to water, 

infiltration rates for landscape fabrics are still 

relatively slow and can lead to surface runoff. 

In contrast, polyethylene films restrict surface 

evaporation, retain available soil moisture much 

later into the growing season, and are much slower 

to recharge soil moisture in the fall. Polyethylene 

films are readily punctured by animal hooves and 

sprouts of some plants, so woven polypropylene 

fabrics should be used in areas with high nutsedge 

(Cyperus spp.) or deer populations. Barrier mulches 

can lead to significant tree damage because these 

mulches provide an ideal habitat for rodents and 

screen them from predators.

Operationally, it is easiest to install barrier 

mulches with machines on recently cultivated sites 

either before or after tree seedlings are planted. 

Rolls 6 feet wide by 300 feet long are usually 

recommended, because this size allows the outside 

edges to be buried and still provide a 4- to 5-foot 

wide vegetation-free strip in which to hand-plant 

trees. Costs for fabric and installation are typically 

$0.25 to $0.40 per linear foot. Tree seedlings can 

also be machine-planted before laying polyethylene 

or polypropylene mulches. In this case, a small cut 

is made next to bent-over seedlings so the stem can 

be pulled out. The heated air under barrier mulches 

usually keeps most weeds from growing through the 

planting hole or cut slits. A shovel full of soil placed 

near each seedling helps to direct rain water toward 

the opening at the base of each tree and prevents 

wind from lifting the mulch. Using side-discharge 

mowers, clippings of resident vegetation can be 

blown into the tree row to cover barrier mulches 

during the summer, thereby, reducing the high 

soil temperatures encountered with dark-colored 

impermeable films. 

COVER CROPS

Cover crops are established ground covers designed 

to reduce the amount of resident vegetation and 

delay normal succession to perennial weeds. 

A number of forage legumes and grasses are 

tolerant of light shade and could be grown in 

managed black walnut plantings (Lin and others 

1999, Alley and others 1999). In walnut plantings 

managed for timber (CCF 100 to 120), light levels 

are approximately 40% of full sunlight (Smith 

1942). In contrast, in more open walnut plantings 

managed for nuts (CCF 80 to 90), light levels are 

approximately 50% of full sunlight. Obtaining 

sufficient understory light can be problematic 

when managing cover crops in mixed hardwood 

plantings or natural stands. The light intensity in 

the understory of a mature mixed hardwood forest 

is generally less than 20% of full sunlight and can 

be as low as 1% (Dey and MacDonald 2001). At full 

leaf expansion, walnuts in plantings with CCF’s of 

100 to 120 (full site occupancy or B-level stocking) 

absorb approximately 60% of the incoming solar 

radiation. Light infiltration is curvilinear-related 

to CCF or residual stocking density (a function of 

the number of trees/acre and their average DBH) 

(Dey 2002). To provide light levels on the forest 

floor of 40% to 60% of full sunlight, may require 

maintaining 30% to 40% residual stocking densities 

and removal of approximately one-third of the 

overstory trees during each thinning (Schlesinger 

and Funk 1977, Sander 1979). 

Cover crops can significantly reduce the amount of 

labor needed for controlling resident vegetation in 

new plantings with only a modest increase in costs. 

Other benefits include the natural accumulation 

of decaying plant residues on the soil surface that 
Agroforestry and Nut Production 93

Black Walnut in a New Century

reduce summer soil temperatures and a rapid 

increase in soil organic matter by organisms that 

naturally till soil (Minnich 1977, Shribbs 1985). 

Besides increasing nutrient availability, oxygen, 

and water infiltration, earthworms also create soil 

pores that facilitate deeper root penetration by the 

trees. A vegetative cover either as a living mulch 

or decaying residues shades the soil resulting in 

lower soil temperatures because less solar radiation 

is absorbed by the soil. Wohlstenholm (1970) 

reported that planting legume or grass covers in 

pecan orchards can delay bud burst by 6 to 12 

days, thereby reducing damage by late spring 

frosts. Cover crops also provide excellent habitat 

for wildlife, including rabbits, mice, and voles that 

frequently damage young trees. 

Legume Cover Crops

The growth of hardwoods is generally better 

when grown with legume cover crops than with 

resident vegetation (Figs. 1 and 2). Legume cover 

crops increase walnut growth on average by 

28% compared to walnut growth in unmanaged 

plantings. Growth increases are similar for other 

hardwoods (30%) and are not statistically different 

from walnut (128% and 130% of unmanaged 

control, respectively; t’ = -0.22, > 100 df, p < 0.50ns). 

Most studies with vegetation-free control treatments 

report reduced hardwood growth with legume cover 

crops; however, they also report legume cover crops 

reduce growth less than does resident vegetation. 

Legume cover crops reduce walnut growth by 31% 

compared to 44% for resident vegetation (69% 

and 56% of vegetation-free control, respectively; 

t’ = 2.36, 35 df, p <0.05*). Responses for other 

hardwoods are similar where legume cover crops 

reduce growth by 32% compared to 37% for 

resident vegetation (68% and 63% of vegetation-free 

control, respectively; t’ = 1.24, >100 df, p < 0.20ns). 

Because nitrogen tends to be the nutrient most 

often limiting tree growth, increased hardwood 

growth with legume cover crops is generally 

attributed to the ability of most legumes to fix 

atmospheric nitrogen (White and others 1981). 

Unless heavily fertilized, forage legumes on average 

obtain about 75% of their nitrogen through the 

fixation process. If the above ground biomass is not 

harvested, some of this nitrogen becomes available 

to the trees when plant residues decompose and 

organic nitrogen is converted to ammonia or 

nitrate nitrogen. Some mineral nitrogen is also 

made available to the trees through atmospheric 

deposition; however, the burning of fossil fuels and 

lightening only account for 5 to 10 pounds per acre 

in the central United States and 15 to 20 pounds 

per acre in the eastern United States. In contrast, 

fixation rates for forage legumes average 100 to 

300 pounds per acre (Table 1). Although fixation 

rates are largely unknown for most native legumes, 

Table 1.—Average growth and standard deviation for black walnut and other hardwoods as 

a percentage of tree growth in plantings without ground covers and the average shade 

tolerance percentile rank and reported nitrogen fixation rates for different forage legumes.

Legume Cover Crop Tree Growth1

%

Shade Tolerance2

%

Nitrogen Fixation Rate3

lbs/acre

Soybeans 99 + 26 ( 5) ND 50

Crownvetch 83 + 23 ( 18) 78 230

Subterranean clover 81 + 66 ( 5) 43 90

Crimson clover 79 + 37 ( 18) 57 90

Hairy vetch 74 + 29 (119) ND 120

Sweet clover 72 + 7 ( 4) ND --

Red clover 70 + 24 ( 91) 44 250

Sericea lespedeza 69 + 12 ( 22) 30 --

Kura clover 66 + 11 ( 9) 66 155

Birdsfoot trefoil 63 + 26 ( 22) 36 150

Striate lespedeza 63 + 14 ( 5) ND --

White clover 62 + 23 ( 43) 34 200

Resident vegetation 60 + 28 (156) -- --

Korean lespedeza 57 + 9 ( 23) 12 --

Alfalfa 44 + 25 ( 7) 32 300 1 Growth is percent of vegetation-free control + standard deviation (approximately 66% of reported 

responses) and in () number of replications extracted from the literature. Mean least significant difference is 

14%.

2 Average percentile rank under moderate and heavy shade estimated from three or more screening trials 

where 0% was assigned to the least and 100% was assigned to the most shade tolerant species within each 

light level and screening trial. ND = not tested.

3

Average of values reported in six publications identified in vegetation management database.
94 Agroforestry and Nut Production

Proceedings of the 6th Walnut Council Research Symposium

several shade tolerant native legumes have been 

identified that could be used as living mulches in 

hardwood plantings and natural stands (Ponder 

1994, Van Sambeek and others 2004). 

Establishing legume cover crops is more 

expensive and requires more labor than seeding 

forage grasses or manipulating the resident 

vegetation. Legumes usually require a well 

prepared seedbed and may require application of 

lime and fertilizers to produce a stand capable of 

suppressing resident vegetation (Jorgenson and 

Craig 1983, Ingels and others 1998). Because 

seed of most legumes are small, they should 

be seeded with companion crops such as oats 

(Avena sativa L.) or barley (Hordeum vulgare L.) 

that quickly germinate providing a plant cover 

to minimize soil erosion and suppress potential 

resident vegetation (Simmons and others 1992). 

Stamps and others (2002) have shown that 

combining trees and alfalfa (Medicago sativa L.) 

will increase the number of insects that prey on 

other insects including those that feed on black 

walnut foliage. Legume ground covers have also 

been shown to interfere with anthracnose spore 

dispersal from diseased leaves or infection of new 

walnut leaves in the spring (Van Sambeek 2003). 

Although most legumes make a better cover crop 

than resident vegetation, substantial variation 

exists among forage legumes as to their effects on 

hardwood growth and relative tolerance to shade 

(Table 1). In general, annual legumes tend to 

reduce tree growth less than perennial legumes. 

Subterranean clover (Trifolium subterraneum L.) 

and crimson clover (Trifolium incarnatum L.), both 

annual legumes, are probably excellent choices for 

walnut plantings; however, neither will overwinter 

throughout most of the black walnut range. 

Likewise, legumes that have decumbent stems 

such as crownvetch (Coronilla varia L.) or are vines 

such as hairy vetch (Vicia villosa Roth) are excellent 

choices for cover crops because they require less 

biomass to effectively smother other vegetation 

than upright legumes such as sericea lespedeza 

(Lespedeza cuneata (Dumont) G. Don). Legumes 

with upright stems may also require mowing after 

a killing frost each fall to reduce the wildfire risk. 

Based on few published reports, alfalfa may be 

the poorest choice for a cover crop in black walnut 

plantings.

Legume cover crops should not be harvested 

for several reasons. Approximately 75% of the 

plant biomass is in the above ground portion; 

thus, harvesting removes nearly all the nitrogen 

obtained through fixation (Watson and others 

1984). If harvested, legumes may be no better 

than unmanaged broadleaved forbs such as 

rapeseed (Brassica napus L.) or resident vegetation. 

Cool-season legumes such as hairy vetch and 

crownvetch that produce most of their vegetative 

growth in the spring when available soil moisture 

is high are better cover crops than warm-season 

legumes, such as sericea lespedeza and Korean 

lespedeza (Lespedeza stipulacea Maxim.). If not 

harvested, most cool-season legumes set seed in the 

spring and remain relatively dormant throughout 

the summer, effectively mulching the soil surface. 

Harvesting stimulates new vegetative growth and 

a continued demand for limited available soil 

moisture during the summer. Lyons and others 

(1952) found evapotranspiration rates for annual 

legumes and forbs average about 4.5 inches of 

precipitation per ton of above ground biomass 

produced per acre. Within the walnut range, 

typical yields for forage legumes are 3 to 4 tons per 

cutting per acre indicating that regrowth following 

harvesting makes a significant demand on limited 

available soil moisture in the summer (Barnes and 

others 1995). 

Grass Sods

The growth of hardwoods when grown in grass 

sods is similar to that of hardwoods grown with 

resident vegetation (Figs. 1 and 2). Growth of 

walnut might be reduced more by grass sods 

than growth of other hardwoods. In studies with 

unmanaged control plots, growth of walnut is 21% 

less than growth in control treatments compared 

to only a 5% reduction for other hardwoods (79 

and 95% of resident vegetation or unmanaged 

control, respectively; t’ = -1.76, 40 df, p < 0.20ns). 

In studies with vegetation-free control plots, growth 

of walnut in grass sods is 47% less than growth in 

control treatments compared to only 32% reduction 

for other hardwoods (53 and 68% of vegetationfree control, respectively; t’ = -3.23, > 100 df, p 

< 0.01**). In these studies, growth of walnut in 

grass sods is slightly less than growth of walnut in 

resident vegetation (53 and 56% of vegetation-free 

control, respectively; t’ = -0.49, df = 40, p < 0.5ns). 

Similar growth differences exist for other hardwoods 

grown in grass sods compared to that of growth for 

other hardwoods in resident vegetation (68 and 63% 

of vegetation-free control, respectively; t’ = 1.24, > 

100 df, p < 0.40ns). 

The small increases or decreases in growth of walnut 

and other hardwoods compared to growth of trees 

in resident vegetation is partially due to differences 

among grass species (Table 2). Preliminary results 

suggest annual grasses such as cheat (Bromus 

secalinus L.), annual ryegrass (Lolium multiflorum

Lam.), and cereal grains are less competitive than 

perennial grasses. Perennial turf-type grasses such as 

red fescue (Festuca rubra L.) and Kentucky bluegrass 

(Poa pratensis L.) also tend to be less competitive 
Agroforestry and Nut Production 95

Black Walnut in a New Century

than perennial forage grasses such as orchard grass 

(Dactylis glomerata L.) and perennial ryegrass (Lolium 

perenne L.). Overall, tall fescue (Festuca arundinacea

Schreb.) is the only grass that, based on available 

data, statistically reduced growth of black walnut and 

other hardwoods below that of resident vegetation 

(32% and 60% of vegetation-free control; t’ = -7.14, 

> 100 df, p < 0.001***). Warm-season forage grasses 

should be avoided because they lack shade tolerance, 

vegetative growth occurs primarily when available soil 

moisture is low, and mowing in not recommended in 

the fall when nuts are to be collected in well-managed 

black walnut plantings (Lin and others 2000, Van 

Sambeek and others 2004).

Besides the reductions in potential tree growth, 

there are a number of other potential problems 

when using grass sods. Most grasses have 

extensive, finely branched root systems that can 

effectively explore greater volumes of soil than forbs 

and trees resulting in drier and nutrient poor soil 

profiles. Deep-rooted grasses like tall fescue exploit 

the same soil horizons as will walnut and can 

significantly reduce available soil nitrogen within 

the tree rooting zone. Van Sambeek and others 

(1989) reported that in stagnated walnut stands, 

tree growth was limited more by low available 

nitrate nitrogen than it was by competition for 

available soil moisture. Lyon and others (1952) 

reported average evapotranspiration rates for 

annual grasses of 4 inches of precipitation per ton 

per cutting. As with forage legumes, regrowth of 

grasses following cutting puts a significant demand 

on limited available soil moisture in the summer. 

Grasses that maintain their dominance through 

the production of phytotoxins such as tall fescue, 

smooth bromegrass, and broomsedge probably 

should also be avoided (Ponder 1986; Miller and 

others 1987; Rink and Van Sambeek 1985, 1987). 

There are a few advantages to managing grass 

sods in hardwood plantings. This plant cover is 

probably the best to slow surface run-off and 

increase water infiltration, especially on steep 

slopes. Because equipment and labor needs for 

mowing are less costly than for disking or tilling, 

grass sods cost less to maintain than legume cover 

crops (Jorgenson and Craig 1983). Mowing shifts 

the competitive advantage toward grasses that 

have their growing points at the soil line and, thus, 

grass sods are easier to maintain than legume cover 

crops. A sod cover provides year-round footing 

for equipment to complete maintenance including 

mowing, spraying, pruning, or harvesting. It may be 

desirable to establish grass sods near the end of the 

rotation to concentrate on hay and nut production 

and allow the wide growth rings within the sapwood 

to convert to the more desirable heartwood if walnut 

trees have been managed for veneer log production. 

Preliminary results from shade-tolerance screening 

trials indicate mixes of orchard grass or Kentucky 

bluegrass with red clover should produce 

acceptable yields in closed canopy walnut plantings 

(Tables 1 and 2).

Managing cool-season grass and legumes offers the 

opportunity to implement the agroforestry practice 

of silvopasture management. Although forage yields 

are variable, several studies have shown higher 

Table 2.—Average growth and standard deviation for black walnut and other 

hardwoods as a percentage of tree growth in plantings without ground covers 

and shade tolerance percentile rank for selected grass species.

Grass Sod Tree Growth1

%

Shade Tolerance2

%

Bromegrass 76 + 20 ( 10) 55

Annual cool-season 75 + 38 ( 8) 41

Red fescue 71 + 26 ( 77) 35

Bluegrass 69 + 36 ( 19) 58

Cereal grains 68 + 39 ( 15) ND

Orchard grass 65 + 24 ( 84) 58

Quackgrass 64 + 19 ( 2) ND

Timothy 64 + 14 ( 9) 37

Redtop 62 + 17 ( 4) 38

Resident vegetation 60 + 28 (156) --

Perennial ryegrass 62 + 15 ( 8) 36

Mixed grass sod 52 + 24 ( 11) --

Tall fescue 32 + 31 ( 93) 50

1

 Growth is percent of vegetation-free control + standard deviation and in () number of 

replications extracted from the literature. Mean least significant difference is 20%.

2 Average percentile rank under moderate and heavy shade estimated from three or more 

screening trials where 0% was assigned to the least and 100% was assigned to the most shade 

tolerant species within each light level and screening trial. ND = not tested.
96 Agroforestry and Nut Production

Proceedings of the 6th Walnut Council Research Symposium

protein content and increase digestibility of most 

forages when grown under moderate shade (Garrett 

and Kurtz 1983; Lin and others 1999, 2001; Huck 

and others 2001). Compared to open pastures with 

artificial shade structures, hardwood plantings 

alleviate heat stress better producing greater weight 

gains and result in more uniform grazing and 

waste deposits (Garrett and others 2004). Under 

silvopasture management, established trees must 

be protected from soil compaction and physical 

damage of roots near the soil surface through the 

use of rotational grazing and livestock removal 

during wet periods. Most silvopastoral studies 

have not examined effects of forage production and 

grazing on hardwood growth. Rotational grazing 

will stimulate regrowth of grasses increasing total 

annual biomass and expected evapotranspiration 

that will result in greater soil moisture depletion 

than may occur with grass sods that are not grazed 

or mowed. Although forage legumes can increase 

growth of hardwoods compared to grasses or 

resident vegetation, no studies were found that 

examined the growth of hardwoods with forage 

legume and grass mixes. We hypothesize that 

grasses will benefit more from the nitrogen fixed 

by the legumes than will the trees resulting in an 

improved livestock forage but even slower tree 

growth.

WOODY NURSE CROPS

Woody nurse crops are trees or shrubs introduced 

into a hardwood planting to improve growth and 

quality of the crop trees during the sapling and pole 

stages. The resulting growth of hardwoods grown 

with woody nurse crops is intermediate between 

the potential growth that occurs under treatments 

with no ground cover vegetation and that with cover 

crops or resident vegetation (Fig. 2). The increase 

in growth of walnut in mixed plantings averages 

22% compared to growth in pure walnut plantings 

usually managed with a ground cover of resident 

vegetation. The growth increase of other hardwoods 

in mixed plantings tends to be greater than for 

walnut, but is not statistically different (122 and 

138% of unmanaged control, respectively; t’ = -1.19, 

50 df, p < 0.30ns). Schlesinger and Williams (1984) 

found that the growth of walnut with woody nurse 

crops is highly dependent on the planting site and 

which nitrogen-fixing tree species were interplanted. 

The planting of other trees and shrubs that have 

relatively dense crowns with walnut will create 

a shady, cooler, more humid microenvironment 

more typical of that found in woodlands (Funk and 

others 1979, Burke and Pennington 1989). More 

soil moisture is made available to the trees because 

shade reduces surface evaporation and decreases 

evapotranspiration by eliminating most grasses and 

forbs. Mixed hardwood plantings will still result 

in more rapid and deeper depletion of available 

soil moisture than in pure walnut plantings with 

resident vegetation (Funk and others 1979). Lull 

and Fletcher (1962) reported average soil water 

depletion rates in the upper 20 inches of 0.099 

inches/day when mulched, 0.166 inches/day with 

resident vegetation, and 0.197 inches/day under 

trees. In contrast, soil water depletion rates in the 

20- to 40-inch depth averaged 0.018 inches/day 

when mulched, 0.039 inches/day with resident 

vegetation, and 0.102 inches/day under trees. 

A shaded microenvironment also moderates the 

extremes and fluctuations in soil temperature and 

frequently supports a wider range of beneficial 

soil fauna and flora than found in exposed soils. 

Although lateral branches still need to be pruned, 

creating a dense canopy results in death of most 

branches while still relatively small in diameter 

growth. Woody nurse crops may be the best 

alternative for ground cover management on sites 

not readily accessible to farm equipment such as 

those along meandering creeks and other riparian 

areas.

The major problem with using woody nurse crops 

in hardwood plantings is matching the growth of 

the woody nurse crops to the crop trees. Maximum 

benefits of planting woody nurse crops are usually 

achieved when the growth rate of the woody nurse 

crop is slightly greater than for the crop tree 

species. This can occur naturally with shrubs 

like autumn olive or culturally through repeated 

coppicing as has been demonstrated with black 

locust. For 2 to 4 years after establishment, mixed 

hardwood plantings will require some other method 

of ground cover management until the dense 

canopy of the nurse crop shades out the competing 

understory vegetation. Differences in sensitivity of 

hardwood species to herbicides may restrict which 

can be used to establish mixed plantings (Seifert 

and Woeste 2002). Compared to plantings with 

well managed ground covers, woody nurse crop 

plantings have an unkempt appearance (Burke 

and Pennington 1989). Pruning, thinning, and nut 

harvesting operations are more difficult in mixed 

plantings than in pure walnut plantings. Growers 

must make a decision balancing appearance and 

growth rate, but in plantings managed for timber 

where optimum growth and quality are the main 

objectives, creating a woods-like condition is 

probably the preferred method for ground cover 

management. 

Currently, the species of choice as a woody nurse 

crop in black walnut plantings is autumn olive 

(Elaeagnus umbellata Thunb.). Unfortunately, 

it is an invasive exotic species and most states 

discourage its planting. Research is needed to 
Agroforestry and Nut Production 97

Black Walnut in a New Century

identify native or less invasive shrubs or trees 

that produce dense canopies capable of shading 

out perennial resident vegetation and are capable 

of fixing atmospheric nitrogen (Van Sambeek 

and others 1985). European black alder (Alnus 

glutinosa (L.) Gaertn.), an actinorhizal tree, black 

locust (Robinia pseudoacacia L.), a nitrogen-fixing 

leguminous tree, and silver maple (Acer saccharum

Marsh.) need frequent pruning and/or coppicing 

to keep them from overtopping walnut (Schlesinger 

and Williams 1984, von Althen 1989). In a recent 

review of the nutritional interactions in mixed tree 

plantings, Rothe and Binkley (2001) concluded 

nutritional benefits of nurse crops are mostly 

additive; however, when using nitrogen-fixing nurse 

crops, they concluded benefits can be synergistic 

with greater than expected yields. Evaluation of 

recently established hardwood plantings under 

the Conservation Reserve Program may yield 

meaningful information in the future as these 

plantings now require a mix of hardwoods that can 

include walnut.

CONCLUSIONS

The vegetation management program chosen 

will be influenced by many factors including 

management objective (timber only, nuts only, or 

timber and nuts), site characteristics (location, 

slope, hydrology), type of planting stock (seed, bareroot seedlings, or large container stock), access 

to equipment (tractors, cultivators, spray rigs), 

and, possibly most importantly, labor availability. 

Ultimately, the primary objective of any program 

is to maximize nut production or growth of 

quality trees while minimizing establishment and 

maintenance costs. The primary objective of any 

ground cover management system is to reduce 

tree competition for water and nutrients, minimize 

labor and equipment costs, and create a ground 

surface that will support equipment needs for 

maintenance and nut collections. After considering 

the advantages and problems of the various 

vegetation management approaches, managers of 

walnut or mixed hardwood plantings should be able 

to develop the most appropriate strategies to meet 

their objectives for their particular plantings.

ACKNOWLEDGMENTS

We express our appreciation to Chris Starbuck, Felix 

Ponder, Jr., and the Ecology and Management of 

Central Hardwoods Project staff for their constructive 

comments on database development and early drafts 

of this paper. A portion of this effort was supported 

by the U.S. Department of Agriculture, Agriculture 

Research Service, Dale Bumpers Small Farms 

Research Center, Booneville, AR.

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Agroforestry and Nut Production 101

Black Walnut in a New Century

GIRDLING EASTERN BLACK WALNUT TO

INCREASE HEARTWOOD WIDTH

Larry D. Godsey, W.D. “Dusty” Walter, and H.E. “Gene” Garrett1

ABSTRACT—Eastern black walnut (Juglans nigra L.) has often been planted at spacings 

that require pre-commercial thinning. These thinnings are deemed pre-commercial 

due to the small diameter of the trees and the low ratio of dark wood to light wood. As 

a consequence of size and wood quality, these thinnings are often an expense rather 

than a source of revenue. In an effort to increase the value of these thinnings it would 

be beneficial to increase the ratio of dark wood to light wood. One way to increase the 

amount of dark wood is through costly processing using steam. However, several nonscientific studies have reported that dark wood can be increased by girdling small trees 

and allowing them to remain on the stump for a certain period of time. This study was 

designed to test this idea. In a black walnut plantation scheduled for thinning, 10 trees 

were randomly selected and double-girdled. At that time, increment cores were taken 

30.48 cm above the top girdle. These trees were allowed to remain on the stump for 21 

months before they were harvested. Results of this study will describe changes in dark 

wood content following the girdling treatment. The results will compare light to dark 

wood ratios of the initial increment cores to the boards sawn from the harvested logs. 

For many Missouri landowners, the idea of planting 

eastern black walnut (Juglans nigra L.) trees for 

the saw log or veneer market may seem like a poor 

investment, since it takes 60 to 80 years before a 

return is realized. From a financial perspective, the 

uncertainty and risk involved with a 60 to 80 year 

investment make it unfeasible. Although harvesting 

black walnut nuts can generate some cash flow, the 

prospect of selling marketable saw logs or veneer is 

often the deciding factor for choosing black walnut 

for planting.

In Missouri, markets for small diameter hardwood 

timber are being explored. By marketing smaller 

diameter trees, the investment period can be 

shortened considerably. Small diameter markets 

may work for native oaks (Quercus spp.); however, 

black walnut’s appeal in the market is due to its 

dark colored heartwood. Small diameter black 

walnut logs have a large amount of light colored 

sapwood in relation to the darker colored heartwood 

(Panshin and DeZeeuw 1980). Because of this ratio 

of light wood to dark wood, black walnut may be an 

unlikely candidate for the small diameter market. 

Although there are commercial methods, such as 

steaming (Chen and Workman 1980), that increase 

the amount of coloration in black walnut, these 

methods are often too expensive for small sawmills. 

The search is on for a low cost method of increasing 

the dark colored wood in small diameter black 

walnut. One method that has been rumored for 

many years is to kill the tree by girdling, and then 

leave it standing for a period of time. When the 

girdled tree is harvested, the light colored sapwood 

will have been darkened to match the color of the 

heartwood, thus increasing the quantity of useable 

wood (Chen and others 1997, 235). This study 

was designed to test the potential for increasing 

the amount of dark colored wood in black walnut 

through girdling. 

METHODS

This study was conducted at the Sho-Neff 

Plantation near Stockton, Missouri. Sho-neff 

consists of about 194.2 ha with 123.8 ha of eastern 

black walnut trees of various ages planted at 

1

Economist (LDG), University of Missouri Center for Agroforestry, 203 Anheuser-Busch Natural Resources Building, Columbia, MO 

65211; Senior Training Specialist (WDW), University of Missouri Center for Agroforestry; and Director (HEG), University of Missouri 

Center for Agroforestry. LDG is corresponding author: to contact, call (573)884-3216 or e-mail at godseyl@missouri.edu.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
102 Agroforestry and Nut Production

Proceedings of the 6th Walnut Council Research Symposium

various spacings. The plantation is divided into 25 

areas for research and management. 

In 2001, Hammon’s Products Company, owner 

of the Sho-Neff, conducted a thinning to remove 

approximately one-third of the plantation. Ten 

trees (Table 1) marked for thinning were chosen 

at random for this study from Area 16A of ShoNeff. These trees were native seedlings, planted in 

1976 at a spacing of 6 m x 12 m. Annual crops of 

soybeans, wheat, and milo had been grown between 

the rows of trees from 1977-1988. A traveling gun 

system irrigated the area for several years while 

crops were being grown. 

On April 4, 2001, these 10 trees from Area 16A were 

double girdled with a chainsaw. Girdling started 

at about 25.4 cm from the ground. Approximately 

a 7.62 cm space was left between each girdle. An 

increment core was taken at approximately 30.48 

cm above the top girdle at the time of girdling. The 

trees were left standing until January 2003. On 

January 24, 2003, the trees were felled, cut to 2.4 

m to 3 m lengths, hauled to the Horticulture and 

Agroforestry Research Center at New Franklin, 

Missouri and placed inside a storage shed. 

On July 8, 2003, a Woodmizer sawmill was used 

to cut the logs lengthwise. Slabs of about 2.54 

cm thickness were removed from the logs until 

the approximate center of the log was exposed. 

By cutting the logs lengthwise, any color change 

could be examined in terms of consistency along 

the length of the log. Photographs of the slabs 

were taken for reference purposes. The slabs were 

numbered the same as the logs. 

In order to analyze the logs, measurements were 

taken from the center of the log to the edge of the 

dark colored wood (C-D) and then from the center 

of the log to the beginning of the bark (C-B). These 

measurements were taken using a digital caliper, 

and were taken at about 30.48 cm above the top 

girdle on the half-log that was left after the slabs 

were cut off. Each log was measured twice, from the 

center out to the left and from the center out to the 

right. After measuring C-D and C-B on both sides 

of the log, an average estimate of the relationship 

between dark colored wood to total wood was 

calculated. Averaging the measurements in two 

directions out from the center allowed for any 

elliptical distortion in the proportion of heartwood 

to sapwood. 

The increment cores were also measured using a 

caliper and measured from the center to the edge of 

the dark wood (C-D), and from the center out to the 

beginning of the bark (C-B). Dark colored wood was 

calculated as a percent of total wood. A comparison 

of the C-D/C-B ratio from the increment cores was 

compared to the average C-D/C-B ratio from the 

logs to determine if any color change had occurred. 

RESULTS

To analyze color movement, changes in the 

percentage of dark colored wood to total wood 

were compared from the increment borer samples 

and the logs. At the time of the girdling, dark 

colored wood and heartwood were synonymous. 

However, after waiting the 22 months from the 

time of girdling, dark colored wood could be a 

combination of heartwood and wood that may be 

dark in color due to stain or various other reasons 

(e.g., movement of the dark heartwood color into the 

lighter colored sapwood) (Bamber and Fukazawa 

1985).

Table 2 shows the ratio of dark wood (C-D) to total 

wood (C-B) from the increment core readings taken 

at the time of girdling. Log VII had no reading 

because a portion of the increment borer sample 

was missing. This made it impossible to determine 

where the center of the log would have been. It is 

interesting to note the variance in the percentage 

of heartwood in trees that are of the same age. One 

of the trees had as much as 78% heartwood, while 

another had as little as 49%. Most of the trees had 

about 72% heartwood; however, the mean ratio was 

approximately 65% heartwood to total wood. The 

proportion of heartwood to total wood was inversely 

related to the DBH of the tree, which supports other 

studies on this topic (Nelson 1976). For example, 

the tree with 49% heartwood had the largest DBH. 

Table 1.—Location, reference number, and DBH for 

the 10 black walnut trees analyzed to measure 

colored wood movement after girdling and 

standing for 22 months.

Location Ref. Number DBH (cm)

Row 4 - #22 I 24.64

Row 4 - #18 II 24.89

Row 4 - #14 III 28.70

Row 2 - #16 IV 23.37

Row 2 - #18 V 27.18

Row 2 - #20 VI 25.91

Row 2 - #22 VII 27.43

Row 2 - #25 VIII 22.61

Row 3 - #19 IX 26.42

Row 3 - #24 X 26.92
Agroforestry and Nut Production 103

Black Walnut in a New Century

Caliper measurements taken from the cut logs after 

the trees were allowed to stand for 22 months are 

shown in Table 3. Those measurements show that 

the percent of dark colored wood ranged from 49% 

to 86%. The mean ratio of dark colored wood to 

total wood is approximately 73%. 

DISCUSSION

The purpose of this study was to determine if 

girdling and leaving trees standing for a period 

of time would provide a cheap, effective way to 

increase the colored wood content in small diameter 

eastern black walnut trees. This idea stemmed from 

many discussions regarding the marketing of trees 

grown in agroforestry configurations that require 

thinning prior to the trees reaching a commercial 

size.

This study was an attempt to answer two main 

questions. First, does killing a black walnut tree 

by girdling and allowing it to dry on the stump 

increase the amount of colored wood? Second, what 

effect does this method have on the market value of 

the small diameter eastern black walnut log? 

Looking at the change in dark wood to total wood 

ratios (Table 4) for both the increment borer 

samples taken at the time of girdling and the log 

readings taken 22 months later, it is evident that 

some of the logs experienced significant increases 

Table 2.—Log number, increment borer reading taken 

at the time of girdling at approximately 30.48 cm 

above the girdle, and ratio of heartwood to total 

wood for each log girdled and let stand for 22 

months. 

Log

Number

Increment Borer

% D-B3 C-D1 (cm) C-B2 (cm)

I 7.8 10.9 71.56%

II 7.1 9.95 71.36%

III 6.7 13.65 49.08%

IV 7.4 9.5 77.89%

V 8.1 11.2 72.32%

VI 7.0 12.5 56.00%

VII No reading

VIII 7.3 10.2 71.57%

IX 6.25 10 62.50%

X 5.3 10.5 50.48%

1

C-D: dark colored wood (heartwood) width in cm measured 

from center of stem to edge of dark colored wood. 

2

C-B: total width in cm measured from center of stem to 

inside of bark. 

3

% D-B: ratio of heartwood width to total width in cm.

Table 3.—Log number, colored wood widths; and 

average dark wood to total wood ratios measured 

from the center of the log to inside the bark on 

both sides of the stem 22 months after girdling.

Log

Number

Log Readings

C-D1 C-B2 % D-B3 Avg. %

I 8.84 10.24 86.33% 86.37% 9.6 11.11 86.41%

II 6.81 10.21 66.70% 67.12%

7.45 11.03 67.54%

III 6.61 13.8 47.90% 49.48%

5.75 11.26 51.07%

IV 6.73 8.68 77.53% 77.48%

7.17 9.26 77.43%

V 10.26 11.71 87.62% 80.67%

9.93 13.47 73.72%

VI 10.16 10.49 96.85% 80.77%

7.86 12.15 64.69%

VII 10.38 13.9 74.68% 72.52%

8.88 12.62 70.36%

VIII 7.01 8.96 78.24% 76.48%

7.83 10.48 74.71%

IX 7.94 10.73 74.00% 69.09%

6.61 10.3 64.17%

X 7.25 10.8 67.13% 65.80%

7.13 11.06 64.47%

1

C-D: dark colored wood width in cm measured from center 

of stem to both edges of dark colored wood.

2

C-B: total width in cm measured from center of stem to 

inside of bark on both sides of the cut log.

3

% D-B: ratio of dark wood width to total width 

in the amount of dark wood content. Logs VI, I, 

and X showed the most change in dark wood to 

total wood ratio. Log VI had a change in colored 

wood of nearly 25%, whereas logs I and X had a 

change of approximately 15%. The remainder of 

the logs showed very little change in the amount of 

colored wood, with some logs even showing a loss in 

colored wood percentage. This may be attributed to 

shrinkage. 

Seven of the nine logs that were compared showed 

at least a slight increase in the proportion of dark 

wood to total wood. However, out of the seven logs 

showing an increase in colored wood, none of the 

logs had consistent color change throughout the 

length of the log. In fact, the color increase in log VI 

was only present in an area approximately 33 cm 

long and only on one side of the log. This significant 

increase in colored wood was in the area of the 
104 Agroforestry and Nut Production

Proceedings of the 6th Walnut Council Research Symposium

log where bark had been separated and fallen off 

during the early stages of the girdling treatment. 

The inconsistency in the color along the length of 

the log leads to the second question regarding the 

economic potential for this type of treatment. If the 

color is inconsistent along the length of the log, 

then the log is no more valuable than it was before 

the treatment was applied. Likewise, the log may 

be less valuable because the coloration may be 

considered a defect. 

Also, although several of the logs showed an 

increase in colored wood, this same increase could 

possibly have been experienced if the trees were 

allowed to grow for another 22 months. These 

trees were growing at a rate of approximately 1 

cm per year, indicating that they could have been 

increasing heartwood at a rate of 1 cm per year as 

well. It is noted that tree growth and heartwood 

formation may only have a correlation coefficient 

of 0.56 (Bamber and Fukazawa 1985). Table 5 

shows the amount of heartwood measured on the 

increment borer samples compared to the amount 

of dark wood measured in both directions from 

the center of the logs 22 months after girdling. An 

average change in dark wood content is calculated 

for each log. 

Three of the logs averaged about a 2 cm increase in 

colored wood. However, looking at log VI, there was 

7 cm of heartwood at the time of girdling and 22 

months later the log had 7.86 cm measuring from 

one side of the log center and 10.16 cm measuring 

from the other side. It is unclear from which side 

of the log the increment bore sample was taken: 

however, the side that had 10.16 cm of dark wood 

was the side that had significant bark loss. The 

other side had only gained .86 cm in dark wood. 

Dividing that growth rate by nearly 2 years makes 

that a 0.43 cm growth in dark wood per year. 

Another visible factor affecting marketability of the 

logs was the distinct presence of insect damage to 

the logs. All the logs that had significant damage to 

the bark had significant insect damage. Stain, or 

spalt, was another problem with the logs. The logs 

had been stored in a dry, protected building after 

they were felled; however, some of the logs showed 

dark stains. 

In conclusion, based on this study, it appears 

that increasing dark colored wood in eastern 

black walnut by girdling the trees and leaving 

them standing does not increase the marketability 

of the log or the market value of the tree. Any 

abnormal increases in dark wood formation are 

Table 5.—Average change in dark wood content from 

the time of girdling (Increment borer) to the time of 

cutting 22 months later (Log).

Log 

Number

Dark Wood (cm)

Change1

(cm)

Avg 

Change 

(cm)

Increment 

Borer Log

I 7.8 8.84 1.0 1.4 9.6 1.8

II 7.1 6.81 -0.3 0.0 7.45 0.4

III 6.7 6.61 -0.1 -0.5 5.75 -1.0

IV 7.4 6.73 -0.7 -0.5 7.17 -0.2

V 8.1 10.26 2.2 2.0 9.93 1.8

VI 7.00 10.16 3.2 2.0 7.86 0.9

VII No 

Reading

10.38 -- ----- 8.88 --

VIII 7.3 7.01 -0.3 0.1 7.83 0.5

IX 6.25 7.94 1.7 1.0 6.61 0.4

X 5.3 7.25 2.0 1.9 7.13 1.8 1

Log measurement minus increment borer measurement from 

both sides of the stem.

Table 4.—Comparison of dark wood to total wood 

ratios at the time of girdling (Increment borer) and 

22 months later (Log). 

Log 

Number

Dark Wood to Total 

Wood Ratio

Color 

Change

Increment 

Borer Log

I 71.56% 86.37% 14.81%

II 71.36% 67.12% -4.24%

III 49.08% 49.48% 0.40%

IV 77.89% 77.48% -0.41%

V 72.32% 80.67% 8.35%

VI 56.00% 80.77% 24.77%

VII No reading 72.52% ---

VIII 71.57% 76.48% 4.91%

IX 62.50% 69.09% 6.59%

X 50.48% 65.80% 15.32%
Agroforestry and Nut Production 105

Black Walnut in a New Century

counterbalanced with increased risk of insect 

infestation and stain formation. Steaming the logs 

to increase dark wood formation would be the 

recommended method of increasing marketable 

wood. More research should be directed at finding 

cheaper methods of steaming and/or utilization 

of small diameter black walnut logs including the 

sapwood. 

ACKNOWLEDGMENTS

This work was funded through the University of 

Missouri Center for Agroforestry under cooperative 

agreement AG-02100251 with the U.S. Department 

of Agriculture, ARS Dale Bumpers, Small Farms 

Research Center, Boonville, AR. The results 

presented are the sole responsibility of the P.I. 

and/or MU and may not represent the policies or 

positions of the ARS. 

LITERATURE CITATIONS

Bamber, R.K.; Fukazawa K. 1985. Sapwood and 

heartwood: a review. Forest Products Abstracts. 

8(9): 265-278.

Chen, P.Y.S.; Stokke D.D.; Van Sambeek, J.W. 

1997. Processing small quantities of walnut 

logs for lumber and craft items. In: Van 

Sambeek, J.W., ed. Knowledge for the future 

of black walnut: 5th Black walnut symposium; 

1996 July 28-31; Springfield, MO. Gen. Tech. 

Rep. NC-191. St. Paul, MN: U.S. Department 

of Agriculture, Forest Service, North Central 

Forest Experiment Station: 233-235.

Chen, P.Y.S.; Workman, E.C., Jr. 1980. Effect 

of steaming on some physical and chemical 

properties of black walnut heartwood. Wood 

and Fiber. 11(4): 218-227.

Nelson, N.D. 1976. Gross influences on heartwood 

formation in black walnut and black cherry 

trees. Res. Pap. FPL 268. Madison, WI: U.S. 

Department of Agriculture, Forest Service, 

Forest Products Laboratory.

Panshin, A.J.; DeZeeuw C. 1980. Textbook of wood 

technology: structure, identification, properties, 

and uses of the commercial woods of the United 

States and Canada, 4th ed. New York, NY: 

McGraw-Hill Book Company.
106 Agroforestry and Nut Production

Proceedings of the 6th Walnut Council Research Symposium

USE OF HERBICIDE TO REDUCE STUMP-SPROUTING FOLLOWING THINNING

OF AN EASTERN BLACK WALNUT AGROFORESTRY PLANTING

W.D. “Dusty” Walter, H.E. “Gene” Garrett, and Larry D. Godsey1

ABSTRACT—When establishing an agroforestry practice, the number of trees planted 

will often exceed the densities needed to achieve a final spacing or configuration. While 

tight spacings may facilitate certain growth parameters, such as height development, 

timely thinnings of plantings are required in order to maintain desirable growth rates. 

In managed plantations especially, the stump sprouts that often result from thinnings 

may be deemed undesirable for aesthetic reasons, and inhibit other management 

activities, such as harvesting nuts and mowing. By applying select herbicides to the 

stump surface following thinning operations, sprouting that often develops can be 

eliminated. This study was established during the thinning of an eastern black walnut 

(Juglans nigra L.) plantation, with stumps treated immediately following tree felling. 

Four treatments, including 3 herbicides (Garlon 3A®, Banvel® and Roundup®) and a 

control (no stump treatment), were applied in a randomized complete block design to 36 

stumps. Treatments were applied during April of 2001, with damage to adjacent crop 

trees assessed later that same growing season. For 2 years following stump treatment, 

sprouts were counted. Garlon 3A® provided the best results with no sprouting observed 

over the 2 year study period.

INTRODUCTION

Of all the hardwood species, eastern black walnut 

(Juglans nigra L.) is one of the most likely to be 

planted in a monoculture (single species) plantation 

setting. As a tree with production opportunities 

for growing both nuts and timber of high value, it 

has been widely planted in agroforestry practices 

that seek to realize the potential of a given land 

area for diversified production, as well as resource 

stewardship. However, embodied within the goals of 

stewardship and productivity is the maintenance of 

healthy trees and forests. 

When striving to promote healthy forest stands, 

and optimize productivity, timely thinnings become 

invaluable tools for forest managers. In hardwood 

forest stands, thinnings are usually accomplished 

by either mechanical means that include cutting 

or girdling tree stems, by chemical release using 

injection and/or basal bark spray treatments, or 

by some combination of the two that will ensure 

removal of select stems and prevent further 

regrowth. The select removal of undesirable woody 

plants represents a low-impact, cost-effective 

means of forest thinning that may also be viewed as 

being more environmentally friendly than general 

broadcast treatment methods. 

Today’s forestry herbicides are rigorously tested 

in order to meet strict standards of environmental 

safety and human health protection. Further, most 

active ingredients in forestry herbicides are below 

lethal-dose levels associated with many household 

chemicals, food additives, and nonprescription 

drugs (McMahon and others 1994). However, 

for safety and to maximize the effectiveness of 

the herbicide on the intended target plant, label 

recommendations should always be followed. 

To best ensure the realization of forest management 

goals and optimize the use of personnel time, 

thinnings should be designed to reduce the 

competition between trees for limited site resources 

(light, moisture, and nutrients). Thinnings that do 

not eliminate resource competition do not optimize 

the investment of time, or the likelihood of achieving 

management goals associated with a thinning 

1

Senior Research Specialist (WDW), University of Missouri Center for Agroforestry, 203 ABNR, Columbia, MO; Director (HEG), School 

of Natural Resources, University of Missouri Center for Agroforestry, Columbia, MO; Economist (LDG), University of Missouri Center 

for Agroforestry, Columbia, MO. WDW is corresponding author: to contact, call (573) 884-7991.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
Agroforestry and Nut Production 107

Black Walnut in a New Century

operation. This can occur when trees identified for 

removal during a thinning practice either, do not 

suffer loss of upper-stem growth, have upper-stem 

die-back but resprout from the stump, or the main 

stem is killed but the tree suckers from the root 

stock. In all cases, the competition for light may 

have been eliminated, yet competition for moisture 

and nutrients continues. This does not optimize 

the outcomes associated with thinning activities. 

Proper application of herbicides can effectively 

minimize the likelihood that thinned trees remain in 

competition for onsite resources.

This study was designed to evaluate the 

effectiveness of three herbicides in eliminating 

stump sprouting following the conventional 

thinning of a black walnut plantation. Following 

tree felling with a chainsaw, cut surface application 

of three herbicide formulations occurred. 

Study Site

Located in Southwest Missouri, the Sho-Neff Black 

Walnut Farm began planting eastern black walnut 

in 1975. It is currently owned and managed by the 

Hammons Products Company of Stockton, MO. The 

farm totals 480 acres that are divided into 25 areas. 

The stump treatment study was applied in area 16B 

which was established in 1976. With the primary 

goal of nut production, trees were planted at an 

initial spacing of 20 x 40 feet. Agroforestry was 

practiced on the site in years 1 through 11, with 

plantings of soybean, wheat and milo in the 40-foot 

alleyways. A thinning was conducted in 1998-1999 

to remove inferior trees, with an additional thinning 

occurring in 2000 to maintain growth rates on the 

residual trees.

METHODS

Four treatments were assessed for their 

effectiveness in minimizing stump sprouting 

following thinning. These included (1) a control with 

no chemical stump treatment, (2) Garlon 3A® having 

44.4% Triclopyr as the active compound, (3) Banvel®

having 48.2% Dicamba as the active compound 

and (4) Roundup® having 41% Glyphosate as the 

active compound. Herbicide sprays were applied 

full strength in order to test the maximum effect 

that each would have on stump sprouting and the 

growth of adjacent trees.

Located across 7 rows (40 feet between rows) within 

a black walnut plantation, 36 trees were cut in 

2001, and 1 of the 4 sprouting control treatments 

applied. Trees were cut with a chainsaw, and 

application of the treatment occurred immediately 

following each cut. Each stump was cut low to the 

ground, at an approximate height of no greater 

than 3 inches. Herbicide was applied to the outer 

2 inches of each stump surface. The herbicides 

were applied using an adjustable spray bottle that 

held 1 quart of chemical. Using a similar setting 

between stream and mist, the outer 2 inches of 

each stump was covered in chemical. At the time 

of thinning, the DBH (diameter at breast height, 

4.5 feet above ground line) of each study tree was 

measured. Study trees ranged in size from 6.6 to 

12.3 inches in DBH across all treatments. However, 

the difference in mean DBH within each treatment 

varied by approximately 1 inch or less (Table 1).

Using a randomized complete block design, 

treatments were applied to the trees marked for 

thinning. As many replicates as possible were applied 

within each tree row. Each cut-stump treatment was 

applied a total of nine times over seven rows.

Table 1.—Comparison of the mean number of stump sprouts during years 1 and 2 for walnut stumps treated with 

Garlon 3A®, Banvel® and Roundup® or left untreated (Control).

Cut-Stump Treatment Active Ingredient

Mean DBH

of Treated Stumps

Mean Number of Stump Sprouts

Year 1 Year 2

Control no chemical treatment 10.47 5.7A 5.7A

Garlon 3A® 44.4% Triclopyr 10.92 0B 0C

Banvel® 48.2%Dicamba 9.83 2.3B 2.8B

Rounup® 41% Glyphosate 10.56 0.2B 2.9B

* Mean number of sprouts followed with the same letter within a given year are not significantly different at alpha = 0.05 as 

compared by Duncan’s Mutliple Range Test. 
108 Agroforestry and Nut Production

Proceedings of the 6th Walnut Council Research Symposium

First in July of 2001 and again in January 2003, 

sprouts from the stump of each tree were counted 

and the height of the tallest sprout measured. 

Statistical analyses were conducted using SAS 

(1999) to determine whether 1 stump treatment 

was superior to the others at reducing the number 

of stump spouts. Means of the number of sprouts 

occurring in year 1 and year 2 were compared using 

Duncan’s Multiple Range Test at alpha = 0.05. By 

using General Linear Model (GLM) the influence 

of chemical and DBH of thinned trees was also 

assessed. 

RESULTS

At the end of years 1 and 2, the number of sprouts 

occurring on each stump was counted. The control 

treatment (no herbicide applied to the cut surface) 

had the greatest number of sprouts in both years 1 

and 2, with a range of 2-18 sprouts in year 1 and 

2-10 sprouts in year 2. The stump with the most 

spouts occurred within the control treatment. A 

9.3-inch DBH stump produced 18 sprouts in year 

1. By year 2, the most sprouts counted on a single 

stump was 10, and they also occurred on a control 

stump. By comparison, Garlon 3A® had the fewest 

sprouts. In both years, zero-sprouts were observed 

across all stumps. 

In year 1 no significant differences were found 

between the three treatments receiving herbicides, 

but all three were significantly different from the 

control (no herbicide) (Table 1). However, by year 2 

treatment differences, as measured by the number 

of sprouts, had changed. The mean number of 

sprouts per stump increased in both the Banvel®

and Roundup® treatments resulting in Garlon 3A®

having significantly fewer sprouts than all other 

treatments (Table 1).

In measuring the influence of chemical treatment 

and DBH on the propensity of a treated stump 

to sprout, chemical treatment exerted greater 

significance. The GLM (General Linear Model) of 

stump sprouting based on chemical treatment 

and DBH had an R-square of 0.52, indicating 

that all variance is not accounted for by the two 

independent variables. However, within the model, 

the chemical treatment was significant at a 0.99% 

confidence level. 

DISCUSSION

Properly applied herbicide’s can be an effective 

tool for maximizing the benefits from a thinning 

operation. Numerous studies have examined 

the use of chemicals for their effectiveness when 

applied as cut-surface, injection and basal sprays. 

Thomas and others (1988) used several cut-surface 

treatments on sugar maple stumps in an effort 

to eliminate sprouting. He identified a change 

over time in the number of sprouts, with stumps 

that initially appeared dead (without sprouts), 

developing sprouts during the second year. Tordon 

RTU® and Garlon 3A® were identified as maintaining 

good control of sprouts for 2 years in the study 

(Thomas and others 1988). 

Other studies, including those by Miller (1993) and 

Van Sambeek and others (1995), have identified the 

effectiveness of injection and basal spray treatments 

as measured by crown reduction and/or tree 

mortality. These studies have identified differences 

in effectiveness based on species, and diameter 

within a species. Their studies tested a variety of 

chemicals and chemical rates, with varied success. 

Pannill (1997) expressed dissatisfaction with 

the results of the hack-and-squirt method when 

applying Roundup®. He identified an associated 

mortality of 75% of treated trees.

Additionally, in plantings of like species trees, there 

should always be a concern with flashback when 

applying herbicides to thin trees in plantations. 

Flashback is the unintended negative impact of 

chemical application on trees adjacent to those 

treated during the thinning process. This occurs 

when a chemical translocates from the stem into 

the root system and via root grafting, moves into an 

adjacent tree. Often flashback is first evidence in 

the dieback or yellowing of the foliage of adjacent 

trees. Our study identified flashback in two 

separate cases. One resulted from a stump treated 

with Banvel® and was evidenced by approximately 

20% crown dieback of the adjacent tree. The second 

occurred with Roundup® and caused 30-40% crown 

dieback. Both trees recovered, but when a thinning 

is designed to enhance growth and development of 

released crop trees, any flashback is undesirable. 

Only the control and Garlon 3A® treatments did not 

result in any occurrences of flashback. 

Our study demonstrates significant differences 

between herbicides in controlling walnut stump 

sprouting. Furthermore, it is clear from our results 

that 1 year is an insufficient time frame within 

which to evaluate the true effects of herbicides. 

While all three herbicides tested were found to 

significantly reduce sprouting after 1 year from 

that observed when no herbicide was applied, no 

significant differences were found between the 

herbicides. However, as a result of the recovery of 

stumps treated with Banvel® and Roundup®, both 

had significantly more sprouts after 2 years than 

were observed on stumps treated with Garlon 3A®. 
Agroforestry and Nut Production 109

Black Walnut in a New Century

Clearly, Garlon 3A® was the superior herbicide 

tested in this trial and a minimum of 2 years 

following the application of the chemical were 

required to make a valid comparison.

LITERATURE CITED

McMahon, C.K.; Miller, J.H.; Thomas, D.F. 1994. 

The role of low impact herbicide treatments in 

ecosystem management. In: Foley, Louise H., 

comp. Silviculture: from the cradle of forestry 

to ecosystem management, proceedings of the 

National Silviculture workshop; 1993 November 

1-4; Hendersonville, NC. Gen. Tech. Rep. SE-88. 

Asheville, NC: U.S. Department of Agriculture, 

Forest Service, Southeastern Forest Experiment 

Station: 242-251.

Miller, J.H. 1993. Tree injection of sweetgum 

(Liquidambar styraciflua L.) with Imazapyr, 

Triclopyr, and Picloram plus 2,4-D. In: Weed science 

in harmony with the environment: 46th annual 

meeting of the Southern Weed Science Society; 

1993 January 18-20; Charlotte, NC. Champaign, IL: 

Southern Weed Science Society: 194.

Pannill, P. 1997. Thinning by injection. Walnut 

Council Bulletin. 24(2): 12.

SAS Institute Inc. 1999. SAS/STAT OnlineDoc, 

Version 8. Cary, NC: SAS Institute Inc.

Thomas, M.W.; Garrett, H.E.; Pallardy, S.G. 1988. 

Susceptibility of sugar maple and oak species 

to chemical control in Missouri’s oak-hickory 

forests. In: Kaufman, J.I., Westerdayl, H.E., eds. 

Chemical vegetation management; proceedings 

of a symposium organized by the Plant Growth 

Regulator Society of America; San Antonio, 

Texas; August 1, 1988. 

Van Sambeek, J.W.; Kia, D.A.; Shenaut, D.B. 1995. 

Chemical release of pole-sized trees in a central 

hardwood clearcut. In: Proceedings, 10th Central 

Hardwood Forest conference; 1995 March 5-8; 

Morgantown, WV. Gen. Tech. Rep. NE-197. 

Radnor, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment 

Station. 152-157.
110 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

DISEASES OF INTENSIVELY MANAGED EASTERN BLACK WALNUT

Manfred E. Mielke and Michael E. Ostry1

ABSTRACT—Eastern black walnut has few serious disease problems in its natural 

woodland setting. However, trees in plantations are subjected to various cultural 

activities that can create stand conditions that increase pathogen populations and 

abiotic injuries that often interfere with landowner’s objectives.

Native diseases are largely a preventable threat 

to stands of eastern black walnut (Juglans nigra). 

Eastern black walnut grows naturally in small 

groups or scattered individuals on a variety of sites 

associated with assorted tree species of the central 

hardwood zone. It grows best on well-drained 

bottomland soils and in coves from the Midwest to 

the Appalachians. However, when large numbers 

of eastern black walnut are grown together and 

managed in plantations, poor site conditions, stress 

caused by weed competition, soil compaction, 

injuries from equipment, improper pruning, and 

wildlife, and adverse microclimatic conditions can 

predispose trees to fungal pathogens.

NURSERIES

Nurseries epitomize intensive management. The 

high density of seedlings in production beds and 

related microclimate conditions leads directly 

to potential high incidences of foliage and stem 

diseases and the buildup of inoculum in soil 

and seed borne pathogens. These conditions 

and pathogens can result in seedling mortality, 

weakened seedlings, or the inadvertent planting 

of infected nursery stock, that in turn can lead to 

plantation failure.

Root Diseases

Several root diseases have been reported that either 

kill nursery-grown seedlings outright or cause 

seedling losses in storage, or during shipping and 

handling. Disease incidence depends primarily on 

high soil moisture and the presence of pathogens 

in the nursery beds or adjacent areas. The fungi 

Phytophthora citricola, P. cactorum, Fusarium 

episphaera, Cylindrocarpon radicicola, C. candidum, 

Pythium vexans, and Cylindrocladium sp. have all 

caused seedling mortality (Berry 1973, Green and 

Ploetz 1979, Roth and Griffen 1979). Fumigation or 

treatment using soil chemicals can have short-term 

benefits, but inevitably beds are re-infested from 

nearby affected areas. Reducing standing moisture 

by growing seedlings in well-drained beds may 

minimize losses (Berry 1973, Kessler 1982).

Seed Borne Pathogens

Recently in Canada, eastern black walnut and 

butternut (J. cinerea) seed that were apparently 

infected by the butternut canker fungus (Sirococcus 

clavigignenti-juglandacearum) were sown in 

nursery beds resulting in cankers developing on 

the seedlings of both walnut and butternut. The 

entire crop of both species was destroyed to prevent 

long distance spread of inoculum by shipping the 

infected planting stock. The butternut canker 

fungus has been reported on mature eastern black 

walnut in natural stands, but the fungus has not 

caused as severe damage as on butternut (Ostry 

and others 1997).

PLANTATIONS

Walnut trees growing in plantations and the 

application of various cultural activities used in 

managing these trees can create stand conditions 

that increase pathogen populations and abiotic 

injuries that are often not present in natural stands 

so disease incidence and severity is usually greater 

in plantations.

1Plant Pathologist (MEM) USDA FS, 1992 Folwell Ave., St. Paul, MN 55108 and Research Plant Pathologist (MEO) USDA FS, 1992 

Folwell Ave., St. Paul, MN 55108. MEM is the corresponding author: to contact, call 651-649-5267 or email at mmielke@fs.fed.us

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
Walnut Pest Management 111

Black Walnut in a New Century

Stem Diseases

Most canker fungi need some type of opening in 

the bark in order to invade and infect susceptible 

tissues of trees. Wounds result from a variety of 

management activities, by wildlife and from several 

abiotic causes. Preventing wounds can minimize 

serious damage caused by many fungi. A few 

examples of walnut stem diseases; their cause and 

ways to minimize tree damage are discussed in the 

next sections.

Wounds Caused by People

Any open wound caused by equipment and 

improperly timed or poorly made pruning cuts can 

lead to infection by Fusarium spp., the cause of the 

annual walnut canker, or infection by the perennial 

target canker fungus Nectria galligena (Berry 1973, 

Cummings and Kuntz 1985, Cummings and others 

1993). Pruning should be done properly (removing 

branches outside the branch collar area taking care 

to not tear bark below the collar) during the late 

winter, although anytime during the leaf-off period 

is acceptable (Bedker and others 1995). 

Slow tree growth caused by grass competition 

results in inadequate or delayed natural pruning 

and may prevent rapid closing of pruning wounds. 

Herbicides will control weeds but soil compaction and 

mechanical wounds to trees associated with chemical 

applications may occur. Care should be taken to avoid 

operating equipment in the field during wet weather 

or in early spring when soils may be saturated to 

avoid soil compaction and root damage. Obviously, 

bumping trees with equipment and otherwise 

scraping off bark should always be avoided. 

Wounds Caused by Wildlife

Damage to tree bark caused by deer rubbing their 

antlers and pecking of the bark by birds can cause 

tree damage directly as well as lead to infection by 

canker organisms. There are myriad products that 

claim to protect trees from deer damage. Various 

chemicals or physical barriers such as tree shelters 

can yield variable results. Experiment with a variety 

of materials to see what works best in your area. 

There is little that can be done to protect trees from 

being fed on by the yellow-bellied sapsucker. It is a 

protected songbird and in large numbers can cause 

widespread damage. A possible approach is to leave 

trees in stands that have been attacked previously 

since the birds seem to be attracted to previously 

attacked trees, thereby minimizing damage to 

remaining trees.

Wounds Caused by Weather Extremes

In the northern extent of its range, and elsewhere 

in cold air drainages during severe winters, freeze 

damage causes dieback and mortality of the above 

ground portions of black walnut, often resulting in 

infection by Fusarium spp. causing annual cankers 

(Cummings and others 1993, Hayes 1995). This 

fungus is soil-borne and gains entrance through 

breaks in the bark anytime during the growing 

season.

In most cases, little can be done to protect trees 

from breakage caused by wind, snow and ice. 

However, timely corrective pruning can help trees 

survive and retain acceptable crown architecture 

while protecting them from breakage and 

subsequent entry by decay fungi.

Decay organisms will eventually infect aging trees, 

and trees with old branch stubs, large open wounds, 

and cankers. Eastern black walnut is host to several 

heart rot fungi. The two most common are Phellinus

igniarius and Laetiporus sulphureous (Berry 1973). 

Maintaining tree vigor and preventing wounds is the 

best way to avoid damage by decay fungi. 

FOLIAGE DISEASES

Foliage diseases are the most serious diseases of 

plantation eastern black walnut. Factors that lead 

to infection and increased disease incidence and 

severity are high humidity (> 98%), free moisture on 

leaves (rain, dew, fog, or from irrigation), low light 

intensity, and temperatures around 21o C. There 

are four common leaf diseases of black walnut, 

Microstroma white mold, bull’s-eye leaf spot, 

Mycosphaerella leaf spot, and walnut anthracnose. 

The most common and most serious of these is 

walnut anthracnose (Weber and others 1980). All 

but Microstroma white mold can cause premature 

leaf loss resulting in reduced growth, increased 

susceptibility to other diseases, and reduced 

quantity and quality of nuts. 

All of these fungal diseases have a similar biology. 

Primary infections occur in the spring, May through 

early June, from ascospores emerging from fruiting 

bodies on over-wintered leaves on the ground. Once 

primary infection occurs, lesions appear on leaves 

within a couple of weeks. Subsequent secondary 

infections occur on leaves throughout the summer 

by wind-dispersed spores (conidia) produced in these 

lesions. Production of spores increases gradually 

during July and reaches maximum numbers in 

August. The infected, fallen leaves serve as the 

reservoir for next year’s inoculum, completing the 

annual life cycles of these fungi.
112 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

Microstroma White Mold

Microstroma white mold (or downy leaf spot), 

caused by Microstroma juglandis, is more unsightly 

than damaging. This disease does not kill the leaf 

and is not known to cause defoliation. The effect to 

the tree is minimal, probably resulting in a minor 

reduction in photosynthesis. Symptoms are a 

yellowish discoloration on the upper surface of the 

leaf, and a whitish growth on the underside of the 

leaf, often concentrated along the veins. 

Bull’s-Eye Leaf Spot

Bull’s eye leaf spot (or zonate leaf spot); caused 

by Grovesinia pyramidalis (asexual state = 

Cristulariella moricola) causes leaf spots and 

premature defoliation. Maple, hickory and many 

common weeds are also infected. Symptoms are 

unique in that the dark lesions on leaves are 

rounded and have concentric white rings, giving the 

spot a target-shaped appearance, hence the name 

bull’s-eye leaf spot.

Mycosphaerella Leaf Spot

Mycosphaerella leaf spot, caused by Mycosphearella

juglandis (asexual state = Cylindrosporium juglandis) 

causes leaf spots that are angular, reaching a 

maximum size as large as 4 mm in diameter. By 

midsummer, lesions from secondary infections result 

in affected trees becoming chlorotic. Coalescing 

lesions produce a vein pattern or a leaf scorch 

symptom. By late summer, especially in dry weather, 

severely diseased leaves fall. 

WALNUT ANTHRACNOSE

The most serious and widespread leaf disease of 

black walnut is walnut anthracnose, caused by 

Gnomonia leptostyla (asexual state = Marssonina

juglandis). The disease results in leaf spots ranging 

in size from a few mm to 1.25 cm in diameter. Dark 

spots first appear on the leaf blades and petioles 

in the spring. The leaves infected early in the year 

are most important for photosynthesis during the 

critical period of nut formation and severe disease 

can cause reduced yield and nut crop failure. 

The fungus may infect the nuts directly, causing 

nutmeats to shrivel and darken. The older leaves in 

the lower and interior portions of the tree crown are 

most severely affected, show the most chlorosis and 

are the first to fall prematurely. 

Adjacent trees in plantations may exhibit varying 

levels of disease, indicating existence of natural 

resistance to anthracnose. This resistance is highly 

heritable, with provenances from the relatively arid 

western edge of the natural range of black walnut 

(Kansas and Oklahoma) being most susceptible. 

This is likely due to the limited natural selection for 

anthracnose resistance in this region since disease 

incidence is much lower under arid conditions.

When evaluating differences among cultivars in 

susceptibility to anthracnose one must take into 

account the nut load (William Reid, personal 

communication). This is because non-fruiting 

shoots have more terminal leaves and these leaves 

have fewer anthracnose lesions because they 

matured later in the season after the primary 

infection period for the fungus had passed. Thus, 

a tree producing a large quantity of nuts will 

look more susceptible to anthracnose than a tree 

producing few or no nuts.

Control of Leaf Diseases

Most of the research on control and management 

of foliar diseases of black walnut has been directed 

toward walnut anthracnose, although many of the 

strategies available to growers probably would be 

effective against all the walnut leaf diseases. 

Cultural management is the best approach to 

minimizing the impact of leaf diseases, particularly 

in plantations. First, if possible, plant only seedlings 

known to be resistant to disease. Presently, growers 

are at the mercy of nut collectors when buying 

seedlings. It is incumbent on nurseries to only sow 

nuts from known sources, preferably from resistant 

trees within the region where seedlings ultimately 

will be planted. Avoid shipping seedlings from the 

western area of the natural range of black walnut to 

other regions. In established plantations, growers 

should observe which seedlings exhibit symptoms 

and select against the most susceptible individuals 

during thinning operations.

Most cultural practices should be directed toward 

reducing free moisture on leaves. Dense stands 

are susceptible to disease because of increased 

humidity, caused by reduced wind flow through 

these stands, resulting in free moisture on leaf 

surfaces for extended periods of time. Thinning 

trees improves drying of leaf surfaces and reduces 

leaf shading between trees thereby reducing 

incidence and severity of disease (Tisserat 1985). 

Ponds, lakes and streams release heat gradually on 

cool nights, so establishing plantations near bodies 

of water, particularly on the lee side, lessens dew 

formation on leaves. Avoid establishing plantations 

in cold seeps or small openings since this leads to 

increased dew formation. If possible, avoid overhead 
Walnut Pest Management 113

Black Walnut in a New Century

irrigation if moisture on leaves will be maintained 

for long periods of time. 

Weeds can contribute to high relative humidity 

around seedlings, increasing leaf disease, so 

growers should control weeds either through 

cultivation or the use of herbicides. Cultivation 

after leaf fall has the added benefit of incorporating 

infected leaves into the soil, thereby promoting their 

decomposition and reducing pathogen populations. 

Depending on the crop, intercropping may also 

contribute to increased humidity. Growers will 

have to weigh the risk of increased potential of 

leaf diseases versus the benefits of intercropping. 

Interplanting autumn olive, hairy vetch, crown 

vetch, or serecia lespedza may reduce infection by 

increasing total foliar nitrogen and by preventing 

primary infections due to increased decomposition 

of fallen black walnut leaves thereby lowering 

ascospore production.

April and June applications of nitrogen may 

reduce infection as well as increase growth of 

trees. However, according to W. Reid, high rates 

of nitrogen stimulate non-fruiting shoots to grow 

late into the summer well after the infection period. 

These leaves give the tree a healthy appearance and 

mask the defoliation of older leaves. Soil application 

of ammonium sulfate, ammonium nitrate, and urea 

all are effective, although foliar application of urea 

is not. The addition of phosphorus and potassium 

has been shown to diminish the benefit of nitrogen 

fertilizer.

Direct control using fungicides can be effective, 

although the availability of suitable materials is 

in question. Since the status of all agricultural 

chemicals is always under review, contact your local 

extension agent for the latest information. Infection 

can occur anytime during the growing season 

so timing of applications presents a problem. A 

practical approach is to use two applications: the 

first during the last week in May to prevent the 

primary infection by ascospores, and the second 

during the first week of July to inhibit secondary 

infection by conidia.

Overall, native insects and pathogens should 

not pose a significant threat to black walnut 

management. Planting quality seedlings from 

geographically suited seed sources, and taking 

care to avoid injurious management practices 

will result in healthy growing stock. Currently no 

known exotic pests threaten black walnut. Given 

the high amounts of global trade, and the frequent 

introduction of forest pests, plantings should be 

monitored closely since they can serve as an early 

detection network for potentially invasive and 

damaging pests.

LITERATURE CITED

Bedker, P.J.; O’Brien, J.G.; Mielke, M.E. 1995. 

How to prune trees. NA-FR-01-95. Radnor, PA: 

U.S. Department of Agriculture, Forest Service, 

Northeastern Area, State and Private Forestry. 12 p.

Berry, F.H. 1973. Diseases. In: Black walnut as a 

crop: 2nd Black walnut symposium; 1973 August 

14-15; Carbondale, IL. Gen. Tech. Rep. NC-4. St. 

Paul, MN: U.S. Department of Agriculture, Forest 

Service, North Central Forest Experiment Station: 

88-90.

Cummings, J.E.; Kuntz, J.E. 1985. Stem 

canker on black walnut caused by Fusarium

sporotrichioides. Phytopathology 75:1279.

Cummings Carlson, J.; Mielke, M.E.; Appleby, J.E.; 

Hatcher, R.; Hayes, E.M.; Luley, C.J.; O’Brien, J.G.; 

Rugg, D.J. 1993. Survey of black walnut canker in 

plantations in five central states. Northern Journal 

of Applied Forestry. 10: 10-13.

Green, R.J.; Ploetz, R.C. 1979. Root rot of black 

walnut seedlings caused by Phytophthora

citricola. In: Walnut insects and diseases 

workshop proceedings; 1978 June 13-14; 

Carbondale, IL. Gen. Tech. Rep. NC-52. St. Paul, 

MN: U.S. Department of Agriculture, Forest 

Service, North Central Forest Experiment Station: 

5-10.

Hayes, E.M. 1995. Freeze injury to black walnut 

plantations in Minnesota during the winter of 

1993-94. 86th Annual Report of the Northern 

Nut Growers Association: 21-27.

Kessler, K.J.; 1982. Control of black walnut root rot 

diseases in nurseries. Res. Pap. NC-228. St. Paul, 

MN: U.S. Department of Agriculture, Forest Service, 

North Central Forest Experiment Station. 8 p.

Ostry, M.E.; Katovich, S.; Anderson, R.L. 1997. First 

report of Sirococcus clavigignenti-juglandacearum

on black walnut. Plant Disease. 81: 830.

Roth, D.A.; G.J. Griffen. 1979. Cylindrocladium 

root rot of black walnut seedlings: root rot of 

black walnut seedlings caused by Phytophthora

citricola. In: Walnut insects and diseases workshop 

proceedings; 1978 June 13-14; Carbondale, 

IL. Gen. Tech. Rep. NC-52. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, North 

Central Forest Experiment Station: 11-21.

Tisserat, N.A. 1985. Foliage and stem diseases of 

black walnut. In: 76th Annual Report of the 

Northern Nut Growers Association: 121-130. 

Weber, B.C.; Anderson, R.L., Hoffard, W.H. 1980. 

How to diagnose black walnut damage. Gen. 

Tech. Rep. NC-57. St. Paul, MN: U.S. Department 

of Agriculture, Forest Service, North Central 

Forest Experiment Station. 20 p.
114 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

SPREAD OF BUTTERNUT CANKER IN NORTH AMERICA, HOST RANGE, 

EVIDENCE OF RESISTANCE WITHIN BUTTERNUT POPULATIONS AND

CONSERVATION GENETICS

M.E. Ostry and K. Woeste1

Abstract—Butternut canker is killing trees throughout the range of butternut in North 

America and is threatening the viability of many populations in several areas. Although 

butternut is the primary host, other Juglans species and some hardwood species 

also are potential hosts. Evidence is building that genetic resistance within butternut 

populations may be exploited for conservation and restoration of the species.

Regional Forester Sensitive Species in the Eastern 

Region on 13 of the 16 National Forests. In Canada, 

butternut was listed endangered by the Committee 

on the Status of Endangered Wildlife in Canada 

(COSEWIC) in November 2003. 

SPREAD OF THE PATHOGEN

The first reported occurrence of butternut canker 

was from southwestern Wisconsin where all but two 

butternut trees in a 40-acre woodlot were diseased 

(WI Conserv. Dept. 1967). A survey of butternut 

in 36 Wisconsin counties in 1976 revealed that 

31 and 9% of the trees were diseased and dead, 

respectively. In contrast, in a 1992 resurvey of 

32 Wisconsin counties 92 and 27% of the trees 

were diseased and dead, respectively (Carlson and 

Guthmiller 1993). 

A survey for butternut canker in the eastern United 

States revealed that the disease was present in at 

least 14 of the 16 states surveyed (Anderson and 

LaMadeleine 1978). In that report the authors 

mention the disease had essentially eliminated 

many populations of butternut in North and 

South Carolina. Early reports of butternut decline 

throughout the northeastern United States were 

attributed to the fungus Melanconis juglandis

Ellis & Everhart Graves (Ostry 1997b) that causes 

branch dieback but not stem cankers. Although 

cankers are obvious, unless close examinations 

INTRODUCTION

Butternut (Juglans cinerea L.) is being killed 

throughout its range by a canker caused by the 

fungus Sirococcus clavigignenti-juglandacearum

Nair, Kostichka, and Kuntz, described as a new 

species in 1979 (Nair and others 1979). Although 

there are no reports of this fungus outside of North 

America, it is thought to be an exotic pathogen 

(Furnier and others 1999). Spores of the fungus 

develop under infected bark in sticky masses and 

are dispersed by rainsplash and wind during the 

growing season.

Butternut is valued for many uses and is important 

for wildlife and forest diversity, however, its 

infrequent occurrence within forest stands and its 

relatively small kernel and hard shell have, in part, 

limited its commercial importance as a timber or 

nut species (Ostry and Pijut 2000). As local supplies 

of healthy butternut trees become scarce the value 

of the wood has increased.

Butternut was listed under Category 2 on the list 

of Endangered and Threatened Plants under the 

Federal Endangered Species Act of 1973, however, 

this category has been eliminated and currently 

butternut has no official listing status. The first 

state to enact a measure to conserve butternut 

was Minnesota where in 1992 a moratorium on the 

harvest of healthy butternut on State lands was 

enacted. Butternut remains a “species of concern” 

or a “sensitive species” in many states and is a 

1

Research Plant Pathologist and Research Plant Molecular Geneticist, North Central Research Station, USDA Forest Service, 1561 

Lindig Avenue, St. Paul, MN 55108 (MEO) and Hardwood Tree Improvement and Regeneration Center, Department of Forestry and 

Natural Resources, Forestry Building, 195 Marsteller St. West Lafayette, IN 47907 (KW). MEO is corresponding author: to contact call 

(651) 649-5113 or e-mail at mostry@fs.fed.us.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
Walnut Pest Management 115

Black Walnut in a New Century

were made of trees, these reports may have 

mistakenly attributed tree decline to M. juglandis

and butternut canker may have been present much 

earlier than reported.

In Canada, butternut canker was first detected and 

confirmed from Ontario and Quebec in 1991 (Davis 

and others 1992) and in New Brunswick in 1997 

where it was thought to have been present for at 

least 7 years (Harrison and others 1998).

The most recent U.S. Department of Agriculture, 

Forest Service, Forest Inventory and Analysis 

survey data examined for butternut (NCRS, FIA Web 

site, October 2003) revealed that overall in seven 

Midwestern states the number of butternut trees 

in all size classes decreased by 23%, however, the 

number of trees recorded increased in three of the 

states. The states with a decrease in the number 

of trees and the inventory interval from which the 

data were collected are as follows: Michigan, 89% 

(1993-2001); Illinois, 87% (1998-2002); Wisconsin, 

44% (1996-2001); and Iowa, 40% (1990-2001). 

An increase in the number of butternut trees was 

recorded in Minnesota, 55% (1990-2002); Indiana, 

41% (1998-2002); and Missouri, 25% (1989-2002). 

This increase in number of trees was predominantly 

in the smallest size class (1.0-2.9 inch); the number 

of trees in all other size classes revealed decreases 

ranging from 13% (11.0-12.9 inch) to 100% (21.0+).

Butternut and eastern black walnut (J. nigra L) seed 

are known to harbor S. clavigignenti-juglandacearum

(Innes 1997). There is no evidence that it can be 

spread on Japanese walnut (J. ailanthifolia Carr.) 

seed or seedlings but this walnut species has been 

widely planted throughout the eastern United States 

(Bixby 1919). One can only speculate whether the 

fungus could have been inadvertently introduced 

into the United States from Asia on seed. There also 

is evidence that several insect species are closely 

associated with healthy and diseased butternut and 

because some of these insects were shown to be 

contaminated with the fungus they may act as vectors 

of the pathogen although the exact mode of spread 

is unknown (Katovich and Ostry 1998, Halik and 

Bergdahl 2002). Birds also may come into contact 

with the sticky spores and spread them from diseased 

to healthy trees within and between forest stands.

NATURAL AND EXPERIMENTAL HOST RANGE

Butternut is the only species that is killed by this canker 

disease. However, other Juglans species and hybrids are 

diseased to varying degrees. Orchard and others (1982) 

inoculated 10- to 20-year-old trees of several Juglans

species and found that Japanese walnut, heartnut 

(J. ailanthifolia var. cordiformis (Maxim.) Rehd.) and 

hybrids between them and butternut expressed greater 

resistance than either eastern black walnut or Persian 

walnut (J. regia L.) with the latter developing the most 

severe disease symptoms. 

In Canada and the United States tree species 

other than butternut have been affected by 

butternut canker over the past several years. It 

was reported eastern black walnut and butternut 

seedlings were naturally infected by S. clavigignentijuglandacearum in a nursery in Quebec (Innes 

1997). Stem cankers were confirmed on a 48 cm 

diameter eastern black walnut in North Carolina, 

and branch cankers were detected on 20-year-old 

eastern black walnut trees in Minnesota (Ostry and 

others 1997). Branch cankers were also found on a 

25-year-old heartnut in Iowa (Ostry 1997a). These 

reports indicate that this fungus may be a potential 

threat to walnut plantations.

Nearly the entire U.S. Persian walnut crop is 

produced in California (Beede and Hasey 1998). 

S. clavigignenti-juglandacearum is not known to be 

present in California and a quarantine on importing 

Juglans species from the eastern U.S. was put in 

place, but it is unknown what impact, if any, this 

pathogen may have on walnut cultivation should it 

become established there. Grafted plants of several 

Juglans species and hybrids have been artificially 

inoculated in the greenhouse, including several 

accessions from the National Clonal Germplasm 

Repository in Davis, California. Three of the leading 

cultivars grown, ‘Hartley’, ‘Chandler’, and ‘Payne’ 

(Beede and Hasey 1998) were among the most 

susceptible Persian walnut selections tested (Ostry 

and Moore, unpublished data) indicating caution 

should be exercised to avoid the movement of the 

pathogen into California. Interestingly, a “Paradox’ 

hybrid (J. hindsii x J. regia), a hybrid commonly 

used as rootstocks for Persian walnut (Beede and 

Hasey 1998), was highly resistant.

Using artificial inoculations of greenhouse 

seedlings, several other hardwood species have 

been shown to be susceptible and may be able to 

harbor the fungus (Ostry 1997b). Species in Carya, 

a genus in the walnut family (Juglandaceae) that 

were demonstrated to be susceptible include pecan 

(C. illinoensis [Wangenh.] K. Koch) and shagbark 

hickory (C. ovata [Mill.] K. Koch). Although not 

causing large cankers, the fungus was recovered 

beyond the inoculation point from northern red 

(Quercus rubra L.), black (Q. velutina Lam.), and 

white oak (Q. alba L.) and black cherry (Prunus 

serotina Ehrh.). Bitternut hickory (C. cordiformis

(Wangenh.) K. Koch) also has been shown to 

support the growth of the pathogen in greenhouse 

tests (Ostry and Moore, unpublished data). These 

preliminary results indicate that species of genera 
116 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

other than Juglans may serve as a reservoir of the 

pathogen within forest stands.

EVIDENCE OF RESISTANCE

In many areas throughout its range, healthy 

butternut have been found growing adjacent to 

trees infected and killed by the disease. Some 

of the trees we have monitored have remained 

healthy for over 12 years despite the severe disease 

on neighboring trees, minimizing the likelihood 

that disease escape is responsible for trees being 

symptom-free. Although these relatively rare 

trees may be disease resistant, we do not have 

experimental data as yet to demonstrate the 

existence of effective resistance.

Our current evidence of resistance mechanisms 

is circumstantial based on examining butternut 

over the years in search of trees that may have 

disease resistance. During our examinations we 

have detected two bark phenotypes on trees of the 

same size and relative age. One is a dark colored 

bark with deep bark fissures resembling the bark of 

eastern black walnut. The other is a light gray bark 

color with shallow bark fissures. These bark types 

and various intermediate types have been found on 

adjacent trees in many woodlots in Minnesota and 

Wisconsin. 

Often the dark/deep bark phenotype is associated 

with healthy trees and the light/shallow bark 

with diseased trees (Ostry and others 2003). Part 

of our research is directed at determining if bark 

phenotype and disease severity are genetically 

based traits that may help elucidate the mechanism 

of host resistance and potentially be used in 

conservation and genetic improvement of the 

species.

Disease resistance screening was initiated in one of 

the five grafted butternut clonal archives in 2003 

(Ostry and others 2003). Three trees 7-11 years 

old from each of 22 accessions propagated from 

diseased and healthy source trees and unselected 

9-year-old butternut trees were wound inoculated 

each month from April through October with two 

isolates of S. clavigignenti-juglandacearum. The 

objective was to mimic natural infection in the field 

to compare time of inoculation and host responses 

of selected grafted lines of butternut with putative 

disease resistance to grafted clones of butternut 

that are known to be highly susceptible. 

Although it is too early for reporting definitive 

results from this screening trial, indications are 

that infection resulted from all inoculation dates 

and several selected butternut lines have limited 

canker development compared to unselected 

or diseased source trees (Ostry and Moore, 

unpublished data). As with inoculations of plants in 

the greenhouse, screening trees in the field this way 

may allow us to separate groups of highly resistant 

selections from those that are highly susceptible.

The potential for plant pathogens to overcome host 

resistance can be high, especially with pathogens 

associated with long-lived trees. Agriculture is in a 

constant race with plant pathogens to develop and 

incorporate new genetic resistance into important 

crops as pathogens evolve and overcome them. 

Pathogens with a high evolutionary potential are 

more likely to overcome resistance compared to 

pathogens with a low evolutionary potential and 

knowledge of the genetic structure of a pathogen 

may be useful in predicting its future evolutionary 

potential (McDonald and Linde 2002). Pathogens 

having both a sexual and asexual reproduction 

system, high genotype flow, large effective 

population size and a high mutation rate will have 

the greatest potential to overcome host resistance.

Evaluating the potential for S. clavigignentijuglandacearum to overcome resistance in butternut 

within the framework outlined above results in 

guarded optimism that resistance may be longlasting. First, a sexual state of S. clavigignentijuglandacearum is not known to be present, 

therefore recombination via outcrossing resulting 

in new gene combinations that could overcome 

resistance is not likely. DNA fingerprinting (Furnier 

and others 1999) revealed limited genotype diversity 

supporting this theory. 

Second, gene flow, exchange of either alleles (genes) 

or individual clones (genotypes) among populations 

is more limited with S. clavigignenti-juglandacearum

than many other tree pathogens because it lacks an 

efficient long-range airborne spore stage. However, 

the sticky spores may be moved considerable 

distances by insects or birds countering this. 

Another mode enabling pathogens to move beyond 

their natural dispersal range is through human 

transport of infected plants or plant parts and S.

clavigignenti-juglandacearum can be seedborne 

and also moved on logs and scionwood. Thus, 

although lacking an efficient airborne state, S.

clavigignenti-juglandacearum can still be dispersed 

long distances.

Another source of genetic variation in pathogens 

is mutation resulting in new strains that could 

overcome host resistance genes. However, these 

mutations are more likely to occur and be selected 

for in pathogens that exist in large populations 

in individual plants, such as with bacteria and 

viruses. Butternut canker is not systemic and small 

populations of the fungus exist within relatively 
Walnut Pest Management 117

Black Walnut in a New Century

few diseased trees in any given area. Thus, the 

potential for a mutant strain of S. clavigignentijuglandacearum to multiply, spread to a susceptible 

host, infect, successfully colonize and then 

reproduce on that host is probably not very high. 

In summary, considering what we know about the 

genetics of S. clavigignenti-juglandacearum, there is 

realistically a low to moderate risk that it will evolve 

strains capable of overcoming disease resistance in 

butternut populations that may exist today or will 

be developed in the future. 

BUTTERNUT CONSERVATION GENETICS

The decline in butternut populations at local, 

regional and national levels raises questions 

about whether the long-term genetic viability 

of the species has been compromised. Stated 

another way, how much genetic variability did 

butternut have historically, how much remains, 

and is there enough for butternut to fulfill its 

ecological functions, resist disease and adapt to 

environmental change (Yang and Yeh 1992)? At 

present, there are few answers.

Available data indicate that butternut has 

considerably lower genetic diversity (as measured 

by percent polymorphic loci and number of alleles 

per locus) than similar species based on allozyme 

and RFLP marker systems (Morin and others 2000, 

Fjellstrom and Parfitt 1994). Morin and others 

(2000) reported that less than 20% of the loci they 

evaluated in butternut were polymorphic, with 1.3 

alleles per allozyme locus or fewer. By comparison, 

eastern black walnut had 42 – 88% polymorphic 

loci and about 2.9 alleles per locus. The preliminary 

findings of Morin and others (2000) indicate that 

butternut may be slightly more genetically diverse 

in the US than in Canada, but they were unable to 

determine the cause or causes of the lower-than 

expected genetic diversity of butternut. 

Microsatellite DNA polymorphisms (SSRs) are 

rapidly becoming the marker system of choice for 

population genetic studies, and several of the SSRs 

originally identified in black walnut (Woeste and 

others 2002) are also polymorphic in butternut 

(Woeste, unpublished data). Nuclear SSRs used in 

tandem with chloroplast markers can potentially 

be used to confirm if butternut has been through 

a genetic bottleneck and to predict whether the 

bottleneck was recent or ancient. SSRs are also 

an excellent tool for evaluating regional and local 

genetic diversity of butternut. Comparative studies 

of allele sizes of SSRs that can be amplified in 

butternut, eastern black walnut, and Japanese 

walnut might also be useful for identifying hybrids.

As previously mentioned, many of the apparently 

canker resistant butternut trees we have examined 

are characterized by deeply fissured, darkly colored 

bark. This phenotype, not typically associated with 

butternut, is similar to the bark of black walnut. 

The origins of this dark-barked phenotype are 

unknown. It is possible that dark-barked trees 

are an ecotype of butternut that was previously 

unnoticed, either because it was rare or because 

dark-barked butternuts were mistaken for walnuts 

by casual observers. Because of the phenotypic 

similarity between dark-barked butternuts and 

black walnut, we investigated whether the ITS 

region of some of the dark-barked trees indicated 

hybrid origins. Published literature was clear that 

the (J. nigra x J. cinerea) hybrid was not possible 

(Funk 1970), but unsubstantiated claims of the 

existence of such hybrids infrequently arise. For 

example, trees catalogued as J. nigra x J. cinerea

hybrids were maintained at the Tree Improvement 

Center (TIC) of the Carbondale work unit of the 

North Central Research Station, and seeds of 

putative J. nigra x J. cinerea have been sold by 

nurseries. The trees in the TIC were grown from 

seeds provided by Michigan State University 

researchers in the late 1950s. We have concluded 

from preliminary analyses of the dark-barked 

butternuts and putative J. nigra x J. cinerea hybrids 

that some of the dark-barked butternuts are true 

butternuts (non-hybrids) and that at least one of 

the many putative J. nigra x J. cinerea hybrids at 

the TIC may in fact be such a hybrid. 

Several unknowns confound these results, but 

first among them is that we do not know if we are 

able to accurately differentiate among J. cinerea

x J. ailanthifolia, J. nigra x J. cinerea and all 

possible three-species hybrids such as (J. nigra x J. 

ailanthifolia) x J. cinerea, in part because there are 

not as yet any positive controls for the experiment. 

Furthermore, it is not known if hybridization that 

may have occurred two or three generations ago can 

be detected accurately using internal transcribed 

spacer (ITS) sequences, although the phenotypic 

impact of such hybridization may still be present. In 

other words, dark-barked butternuts with greater 

canker resistance may be butternuts; they may be 

the product of a rare, natural hybridization between 

eastern black walnut and butternut that occurred a 

few generations ago; or they may be something else. 

Simple analysis of ITS regions only may not be able 

to demonstrate or rule out any of the possibilities. 

Butternut may have hybridized with eastern 

black walnut in the recent past, the more distant 

past, or both. Detailed analysis of the chloroplast 

sequences of both species and the putative hybrids 

may provide some insight into this question. Since 

chloroplasts are strictly maternally inherited in 

Juglans, the presence in dark-barked trees of 

eastern black walnut chloroplast sequences would 
118 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

indicate a hybrid between eastern black walnut and 

butternut in which butternut was the male. 

It should be noted that hybridization between 

butternut and eastern black walnut seems 

ecologically unlikely since interspecific hybridization 

usually happens in the contact zones of spatially 

separated species, and the ranges of eastern black 

walnut and butternut overlap across almost the 

entire northeast quarter of the US (Funk 1970).

Butternut hybridizes with Persian walnut to 

produce J. x quadrangualata (Carr.) Rehd., with 

Japanese walnut to produce J. x bixbyi Rehd and 

with heartnut to produce “buartnut”. The striking 

vigor of the F1 hybrid between butternut and 

Japanese walnut is one phenotype that can be used 

to distinguish these hybrids, also called buarts or 

butterjaps, from butternuts. Field observations 

indicate that buarts are more common in old, 

abandoned farmyards, on pasture edges, and in the 

yards of houses in small, rural towns. The leaves 

of buarts may be greener and more persistent than 

those of butternut, not abscising until well into 

October; whereas butternut leaves typically turn 

yellow and abscise in early to mid-September. There 

are reports that butternut also hybridizes with little 

walnut (J. microcarpa Berland) and Manchurian 

walnut (J. mandshurica Maxim.). 

To identify and characterize hybrids it will 

be essential to develop markers that clearly 

differentiate among Juglans species, specifically 

eastern black walnut, butternut, and Japanese 

walnut. A full study of the variability of the nuclear, 

chloroplast and mitochondrial genomes of all 

three species will probably be necessary before 

any marker system can be used to detect and 

identify the parents of hybrids with a high level of 

confidence. One method for finding species specific 

DNA sequence signatures is the analysis of DNA 

sequences from highly conserved genes. It may be 

possible to identify species-specific polymorphisms 

in the introns or nearby non-coding regions from 

six or seven genes, and these could be used to 

detect backcrosses or even three-species hybrids. 

Unfortunately, there is almost no DNA sequence 

data available in public databases for butternut and 

Japanese walnut, and very little for eastern black 

walnut. All methods for evaluating species diversity 

and hybridity have drawbacks and blind spots, 

and as such a combination of morphological and 

molecular techniques seems likely to produce the 

most reliable results.

At present, the best tool for discriminating 

Juglans species and their hybrids is the sequence 

polymorphism found within the ITS regions of the 

nuclear ribosomal DNA. The ITS region is present 

in most genomes as thousands of copies of tandem 

repeats at one or many loci (Baldwin and others 

1995). Because the sequence undergoes rapid, 

concerted evolution leading to a high level of 

intergenic uniformity, the ITS region has proven 

generally useful in studies of angiosperm taxonomy 

(Baldwin and others 1995) and it has been used 

specifically to decipher the species identities and 

hybrid origins of Juglans rootstocks used in the 

California walnut nursery industry (Potter and 

others 2002). We have identified primers that 

amplify the ITS region of eastern black walnut, 

butternut, and Japanese walnut. By digesting 

the PCR products with restriction enzymes, DNA 

fragments of diagnostic sizes are produced for 

each species. This marker system is imperfectly 

co-dominant because there can be within genome 

length variation in the ITS (Sytsma and Schaal 

1990 and our preliminary results from eastern 

black walnut), because the ITS regions of hybrids 

may homogenize to a single sequence at different 

rates, and because there may not be sufficient 

polymorphism within the ITS to determine which 

species among several possible are represented in a 

hybrid and at what percentage. 

Taxonomists and dendrologists have traditionally 

used morphology to distinguish species and 

their hybrids. This approach is complicated in 

the genus Juglans because there are few readily 

discernable traits that distinguish the species. 

A careful evaluation of trichomes or other more 

subtle features may yet prove valuable in the 

identification of Juglans species and first generation 

hybrids. One potential problem with any approach 

to distinguishing hybrids is that Japanese walnut 

has been widely propagated in the US for over 150 

years (Crane and Reed 1937). Some features that 

we now associate with butternut may have been 

introduced by gene flow from Japanese walnut 

to butternut a generation ago. Similarly, Persian 

walnut, a species that can hybridize with eastern 

black walnut, butternut, and Japanese walnut, 

has been propagated in the US since colonial 

times. In areas where butternut populations have 

undergone a severe decline, one may be justifiably 

skeptical concerning the identification of the 

remaining ‘butternuts’, especially if these trees 

express some morphological or genetic variability 

that is uncommon elsewhere. Do remnant trees 

represent the best opportunity to capture rare local 

diversity, or are they Trojan horses carrying genes, 

including perhaps genes for resistance to butternut 

canker, from other species? The study of herbarium 

sheets of butternut collected before 1860 may be 

one way to evaluate the morphological diversity of 

butternut before there was any potential impact by 

hybridization with other Juglans species. 
Walnut Pest Management 119

Black Walnut in a New Century

CONCLUSIONS

Since the detection of butternut canker in 1967 

researchers have clarified several aspects of the 

disease, including the description of the causal 

agent, its biology, a partial host range, and they 

have documented limited examples of potentially 

disease resistant trees. However, many gaps 

remain in our knowledge including the origin 

of the pathogen, the level of genetic diversity 

in butternut across its range, and silvicultural 

techniques to retain butternut in our forests 

and to restore the species where it has been 

eliminated by the disease. 

Butternut canker and its spread raises fundamental 

issues with respect to the productivity and health 

of the central hardwoods landscape. Exotics invade 

native landscapes on several levels: physically, 

they occupy the space where endemics once grew, 

modifying the environment there and often making 

it less hospitable to native flora and fauna; they 

invade by introducing pests and they invade at 

the genetic level by hybridizing with endemic flora 

or other introduced species. The genetic invasion 

is often unseen and difficult to monitor unless 

(or until) the hybrids themselves become invasive 

weeds. More sensitive genetic and phenotypic 

marker systems are needed to monitor the genetic 

invasion of the exotics such as Japanese walnut 

into the central hardwoods region. Butternut has 

been the most affected by Japanese walnut. But 

over the long-term the possibility exists that black 

walnut could also be adversely affected by exotic 

invasion at the genetic level.

Butternut is rapidly being lost in our forests from 

a variety of causes in addition to butternut canker. 

Genetic diversity in species such as butternut is 

needed for its long-term survival, future adaptation 

and evolution. There is an urgent need to conserve 

genetic diversity among butternut populations 

before valuable populations are lost. 

ACKNOWLEDGMENTS

We thank the many individuals in various state 

and federal agencies and the numerous private 

landowners who continue to support our research 

efforts. We especially thank M. Moore and Bo Li 

for technical support in the conduct of laboratory, 

greenhouse and field research.

LITERATURE CITED

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distribution of butternut decline in the 

Eastern United States. For. Serv. Rep. 

S-3-78. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 5 p.

Baldwin, B.G.; Sanderson, M.J.; Porter, J.M.; 

Wojciechowski, M.F.; Campbell, C.S.; 

Donoghue, M.J. 1995. The ITS region of nuclear 

ribosomal DNA a valuable source of evidence on 

angiosperm phylogeny. Annals of the Missouri 

Botanical Garden. 82: 247-277.

Beede, R.H.; Hasey, J.K. 1998. The history of 

the walnut in California. In: Ramos, D.E., 

ed. Walnut production manual. Publ. 3373. 

University of California, Division of Agriculture 

and Natural Resources: 8-15.

Bixby, W.G. 1919. The butternut and Japan 

walnut. American Nut Journal. 10: 76-83.

Carlson, J.C.; Guthmiller, M. 1993. Incidence and 

severity of butternut canker in Wisconsin in 

1976 and 1992. Phytopathology. 83: 1352. 

Abstract.

Crane, H.L.; Reed, C.A. 1937. Nut breeding. In: 

Yearbook of agriculture. U.S. Department of 

Agriculture, 75th Congress, 1st Session, House 

document No. 28. 827-889.

Davis, C.N.; Myren, D.T.; Czerwinski, E.J. 1992. 

First report of butternut canker in Ontario. 

Plant Disease. 76: 972.

Funk, D. 1970. Genetics of black walnut. Res. Pap. 

WO-10. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 13 p.

Fjellstrom, R.G.; Parfitt, D.E. 1994 Walnut (Juglans

spp.) genetic diversity determined by restriction 

fragment length polymorphisms. Genome. 37: 

690-700.

Furnier, G.R.; Stolz, A.M.; Raka, M.M.; Ostry, M.E. 

1999. Genetic evidence that butternut canker 

was recently introduced into North America. 

Canadian Journal of Botany. 77: 783-785.

Halik, S.; Bergdahl, D.R. 2002. Potential 

beetle vectors of Sirococcus clavigignentijuglandacearum on butternut. Plant Disease. 

86: 521-527.

Harrison, K.J.; Hurley, J.E.; Ostry, M.E. 1998. First 

report of butternut canker caused by Sirococcus 

clavigignenti-juglandacearum in New Brunswick, 

Canada. Plant Disease. 82: 1282.

Innes, L. 1997. Sirococcus clavigignentijuglandacearum on butternut and black walnut 
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fruit. In: Foliage, shoot and stem diseases of 

trees: proceedings of the IUFRO WP 7.02.02 

meeting. Quebec City, Quebec, Canada: Natural 

Resources Canada, Canadian Forest Service, 

Laurentian Forestry Centre: 129-132.

Katovich, S.A.; Ostry, M.E. 1998. Insects associated 

with butternut and butternut canker in 

Minnesota and Wisconsin. The Great Lakes 

Entomologist. 31: 97-108.

McDonald, B.A.; Linde, C. 2002. Pathogen 

population genetics, evolutionary potential, 

and durable resistance. Annual Review of 

Phytopathology. 40: 349-379.

Morin, R.; Beaulieu, J.; Deslauriers, M.; Daoust, 

G.; Bousquet, J. 2000. Low genetic diversity 

at allozyme loci in Juglans cinerea. Canadian 

Journal of Botany. 78: 1238-1243.

Nair, V.M.G.; Kostichka, C.J.; Kuntz, J.E. 1979. 

Sirococcus clavigignenti-juglandacearum: 

an undescribed species causing canker on 

butternut. Mycologia. 71 :641-646.

Orchard, L.P.; Kuntz, J.E.; Kessler, K.J., Jr. 1982. 

Reactions of Juglans species to butternut 

canker and implications for disease resistance. 

In: Black walnut for the future: 3rd walnut 

symposium; 1981 August 10-14; West 

Lafayette, IN. Gen Tech. Rep. NC-73. St. Paul, 

MN: U.S. Deparment of Agriculture, Forest 

Service, North Central Forest Experiment 

Station: 27-31.

Ostry, M.E. 1997a. Sirococcus clavigignentijuglandacearum on heartnut (Juglans 

ailanthifolia var. cordiformis). Plant Disease. 81: 

1461.

Ostry, M.E. 1997b. Butternut canker in North 

America 1967-1997. In: Foliage, shoot and stem 

diseases of trees: Proceedings of the IUFRO WP 

7.02.02 meeting. Quebec City, Quebec, Canada: 

Natural Resources Canada, Canadian Forest 

Service, Laurentian Forestry Centre: 121-128.

Ostry, M.E.; Katovich, S.; Anderson, R.L. 1997. 

First report of Sirococcus clavigignentijuglandacearum on black walnut. Plant Disease. 

81: 830.

Ostry, M.E.; Pijut, P.M. 2000. Butternut: an 

underused resource in North America. 

HortTechnology. 10: 302-306.

Ostry, M.E.; Ellingson, B.; Seekins, D.; Ruckheim, 

W. 2003. The need for silvicultural practices 

and collection of butternut germplasm for 

species conservation. In: Van Sambeek, J.W.; 

Dawson, J.O.; Ponder, F., Jr.; Loewenstein, 

E.F.; Fralish, J.S., eds. Proceedings, 13th

Central Hardwood Forest conference; 2002 

April 1-3; Urbana, IL. Gen. Tech. Rep. NC-234. 

St. Paul, MN: U.S. Department of Agriculture, 

Forest Service, North Central Research Station: 

551-555.

Potter, D.; Gao, F.; Baggett, S.; McKenna, J.R.; 

McGranahan, G.H. 2002. Defining the source 

of Paradox: DNA sequence markers for North 

American walnut (Juglans L.) species and 

hybrids. Scientia Horticulturae. 94: 157-170.

Sytsma K.J.; Schaal B.A. 1990. Ribosomal DNA 

variation within and among individuals of 

Lisianthius (Gentianaceae) populations. Plant 

Systematics and Evolution. 170: 97-106.

Woeste, K.; Burns, R.; Rhodes, O.E., Jr.; Michler, C. 

2002. Thirty polymorphic nuclear microsatellite 

loci from black walnut. The Journal of Heredity. 

93(1): 58-60.

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of in situ and ex situ conservation of forest trees. 

The Forestry Chronicle. 68: 720-728.
Walnut Pest Management 121

Black Walnut in a New Century

INSECTS ATTACKING BLACK WALNUT IN THE MIDWESTERN UNITED STATES

Steven Katovich1

ABSTRACT—Black walnut has only a handful of insects that would be considered 

significant pests. Of the leaf feeders, the walnut caterpillar is the most likely to cause 

significant defoliation and damage to trees. However, severe infestations are infrequent 

and tend to be restricted to small geographic areas. Two other commonly encountered 

defoliators are the yellow necked caterpillar and the fall webworm. The black walnut 

curculio is the major nut pest on black walnut. The walnut shoot moth attacks buds 

and shoots of black walnut. Destruction of the terminal bud or shoot on young trees can 

cause excessive branch forking that can change the shape of a tree. Walnut does have 

several wood boring insects that will invade the main trunk and larger branches. Most 

of them infest trees that are in poor health. Many can also invade freshly cut logs. An 

ambrosia beetle, Xylosandrus germanus, can attack apparently healthy trees and attacks 

are often associated with the pathogen Fusarium. Management practices are provided 

that should minimize insect caused impacts 

INTRODUCTION

Black walnut has an array of insects that feed on its 

leaves and developing nuts, and tunnel into buds, 

shoots, twigs, and through bark into the wood. 

However, only a few insects would be considered 

significant pests. Perhaps the most important are 

those that tunnel into developing shoots and buds 

on younger trees. The destruction of the terminal 

bud or shoot on a young tree can cause excessive 

branch forking that can change the shape of a tree 

and reduce its timber value. There are also insects 

that can infest the main stem of living trees though 

generally this occurs on weakened or damaged 

trees. Freshly cut logs can also be invaded. These 

wood boring insects can introduce a number of 

fungi and other pathogens, and their tunnels can 

cause degrade in the value of wood products. Leaf 

feeding insects are not uncommon and outbreaks 

do occur. But, most outbreaks tend to be shortlived on walnut and widespread tree mortality is not 

often reported. Nut production can be reduced by 

weevil attacks on developing nuts and insects that 

infest the husk. 

SPECIFIC DAMAGING INSECTS

Weber and others (1980) developed a publication on 

diagnosing the most common damaging agents on 

black walnut. This publication is no longer in print, 

but can be accessed via the internet at: http://

www.na.fs.fed.us/spfo/pubs/howtos/ht_walnut/

cover.htm.

Information on identification of many walnut 

insects can also be obtained by using Johnson 

and Lyon (1988) and USDA (1985). Information on 

wood boring insects attacking walnut is available in 

Solomon (1995). 

Insects Damaging Foliage

Loss of leaf tissue is referred to as defoliation. The 

impact that insect caused defoliation has on a tree 

can include growth loss, mortality, and increased 

susceptibility to other insects and pathogens 

(Kulman 1971). But, leaf feeding and leaf loss may 

not always be a problem, in some situations it may 

even be beneficial by stimulating growth through 

increased nutrient cycling or by acting as a thinning 

agent (Schowalter and others 1986). Therefore, leaf 

feeders do not always need to be controlled. Control 

activities should be limited to protecting young 

trees and trees weakened by previous defoliation 

or drought stress. Though not well documented 

in black walnut, reduced tree vigor can make 

trees more susceptible to wood boring insects and 

some pathogens. In most situations, trees that are 

1

Forest Entomologist, USDA Forest Service, Forest Health Protection, 1992 Folwell Avenue, St. Paul, MN 55108. To contact, call 651-

649-5264, or e-mail at skatovich@fs.fed.us. 

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
122 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

healthy or vigorous will survive defoliation events 

and probably do not need to be protected. 

A number of defoliators of black walnut are listed 

in USDA (1985) and Johnson and Lyon (1988). 

The most commonly encountered defoliators on 

black walnut are the walnut caterpillar, Datana

integerrima, Grote & Robinson, yellow necked 

caterpillar, Datana ministra (Drury), and the fall 

webworm, Hyphantria cunea (Drury). These are all 

moth species. They are discussed in more detail 

below. Two other moth species that can on occasion 

be locally abundant include the variable oakleaf 

caterpillar, Heterocampa manteo (Doubleday), and 

the pecan leaf casebearer, Acrobasis juglandis

(LeBaron). 

In addition, there are several very large caterpillars 

that feed on walnut, the adult moths are some of 

our largest native insects. They include the walnut 

sphinx, Cressonia juglandis (J.E. Smith), hickory 

horned devil, Lophocampa caryae (Harris), and 

luna moth, Actias luna (L.). These species are rarely 

damaging and in most cases they do not need to be 

controlled. 

A few beetle adults and sawfly larvae eat black walnut 

leaves, though none are considered major pests. June 

beetles, Phyllophaga sp., and the Japanese beetle, 

Popillia japonica Newman, can defoliate trees. June 

beetles often feed at night and large groups tend 

to congregate on a single tree. Widespread damage 

is unusual. The butternut woollyworm, Eriocampa

juglandis (Fitch), is a sawfly that feeds on Juglans. 

Larvae feed in groups and can defoliate individual 

branches or small trees. The butternut woollyworm 

can be conspicuous because of white “woolly” tufts 

that cover the larval body. 

Some leaf damage can occur from aphid, lace bug 

and mite feeding that can cause leaves to become 

distorted, discolored or to senesce and drop. 

Aphids and lace bugs are small insects that use 

their straw-like mouthparts to suck sap. Mites also 

remove sap and can scrape away leaf tissue. The 

walnut lace bug, Corythuca juglandis (Fitch), is a 

common species found on black walnut leaves. Both 

adults and immatures feed on the undersurface of 

the leaf creating small necrotic spots. The velvet 

gall mite, Eriophyes caulis Keifer, forms a twisted 

velvety red growth on the stem of leaflets. This is 

conspicuous, but relatively harmless. 

The walnut caterpillar is the most likely to cause 

significant defoliation and damage to trees (Farris 

and others 1982). Trees of all sizes can be attacked 

though damage is often greatest on isolated 

individual trees. Severe infestations are infrequent 

and tend to be restricted to small geographic 

areas. It generally requires 2-3 consecutive years 

of heavy defoliation before trees begin to die and 

most outbreaks last no longer than 2 years. Tree 

mortality has not been widely reported following 

walnut caterpillar outbreaks. The number of 

generations varies from one in northern portions 

of the U.S. to three in southern regions. A number 

of natural enemies appear to be important in 

maintaining walnut caterpillar populations at 

reasonable levels (Farris and others 1982). On 

small trees hand removal of larvae can be helpful, 

especially since caterpillars tend to congregate 

when young. Insecticide treatments can be 

successful, but are rarely warranted. The yellow 

necked caterpillar is a close relative of the walnut 

caterpillar and outbreaks should be treated in a 

similar manner to walnut caterpillar. 

Fall webworm outbreaks occur in late summer. 

The caterpillars form large webs that can enclose 

branches and even entire trees. The large webs 

can bring attention to the presence of this insect. 

However, late season defoliation (September) does 

not often impact trees as much as early season 

defoliation (May and June) and damage. Growth 

loss or branch and tree mortality from this insect is 

rarely reported. 

Insects Damaging Nuts

There are insects that specifically feed on developing 

nuts. A weevil, the black walnut curculio, 

Conotrachelus retentus (Say), is the major insect 

pest on black walnut in the eastern U.S. In addition, 

the codling moth, Cydia pomonella (L.) and two 

closely related fly species, the walnut husk maggot, 

Rhagoletis suavis (Loew) and walnut husk fly, R. 

completa Cresson, can infest the husk that covers 

nuts. Codling moth larvae can attack at an early 

stage in nut development leading to premature nut 

drop. On mature nuts, the husks can be infested 

with codling moth larvae or the maggot stage of the 

husk flies. This can result in husks turning black and 

shriveling making nut extraction difficult. 

The adult black walnut curculio is a relatively 

small weevil, about 1/5 inch long. Larvae feed in 

developing nuts on black walnut causing nuts to 

drop prematurely (Blair and Kearby 1979). This is 

referred to as the “June drop” of walnuts (Weber 

and others 1980). Egg laying occurs on developing 

nuts shortly after flowers are fertilized. Larvae 

tunnel into developing nuts and within a few weeks 

infested nuts drop from the tree. On the ground, 

larvae remain in the nuts for about 2 weeks after 

which they chew an exit hole in the husk and enter 

the soil. In the soil they pupate into adults. The 

adults emerge in mid to later summer and spend 

the remainder of the year feeding on black walnut 
Walnut Pest Management 123

Black Walnut in a New Century

foliage. When leaf drop occurs, adults move into the 

soil where they spend the winter. 

A 2-year study done in Missouri, reported 51 percent 

of the annual nut production dropped early as a 

result of black walnut curculio infestations (Blair and 

Kearby 1979). In a 10-year study on the impact of 

the black walnut curculio in southwestern Missouri, 

Linit and Stamps (personal communication) reported 

that curculio-caused nut loss ranged from virtually 

none to 26% of the available nuts. For years in which 

the final harvest nut crop was known, loss of nuts 

from weevil oviposition was high in years with small 

nut crops and low in years with large nut crops. This 

suggests weevil populations may remain relatively 

stable and destroy a constant number of nuts from 

year to year. 

A sanitation program based on rapid removal 

and destruction of nuts that have fallen in early 

summer should reduce the local populations of 

weevils. However, this must be done before weevils 

migrate to the soil. Sanitation can also help reduce 

codling moth and husk fly populations. Early 

summer insecticide applications can be targeted at 

these insects. A pheromone attractant is available 

for adult codling moths that can help in the proper 

timing of any insecticide application. Baits are also 

available for attracting the husk flies. 

Insects Damaging Buds and Shoots

Bud and shoot damage can appear rather 

innocuous. However, the destruction of buds or 

shoots, especially the terminal bud or shoot on 

young trees, can cause excessive branch forking 

that can change the shape of a tree and reduce its 

timber value. The lack of a single dominant straight 

stem can be a major impediment to growing wellformed trees. 

There are several insects that can damage shoots 

and buds, but only one appears to be a significant 

pest. The walnut shoot moth, Acrobasis demotella

Grote, is commonly found attacking buds and 

shoots of black walnut (Kearby 1979, Martinat and 

Wallner 1980). On occasion, a related species, the 

pecan leaf casebearer, A. juglandis (Le Baron), may 

be problematic. It does not feed in shoots but can 

kill significant numbers of buds. The butternut 

curculio, Conotrachelus juglandis LeConte, a 

weevil, can infest black walnut shoots though it 

prefers butternut (Solomon 1995). 

Females of the walnut shoot moth are present 

in mid-summer and lay eggs on the underside of 

walnut leaflets. Larvae soon emerge and begin 

feeding on leaf tissue. When foliage begins to 

change color larvae migrate to twigs and construct 

small silken shelters called hibernacula, where 

they spend the winter. In the early spring, larvae 

emerge and begin to bore into and hollow-out buds. 

As leaves begin to emerge and new shoots expand, 

larvae tunnel into the pith of developing shoots. 

Entrance holes are covered with silk and frass. 

Tunnels can reach 18 inches in length, shoots die 

above the tunnel. By early summer larvae finish 

feeding, leave the shoots, drop to the ground, and 

burrow into the litter layer and pupate inside an 

earthen cell. 

Following damage to terminal buds or shoots, 

McKeague and Simmons (1979) recommended 

corrective pruning. They defined corrective pruning 

as the removal of new shoots developing along a 

branch, leaving one to become the new terminal 

leader. This should be done in early summer when 

bud damage is apparent and shoot growth has not 

been completed. A benefit of pruning is that it helps 

correct for both frost and insect caused injury. 

Frost is a common damaging agent to buds. 

Insects Damaging Twigs and Small Branches

In contrast to new shoot growth discussed in the 

section above, a few insects can damage older 

twigs and small branches. Injury to twigs and 

small branches would rarely impact tree form or 

overall tree health. The most common injuries are 

oviposition wounds from cicadas, planthoppers, 

and treehoppers, all members of the insect order 

Homoptera. Females slit the bark on twigs and 

small branches and imbed eggs within the slits. 

These wounds can kill twigs and branches. 

Twig and branch dieback can also occur due to 

infestations of scale insects although they are not 

widely reported on black walnut. Marshall (1989) 

did list the oystershell scale, Lepidosaphes ulmi (L.) 

as a common walnut pest. 

Insects Damaging the Trunk and Large Branches

Walnut does have several wood boring insects that 

will invade the main trunk and larger branches 

(Solomon 1995). Most are beetles, a few are moth 

larvae. Only a few species are considered significant 

pests and most of these do not kill trees but 

cause problems by providing sites where disease 

organisms can become established or by causing 

defects in wood products through tunnels or 

associated staining. As mentioned, these insects 

rarely kill trees but most of them are associated 

with trees that are declining, dying or recently 

killed, including cut logs. Only Xylosandrus

germanus (Blandford), an ambrosia beetle, has been 
124 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

reported attacking apparently healthy trees (Weber 

and McPherson 1984). 

Ambrosia beetles are relatively small beetles that 

burrow through the bark and into the sapwood and 

feed both as an adult and larvae on fungi that they 

cultivate in their tunnels. These fungi often impart 

a dark blue or black stain along the tunnels. This 

staining can cause a degrade in sawn lumber. Black 

walnut has several species of ambrosia beetles that 

can infest the trunk and larger branches (Table 

1). Most of these attack trees that have existing 

damage, are growing slowly or have recently died or 

been harvested. As mentioned earlier, Xylosandrus

germanus can attack apparently healthy trees and 

attacks by this beetle have been found associated 

with the fungus Fusarium (Weber and McPherson 

1985). Xylosandrus germanus was introduced into 

North America and now occurs throughout the 

Midwest. Young trees up to 8 years old are most 

often attacked. The adults bore small entrance 

holes, about 1/32 inch in diameter. Attacked trees 

often have top dieback and profuse sprouting from 

the base of the tree.

Pinhole or wormhole type damage, diagnostic 

of ambrosia beetles, can also be caused by the 

sapwood timberworm, Hylecoetus lugubris Say. 

This beetle often infests green saw logs as well as 

weakened and dying trees. In addition to pinholes 

that penetrate into the sapwood, there are also 

many small laterally oriented galleries across the 

surface of the sapwood. 

Larger beetles that infest black walnut in the 

eastern U.S. include the flat-headed apple tree 

borer, Chrysobothris femorata (Oliv.), the pole 

borer, Parandra brunnea (Fabricius), and the ash 

and privet borer Tylonotus bimaculatus Haldeman. 

In almost all cases, these insects infest wounded, 

weakened or dying trees. Externally, infestations 

can be found via entry wounds that bleed sap or 

have extruding wood fibers or sawdust. 

Recommended control measures for wood-boring 

insects revolve around maintaining tree vigor. 

Most of these insects are incapable of successfully 

invading healthy, vigorous trees. Therefore, 

management practices such as weed control and 

thinning can be beneficial. Sanitation, or the 

removal of infested material can also be useful in 

reducing local populations of wood boring insects. 

In addition, limiting wounds on the main stem 

should reduce attacks from some of these insects. 

Wounds caused by mowing or logging equipment 

can be attractive to many wood-boring insects. 

Pruning wounds tend to heal quickly and are 

probably not overly attractive. Pruning in the fall 

and winter would further eliminate the risk of 

attracting these insects. 

Freshly cut logs should be removed rapidly from 

the woods to avoid attracting wood boring insects. 

In addition, harvesting activities should be done 

during colder periods of the year, thus reducing the 

likelihood of insect activity.

Table 1.—Ambrosia beetles reported attacking the trunk and larger branches of walnut by Solomon (1995). 

Name Importance

Black stem borer,

Xylosandrus germanus

Considered a significant walnut pest, attack sites are often associated with 

Fusarium cankers. Can attack vigorous trees, especially young walnut trees in 

plantations. Also attacks dying and diseased trees. Sap may ooze from pinhole 

sized entrance holes. Fine dust-like frass is often present. Infested trees often 

suffer top dieback.

Lesser shothole borer, 

Xyleborinus saxeseni

Not considered a major walnut pest. Infests dying or dead material, can attack 

fresh-cut logs causing a stained pinhole defect. White frass on bark.

Pear blight beetle, 

Xyleborus dispar

Not considered a major walnut pest. More common on fruit trees. Infests injured 

or dying trees. 

Cosmopolitan

ambrosia beetle, 

Xyleborus ferrugineus

Not considered a major walnut pest, has a host range of more than 180 species 

worldwide. Most common attacking stumps and logs on the ground. White frass 

on bark.

Oak timber beetle,

Xyleborus xylographus

Not considered a major walnut pest, prefers oaks. Beetles favor lower stumps 

and lower portions of dead and dying trees. Often attacks around wound sites. 

White frass on bark. 
Walnut Pest Management 125

Black Walnut in a New Century

MANAGEMENT RECOMMENDATIONS

Management of black walnut is largely driven by 

the desire to develop well-formed trees that have 

a straight stem. Management activities may be 

needed to protect young trees from insects that kill 

the terminal buds or shoots, specifically the walnut 

shoot moth. Unfortunately, limited direct control 

options are available for this insect. 

Corrective pruning can develop a single terminal 

bud after insect attack or frost injury. 

Younger trees will often self-correct or 

straighten if neighboring trees provide 

competition for light. Therefore, maintain wellstocked stands.

Wood-boring insects tend to infest trees weakened 

by poor growing conditions or injuries to the main 

stem. In addition, repeated defoliation by insects 

or leaf diseases can weaken trees. Therefore, 

management tactics should target maintaining 

healthy, vigorous trees. Further, trees should not be 

damaged during mowing or logging activities.

Proper weed control can allow for rapid early 

growth developing healthy trees.

Thinning older stands to develop trees with 

large vigorous crowns can maintain tree health.

Insecticide applications targeted at defoliating 

insects are rarely warranted. Most healthy trees 

should withstand a single defoliation event, 

especially if it occurs in late summer. But, 

repeated defoliation events may need to be more 

actively managed. In addition, young trees and 

trees stressed by drought events may also require 

protection. 

Monitor local populations of insects such as 

the walnut caterpillar. Seek the advice of a 

professional forest entomologist if outbreak 

populations persist for more than 1 year. 

Newly harvested logs should be removed from the 

woods as quickly as possible, especially during the 

warmer months of the year. 

LITERATURE CITED

Blair, L.M.; Kearby, W.H. 1979. The black walnut 

curculio and its impact on nut production. 

In: Kessler, K.J.; Weber, B.C., coords. Walnut 

insects and diseases workshop proceedings; 

1978 June 13-14; Carbondale, IL. Gen. Tech. 

Rep. NC-52. St. Paul, MN: U.S. Department 

1.

2.

1.

2.

1.

of Agriculture, Forest Service, North Central 

Forest Experiment Station: 51-54.

Farris, M.E.; Appleby, J.E.; Weber, B.C. 1982. 

Walnut caterpillar. For. Insect and Dis. Leafl. 

41. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 6 p.

Johnson, W.T.; Lyon, H.H. 1988. Insects that 

feed on trees and shrubs. Ithaca, NY: Cornell 

University Press. 556 p.

Kearby, W.H. 1979. The life history of Acrobasis 

demotella and its role in seedling and sapling 

stem deformities in Missouri. In: Kessler, 

K.J.; Weber, B.C., coords. Walnut insects and 

diseases workshop proceedings; 1978 June 

13-14; Carbondale, IL. Gen. Tech. Rep. NC-52. 

St. Paul, MN: U.S. Department of Agriculture, 

Forest Service, North Central Forest Experiment 

Station: 55-57.

Kulman, H. 1971. Effects of insect defoliation on 

growth and mortality of trees. Annual Review of 

Entomology. 16: 289-324.

Marshall, P.T. 1989. Insects that affect growth 

and quality of black walnut. In: Phelps, 

J.E., ed. The continuing quest for quality: 4th

Black walnut sympoisum; 1989 July 30-August 

2; Carbondale, IL. Indianapolis, IN: Walnut 

Council: 193-202.

Martinat, P.J.; Wallner, W.E. 1980. Notes on the 

biology and damge of two Acrobasis species

(Lepidoptera: Pyralidae) on black walnut in 

Michigan. Great Lakes Entomologist. 13: 41-48.

McKeague, M.; Simmons, G. 1979. Effects of 

insecticidal and cultural methods to reduce 

Acrobasis spp. and frost injury on black walnut. 

In: Kessler, K.J.; Weber, B.C., coords. Walnut 

insects and diseases workshop proceedings; 

1978 June 13-14; Carbondale, IL. Gen. Tech. 

Rep. NC-52. St. Paul, MN: U.S. Department 

of Agriculture, Forest Service, North Central 

Forest Experiment Station: 44-50.

Schowalter, T.D.; Hargrove, W.W.; Crossley, D.A. 

1986. Herbivory in forested ecosystems. Annual 

Review of Entomology. 31: 177-196. 

Solomon, J.D. 1995. Guide to insect borers of North 

American broadleaf trees and shrubs. Agric. 

Handb. 706. Washington, DC: U.S. Department 

of Agriculture, Forest Service. 735 p.

USDA. 1985. Insects of eastern forests. Misc. Publ. 

1426. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 608 p.
126 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

Weber, B.C.; Anderson, R.L.; Foffard, W.H. 1980. 

How to diagnose black walnut damage. 

Gen. Tech. Rep. NC-57. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, 

North Central Forest Experiment Station. 20 p.

Weber, B.C.; McPherson, J.E. 1984. Attack on 

black walnut trees by the ambrosia beetle 

Xylosandrus germanus (Coleoptera: Scolytidae). 

Forest Science. 30: 864-870.

Weber, B.C.; McPherson, J.E. 1985. Relation 

between attack by Xylosandrus germanus

(Coleoptera:Scolytidae) and disease symptoms 

in black walnut. Canadian Entomologist. 117: 

1275-1277.
Walnut Pest Management 127

Black Walnut in a New Century

CONTROL OF DEER DAMAGE WITH CHEMICAL REPELLENTS IN

REGENERATING HARDWOOD STANDS

Brian J. MacGo-wan, Larry Severeid, and Fred Skemp, Jr.1

ABSTRACT—Wildlife damage can be a major problem in natural tree regeneration or 

tree plantings. In the North Central Hardwoods region, white-tailed deer (Odocoileus 

virginianus) are a significant cause of damage to hardwood seedlings. We evaluated 

the use of a combination of chemical repellents (Hinder®, Tree Guard®, chicken eggs, 

and Plantskydd®) in restoring a poor hardwood stand to higher-valued, native trees in 

Hixon Forest, La Crosse, Wisconsin. Chemically treated oak (Quercus spp.) and black 

cherry (Prunus serotina) seedlings had less terminal browse damage (P < 0.005) during 

the growing season than untreated seedlings. Results during the winter will also be 

presented. Preliminary results support further study of an integrated use of chemical 

repellents in regenerating hardwood stands.

caused by wildlife, the factors that influence 

the severity of damage, and scientifically-based 

methods to reduce wildlife damage. 

Chemical repellents have been evaluated in 

deterring deer damage in pen (Andelt and others 

1992, Lutz and Swanson 1997, Witmer and others 

1997, Wagner and Nolte 2000) and field (Conover 

1984, Swihart and Conover 1990, Fargione and 

Richmond 1995, Witmer and others 1997, Lemieux 

and others 2000b) trials. However, few studies 

tested the efficacy of repellents in an integrated 

approach to minimizing deer damage. Switching 

among repellents has the potential to maximize the 

strengths of each repellent. Also, strategies using 

multiple techniques have a better chance of success 

than those that incorporate a single technique 

(Conover 2002:375). Jordon and Richmond 

(1992) found that the use of repellents increased 

the effectiveness of fencing alone. In our study, 

we evaluated the combined use of Hinder®, Tree 

Guard®, chicken eggs, and Plantskydd® to minimize 

deer damage to tree seedlings in a natural stand 

and present preliminary results in this paper. We 

also summarize and discuss research on available 

options to control deer damage in commercial tree 

plantings and natural hardwood regeneration. 

In the Central Hardwoods Region, white-tailed deer 

are arguably the most important species causing 

damage to forest resources (Conover and Decker 

1991). Their social and economic importance as a 

game species (USFWS 2001), reproductive capacity 

(McCullough 1997), and wide distribution (Fig. 1) 

are all contributing factors. 

Deer can impact natural tree regeneration 

(Tilghman 1989, Trumbull and others 1989, Jones 

and others 1993, Cornett and others 2000), tree 

plantings (Beringer and others 1994), nurseries 

(Conover and Kania 1995), or orchards (Mower and 

others 1997, Lemieux and others 2000b). Repeated 

browsing of terminal shoots distorts growth and 

suppresses tree seedling height and longer rotations 

and mortality can result (Nolte and Dykzeul 2002). 

Conover and others (1995) estimated total losses 

from wildlife damage to the timber industry in the 

U.S. to be $3.4 million annually. Annual economic 

losses due to wildlife damage to all forest resources 

in Oregon were estimated at $333 million (Nolte 

and Dykzeul 2002). With no animal damage 

management, the total predicted reduction in value 

for Oregon forests alone was estimated at $8.3 

billion. These figures underscore the importance for 

understanding the types and amounts of damage 

1

Extension Wildlife Specialist (BJM), Department of Forestry and Natural Resources, Purdue University, 1250 North Franklin Avenue, 

P.O. Box 265, Brookville, IN 47012; (LS), 2524 Zephyr Circle, La Crosse, WI 54601; and (FS), 2210 Park Drive, La Crosse, WI 54601; BJM 

is corresponding author: to contact, call (765) 646-3538 or e-mail at bmacgowan@fnr.purdue.edu.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
128 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

METHODS

The Hixon Forest is a 291 ha woodland located 

within the city of La Crosse, Wisconsin. The 

forest has been uncut for the past 90 years and is 

dominated by a mixture of timber oaks (Quercus

spp.), hickory (Carya spp.), bigtooth aspen (Populus

grandidentata), black walnut (Juglans nigra), 

black cherry (Prunus serotina), and basswood (Tilia

americana). Because of it is located within the 

city limits of La Crosse, deer densities and browse 

pressure are presumably high, although we did not 

measure this. 

We were interested in regenerating shade intolerant 

species that would be available for education 

and research. We chose an area accessible to the 

public that was dominated by black locust (Robinia

pseudoacacia), an invasive tree not native to 

the region. In winter of 2002, a 0.6 ha opening 

was cleared in an initial attempt to evaluate the 

feasibility of regenerating more economically 

valuable tree species using a combination of 

chemical repellents and herbicide treatment. All 

trees and shrubs were cut except a few trees which 

were left for aesthetics at the request of the park. 

Dominate species removed included black locust, 

bigtooth aspen, birch (Betula spp.), box elder (Acer

negundo), and buckthorn (Rhamnus spp.). The 

opening was bordered on the west by a maintained 

mowed area (approx. 1 acre). 

The clearing was divided into one treatment plot 

(approx 0.5 ha) and two control plots (each approx. 

0.05 ha). We maintained several access trails within 

the treatment block. Control plot A was planted 

with tree seedlings, and both controls had no weed 

control or repellent applications. 

During the spring of 2003 in the treatment block, 

control plot A, and the perimeter of control plot 

B we planted 1,157 2-year-old seedlings of white 

oak (Quercus alba), swamp white oak (Quercus 

bicolor), red oak (Quercus rubra), shagbark hickory 

(Carya ovata), black cherry, white ash (Fraxinus 

americana), black walnut, butternut (Juglans 

cinerea), and white pine (Pinus strobus) in a 6 

ft by 7 ft spacing. Each seedling was mulched 

with woodchips and marked with a wood stake. 

Seedlings were planted in clusters of 7-9 seedlings 

of the same species. In the treatment plot, we 

sprayed a glyphosate herbicide (Round-Up®) at the 

labeled rate in an approximately 50 cm radius band 

around each seedling. Thirty-four oaks, walnuts, 

and butternut were replanted in the fall of 2003 to 

replace early seedling mortality. 

Figure 1.—Total deer harvested per square mile (region for Michigan) in the Midwest in 1980 and 

2000. The total number of whitetail deer harvested increased by nearly 250 percent during this 

time period. (USDA Forest Service, North Central Region)
Walnut Pest Management 129

Black Walnut in a New Century

Prior to planting the treatment plot, we brushed 

Tree Guard® (a Bitrex formulation, [0.2% 

denatonium benzoate]) onto each terminal bud. 

When the seedlings began to break bud post 

planting, we sprayed the terminals with a premixed 

formulation of Plantskydd® (87% edible animal 

protein), a taste and smell deterrent. Beginning 

in May, using a hand-held sprayer we applied a 

mixture of 200 ml Hinder® (0.66% ammonium soaps 

of higher fatty acids), six blenderized chicken eggs, 

and 3.78 L water at approximately monthly (4-6 

weeks) intervals until the seedlings went dormant 

in October. Each application took approximately 1.5 

man-hours. After all seedlings were dormant, we 

reapplied Tree Guard® with a paint brush on each 

terminal bud. A second application was applied in 

February 2004. 

In the treatment plot, competing vegetation was 

controlled once it began to shade planted seedlings. 

Vegetation was removed by hand-pulling or with 

the use of brush cutters. A glyphosate herbicide 

(Round-Up®) at the labeled rate was applied with 

a wick applicator or hand-held sprayer onto all 

buckthorn and honeysuckle bushes, and stump 

sprouts of any species.

In all plots, we tallied tree survival during the first 

year of growth. In the fall of 2003 and spring of 

2004, we inspected the terminal of each tree for 

browse damage. Any sign of damage was counted 

as being browsed and recorded for each species or 

species group. We present data from the treatment 

and control plot A. 

RESULTS

Until November 2003, 9.4% of the seedlings died 

(n = 112). Seedling survival was similar in both 

groups. The hickory seedlings had the lowest 

survival rate (68%) and accounted for most of the 

seedlings that failed the first growing season. Only 

black walnut seedlings (86%) had a survival rate 

below 90%. 

Summer browse damage was confined mainly to 

black cherry and white ash, and occasionally oaks 

(Table 1). Percent browsed was much higher in the 

control plot for oaks (χ2 = 13.1, df = 1, P = 0.0003) 

and black cherry (χ2 = 8.2, df = 1, P = 0.004). 

Percent browsed for shagbark hickory and white 

ash in the treatment plots were 1.7% and 11.9%, 

respectively. Neither of these species showed browse 

damage in the control plot. 

Preliminary inspection this winter indicates that 

most of the white pine seedlings have browse 

damage while other species had an increased 

damage rate during this period. This data has yet to 

be collected and will be presented during our oral 

presentation.

DISCUSSION

Our data suggest that a combination of chemical 

repellents and herbicide treatments has the 

potential to provide satisfactory results in 

controlling deer browsing of tree seedlings in the 

first year post harvest. However, conclusions 

should be drawn with caution since our results are 

preliminary and sufficient replication was lacking. 

The ultimate measure of success will be the number 

of tree seedlings that survive to a height beyond the 

reach of a deer. 

Repellents are more appropriate for short-term 

problems (Conover 2002) and are typically reserved 

for smaller areas because it is cost-prohibitive. For 

Table 1.—Percent of tree seedlings with terminal bud browsed by white-tailed deer during 2003 growing 

season. 

Control Plot A Treatment

Species Total Trees

Number of 

Trees Browsed

Percent 

Browsed Total Trees

Number of 

Trees Browsed

Percent 

Browsed

Oak spp. 29 7 24.1 253 14 5.5

White pine 18 0 0.0 115 0 0.0

White ash 13 0 0.0 67 8 11.9

Butternut 15 0 0.0 98 0 0.0

Black walnut 16 0 0.0 137 0 0.0

Black cherry 16 7 43.8 166 40 24.1

Shagbark hickory 15 0 0.0 58 1 1.7

 Total 122 14 11.5 894 63 7.0
130 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

our study, an annual cycle of repeated applications 

of chemical repellents on the treatment plot (0.6 ha) 

was approximately $71.00. Many studies site cost as 

a downside to using chemical repellents. However, 

our costs were relatively inexpensive, although cost 

of labor would significantly increase our costs. Costs 

would also be higher for remote sites. 

In a review of the current literature, El Hani 

and Conover (1997) did not find a repellent that 

consistently reduced deer damage > 50% in field 

trials. Despite the wide variance in study design, 

location, deer and plant species, trial duration, 

criteria of success, and time of year, they found 

BGR® was the most effective repellent. Predator 

urine, although not yet labeled by the EPA for 

use as repellents, showed promise. We used 

a combination of repellents that had mixed to 

favorable results in previous studies. In studies 

reviewed by El Hani and Conover (1997), Hinder®

was ranked as effective (n = 3), intermediate (n = 

2), or slightly effective (n = 1); chicken eggs was 

effective (n = 2). In a test of 20 repellents and 2 

delivery systems, Wagner and Nolte (2001) found 

only BGR® and Plantskydd® reduced browse damage 

to western red cedar by penned black-tailed deer. In 

their study, Hinder® protected seedlings up to only 

4 weeks and Tree Guard® up to 12 weeks. In a trial 

of Hinder®, Tree Guard®, Miller Hot Sauce®, Deer 

Away®, chicken eggs, and predator urine, Lutz and 

Swanson (1997) found Tree Guard® to be the most 

ineffective while the others had mixed results. Our 

study involved the use of several repellents with 

repeated applications (7) per year. 

We cannot differentiate if one, two, or all three 

repellents were effective. Our study site was located 

in an area with presumably high deer populations 

typical of urban areas not open to hunting. Deer 

browse pressure is at its highest during the winter 

when alternative forage is limited. Chemical 

repellents have been found to loose efficacy when 

presented to hungry cervids (Andelt and others 

1991, 1992). Efficacy of our methods may be 

reduced during the winter and may approach 

results of other studies (i.e., < 50%).

Methods other than chemical repellents have 

been used to minimize deer damage in natural 

stands and commercial nurseries. Combining 

one or more of these methods in conjunction with 

chemical repellents may reduce deer browsing than 

any method used alone. A summary of common 

techniques and their results is presented below.

Cultural Methods

Tree seedlings are vulnerable to deer browsing up 

to about 1.8 m (Craven and Hyngstrom 1994). One 

strategy to minimize deer damage in regenerating 

stands is to decrease the length of time terminal 

shoots are available to deer or physical prevent 

access to tree seedlings. Cultural methods to 

reduce deer damage to tree seedlings in harvested 

stands include leaving slash piles (Bergquist and 

Örlander 1998) or management of the surrounding 

vegetation. Gourley and others (1990) found that 

vegetation control helped minimize the effects of 

deer browsing by reducing competition for the tree 

seedlings, allowing for increased growth rates. 

However, surrounding vegetation may also protect 

seedlings by concealing them or by providing an 

alternate food source (Campbell and Evans 1978). 

Diversionary foods may deter small herbivore 

damage to tree seedlings (Sullivan and others 

2001).

The size of forest opening may influence the amount 

of deer damage. Akins and Michael (1997) found 

that percent browsed for all tree species groups 

was generally lower in 0.8 ha clearcuts compared 

to 0.2 ha clearcuts. Our openings approached the 

lower end of their study. Further research needs 

to done to test the effectiveness of our methods or 

the amount of repellents and treatments on larger 

openings. If larger openings have lower browse 

pressure, chemical repellents can potentially be 

more cost efficient on a per acre basis, although 

total costs may still be prohibitive. 

Fencing

Electric fences can reduce deer damage (Craven 

and Hyngstrom 1988, Jordan and Richmond 1992). 

Fencing has the advantage of being a long-term 

solution. Cost varies by fence type, but all are 

relatively expensive compared to other techniques 

(Table 2). Fences require regular maintenance, 

are effective on areas 16 ha or less, and success 

is maximized if installed prior to planting (Craven 

and Hyngstrom 1994). A temporary electric fence 

provides inexpensive protection when the goal is to 

minimize browse damage to terminal buds of trees 

until they have outgrown the reach of deer. 

 Lethal Control

Have been used successfully in urban areas and 

lands previously protected from deer harvest. A 

shotgun-archery hunt reduced a residential deer 

herd by 92% in 6 days (Kilpatrick and others 2002). 

Success would likely be lower in more rural areas 

with more abundant habitat and would depend in 

part on surrounding hunting pressure and hunter 

skill. Lethal control is often sited as the most costeffective method of controlling wildlife populations 

(Conover 2002). Most states have a program where 
Walnut Pest Management 131

Black Walnut in a New Century

deer can be taken out of season with a permit when 

damage exceeds a set economic threshold. 

Methods used to control deer damage in natural 

stands are commonly utilized in tree plantations 

and nurseries (Lemieux and others 2000a). Damage 

caused by deer is difficult to control in nurseries 

because their home range likely will include the 

nursery and the surrounding area. Because of 

their small home ranges, damage caused by 

smaller herbivores are less influenced more by 

physical characteristics of a plantation rather 

than surrounding habitat (Pietrzykowski and 

others 2003). Annual variation of deer browsing 

in Connecticut nurseries was not associated 

with local deer densities, the availability of native 

browse, or winter weather conditions (Conover and 

Kania 1995). Physical size of nurseries, the degree 

bordered by woodlots, or level of remoteness has 

not been found to influence observed levels of deer 

browsing, but the size of adjacent woodlots and 

the combined area of woodlots within 2 km has 

(Conover 1989). 

In conclusion, our preliminary results support 

further study of an integrated use of herbicides 

and chemical repellents in regenerating natural 

hardwood stands. Their success will likely depend 

on use of other methods, local deer browse 

pressure, surrounding habitat, and weather and 

other stochastic variation. Further research is 

needed to identify influencing factors that explain 

the level of deer browsing in different landscapes at 

different temporal scales. As development pressure 

increases and forestland ownership continues 

to become increasingly fragmented, economic 

pressures to regenerate nature stands of highvalued timber may necessitate more intensive and 

costly control of deer damage and other limitations 

to regeneration. 

LITERATURE CITED

Andelt, W.F.; Burnam, K.P.; Manning, J.A. 

1991. Relative effectiveness of repellents for 

reducing mule deer damage. Journal of Wildlife 

Management. 55: 341-347.

Andelt, W.F.; Baker, D.L.; Burnam, K.P. 1992. 

Relative preference of captive cow elk for 

repellent-treated diets. Journal of Wildlife 

Management. 56: 164-173.

Anderson, R.C.; Loucks, O. L. 1979. White-tailed 

deer (Odocoileus virginianus) influence the 

structure and composition of Tsuga canadensis

forests. Journal of Applied Ecology. 16: 355-

361.

Akins, J.W.; Michael, E.D. 1997. Impact of 

clearcut size on white-tailed deer use and tree 

regeneration. Proceedings, 6th Eastern Damage 

Control Conference. 6: 185-195.

Bender, B.A. 1998. Reducing deer damage to 

forest crops. Michigan Agricultural Experiment 

Station Publication. East Lansing, MI: Michigan 

State University.

Bergquist, J.; Örlander, G. 1998. Browsing damage 

by roe deer on Norway spruce seedlings planted 

on clearcuts of different ages: 2. Effects of 

seedling vigor. Forest Ecology and Management. 

105: 295-302.

Table 2.—Cost comparison of different fencing types (from Craven and Hyngstrom 1994, Bender 1998).

Fence Type Cost Per Linear 

Foot

Cost Per 

Acre Items Included in Cost Year

8-ft. mesh / woven wire fence $1.05

$2-$4

$1222 Materials 1990

1994

$4-$6 Materials & installation

High tensile electric fence (5-ft.) $0.89-$2.03 Materials & installation 1998

$30/year Maintenance 1998

High tensile electric fence (6.5-

ft.)

$2.15-$2.70 $368 Materials & installation 1992

High-tensile fence (8-ft.) $0.75-$1.50 Materials 1994

Polytape electric fence $0.50-$1.20 Materials 1997

$0.11 Materials 1994

Slanted seven-wire fence $0.75-$2.00 Materials 1994
132 Walnut Pest Management

Proceedings of the 6th Walnut Council Research Symposium

Beringer, J.; Hansen, L.P.; Heinen, R.A.; Giessman, 

N.F. 1994. Use of dogs to reduce damage by 

deer to a white pine plantation. Wildlife Society 

Bulletin. 22: 627-632.

Campbell, D.L.; Evans, J. 1978. Establishing native 

forbs to reduce black-tailed deer browsing 

damage to Douglas-fir. In: Proceedings, 8th

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Conover, M.R. 1984. Effectiveness of repellents in 

reducing deer damage in nurseries. Wildlife 

Society Bulletin. 12: 399-404.

Conover, M.R. 1989. Relationship between 

characteristics of nurseries and deer browsing. 

Wildlife Society Bulletin 17: 414-418. 

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conflicts: the science of wildlife damage 

management. Boca Raton, FL: Lewis 

Publishers. 418 p.

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to crops: perceptions of agricultural and wildlife 

professionals in 1957 and 1987. Wildlife Society 

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on ornamentals in nurseries. Journal of 

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damage to woody and herbaceous plants. 

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research center special symposium; 1995 

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orchards in Ohio. Wildlife Society Bulletin. 25: 

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Walnut Pest Management 133

Black Walnut in a New Century

Nolte, D.L.; Dykzeul, M. 2002. Wildlife impacts 

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by mammalian herbivores. Journal of Applied 

Ecology. 40: 458-469.

Sullivan, T.P.; Sullivan, D.S; Hogue, E.J. 2001. 

Influence of diversionary foods on vole 

(Microtus montanus and Microtus longicaudus) 

populations and feeding damage to coniferous 

tree seedlings. Crop Protection. 20: 103-112.

Swihart, R.K.; Conover, M.R. 1990. Reducing 

deer damage to yews and apple trees: testing 

Big Game Repellent®, Ro-pel®, and soap as 

repellents. Wildlife Society Bulletin. 18: 156-162. 

Tilghman, N.G. 1989. Impacts of white-tailed 

deer on forest regeneration in northwestern 

Pennsylvania. Journal of Wildlife Management. 

53: 524-532.

Trumbull, V.L.; Zielinski, E.J.; Aharrah, E.C. 1989. 

The impact of deer browsing on the Allegheny 

forest type. Northern Journal of Applied 

Forestry. 6: 162-165.

USFWS. 2001. National survey of fishing, hunting, 

and wildlife-associated recreation. Washington, 

DC: U. S. Department of Interior, Fish and 

Wildlife Service.

Wagner, K.K.; Nolte, D.L. 2000. Evaluation of Hot 

Sauce® as a repellent for forest mammals. 

Wildlife Society Bulletin. 28: 76-83.

Wagner, K.K.; Nolte, D.L. 2001. Comparison of 

active ingredients and delivery systems in deer 

repellents. Wildlife Society Bulletin. 29: 322-

330.

Whitmer, G.W.; Sayler, R.D.; Pipas, M.J. 1997. 

Repellent trials to reduce reforestation damage 

by pocket gophers, deer, and elk. In: Mason, 

J.R., ed. Repellents in wildlife management, 

proceedings, 2nd National wildlife research 

center special symposium; 1995 August 8-10; 

Denver, CO. [Denver, CO]: U.S. Department of 

Agriculture, Wildlife Services, National Wildlife 

Research Center: 321-332.
134 International Walnut Research

Proceedings of the 6th Walnut Council Research Symposium

A NEW TREE IMPROVEMENT PROGRAMME FOR

BLACK WALNUT IN THE UNITED KINGDOM

Karen Russell and Gabriel E. Hemery1

ABSTRACT—Black walnut is an introduced species in the United Kingdom (UK) and 

economically one of the more productive broadleaved species. Currently it is not widely 

planted in the UK as there is insufficient knowledge about the species among foresters 

and very little, if any, improved material is available. A research programme was initiated 

in 2001 to address both these issues. A range-wide seed collection from plus trees 20 

US provenances and 10 European provenances and 2 European populations has been 

undertaken with a view to establishing two provenance/progeny trials. A collection of 

trees grafted from 63 individual plus trees has also been planted for assessment of form, 

vigour and disease resistance.

INTRODUCTION

Black walnut (Juglans nigra L.) was introduced 

to Britain from North America in the early 17th 

Century (Bean 1981) and was widely planted for 

its valuable timber. The species can produce high 

quality timber on a relatively short rotation of 

about 60 years and is consistently in great demand 

by end-users. During 2001 alone, UK imports of 

this valuable timber rose by 51% (Buckley 2002). 

Currently, the species is often disregarded by UK 

foresters because of its reputation as being sitedemanding, usually of poor form and vulnerable to 

frost. No research has effectively addressed these 

problems in the UK.

The Forestry Commission established a series of 

black walnut provenance experiments between 

1986 and 1987; a limited range of material from 

four provenances (Wisconsin, Southern Illinois, 

Ohio and Vermont) was tested (Kerr 1993). 

Insufficient seed was available for testing all 

material across four low-quality UK trial sites. A 

more comprehensive selection of material, selecting 

desirable, straight-stemmed and lightly-branched 

trees suitable to the climatic conditions of the UK 

should encourage a renewed interest in the species.

In 2001, Horticulture Research International 

(HRI) and the Northmoor Trust (NMT) initiated a 

new project for the improvement of black walnut 

in the UK to provide information and material. 

This is a collaborative project under the auspices 

of the British and Irish Hardwoods Improvement 

Programme (BIHIP) with sponsorship from the 

National Forest Company, the Department for 

Environment, Food and Rural Affairs (Defra) and 

the Forestry Commission.

Seed material was collected across the current 

natural US and introduced European ranges of 

the species and is being tested in a long-term 

provenance/progeny research programme. Field 

trials containing European and North American 

seedlots will be established during winter 2003 and 

winter 2004 with the aim of identifying superior 

black walnut trees that can act as seed trees or be 

cloned for timber production. A clonal collection 

of superior ‘plus’ trees has been established at 

HRI. This paper summarizes progress made to the 

autumn of 2003 and outlines expected outcomes 

from the research programme.

FIELD TRIAL METHODS

Selection of Seed Sources

Research institutes in 27 European countries 

were contacted to establish the importance of 

black walnut in their country and their interest 

and willingness to participate in the black walnut 

1

Woodland & Forestry Specialist (KR), Horticulture Research International, East Malling, Kent, ME19 6BJ, UK; and Forestry Research 

Manager (GEH), Northmoor Trust, Little Wittenham, Oxfordshire, OX14 4RA, UK. KR and GEH are corresponding authors, to contact 

call ++44 (0)1732 843833 or email Karen.Russell@hri.ac.uk or ++44 (0)1865 407792 or e-mail Gabriel.Hemery@NorthmoorTrust.co.uk. 

website: www.NorthmoorTrust.co.uk.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.; Coggeshall, M.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. 

Gen. Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p. 
International Walnut Research 135

Black Walnut in a New Century

improvement programme. Institutes that wished to 

participate then identified suitable stands and trees 

for seed collection. In the UK, individual landowners 

were targeted through the forestry press (Hemery 

and Russell 2002). Within the programme, 

provenance is defined as a seed collection unit 

composed of a community of potentially interbreeding 

trees in an area subject to uniform environmental 

conditions (adapted from Turnbull and Griffin 

1986). In countries with scattered individual trees 

the term ‘population’ is used. Progeny refers to halfsib families.

During July 2001, the authors attended the annual 

meeting of the Walnut Council in Wisconsin, 

USA, and visited staff of the Hardwood Tree 

Improvement and Regeneration Centre (HTIRC) 

at Purdue University to encourage the search 

for North American provenances (Hemery 2002). 

HTIRC agreed to coordinate the identification and 

collection of seedlots from across the natural range 

of black walnut in the USA.

Seed Collection

Thirty-two provenances were targeted, 20 from the 

United States and 12 (including two populations) 

from Europe, each ideally comprising 20 half-sib 

progenies, or otherwise as bulked provenances/

populations. Twenty viable seeds from each of 20 

mother trees of good or outstanding phenotypic 

character (Fig. 1) were required per provenance. 

The target number of seeds required per mother 

tree was 30 assuming a 60% germination rate 

(Brinkman 1974), giving approximately 600 seeds 

or approximately 7 kg of seed (cleaned weight) per 

provenance.

A minimum distance of 100 m was specified 

between mother trees within provenances and a 

wide geographic distribution among provenances 

within each country was requested of collaborators 

in each country. The identity of each mother tree 

was maintained for each. seedlot. The phenotypic 

characters of each mother tree was assessed and 

their environmental location recorded. Seeds 

were collected at seed fall during both 2002 and 

2003. Where black walnut trees were scarce, a 

lower number of progenies or a bulked population 

collection was accepted.

The European seedlots were sent to HRI-East 

Malling where the seed was cleaned and sorted 

before distribution to a commercial nursery. 

The North American seedlots were cleaned and 

dispatched from HTIRC directly to Alba Trees plc, 

the UK nursery. After a minimum of 120 days of 

stratification, the seeds were sown into root-trainers 

of 400 cc volume and 17 cm depth and germinated 

in a protected environment (polytunnel). One-yearold seedlings will be used in the establishment of 

the field trials.

Experimental Design

The main purpose of the field trials is to assess the 

suitability of provenances for timber production 

under UK conditions, with particular emphasis on 

selection for good form and late flushing. The trials 

will also provide a resource for genetic diversity 

studies. The trees will be planted out in combined 

provenance/progeny experiments at two sites 

during 2003 and 2004. Seedlots were received 

during 2 years, therefore, one UK progeny will be 

established at each field trial in both years as a 

control population.

Field trials will be established at two sites in 

central and southern England (Fig. 2). The central 

England site is at the National Forest at Lount 

in Derbyshire (52° 36’ N/1° 25 W, soils pH 7.0, 

altitude 120 asl, annual precipitation 600 mm). 

The southern England site is at the Northmoor 

Figure 1.—The ideal black walnut plus-tree or 

‘ideotype’.
136 International Walnut Research

Proceedings of the 6th Walnut Council Research Symposium

Trust’s Forest Research Centre in Oxfordshire. (51° 

38’ N/1° 12’ W, soils pH 6.2 to 7.7, altitude 50 m 

asl, annual precipitation 570 mm). The proposed 

design combines both provenance and progeny 

assessments in a single experiment. Provenances 

will be distributed in non-contiguous multiple-tree 

plots. Progenies will be distributed as single-tree 

plots within a randomised complete block design, 

the replicate (block) size being dependent on the 

final number of progenies available (usually √n

progenies). The 30 provenances will provide 411 

progenies for evaluation at each site, the remainder 

will consist of bulked population collections.

Plantation Design

Trees will be spaced at 2 x 2 m (2,500 trees ha-1) 

and protected by 0.75 m tree shelters. The noncontiguous provenance plots will allow thinning 

whilst minimizing progeny losses. A study using 

molecular markers of the genetic diversity of 

genotypes within the trials is planned early in the 

life of the programme to minimise the impact of 

thinning.

RESULTS

Out of a total of 16 European countries who 

expressed interest in the research programme, 

seven provided seedlots. Four countries provided 

seedlots (number in brackets) during 2002: Austria 

(2), Czech Republic (3), Slovak Republic (2) and 

the UK (1). The seedlots from the UK were typically 

collected from individual trees, often distributed 

in parkland and only rarely in woodland and are 

termed a population.

In the autumn of 2003, seedlots were provided by 

France (1), Italy (2), Serbia and Montenegro (2), and 

additional seedlots were collected in the UK (1). 

The seedlot from France was collected from a seed 

orchard as was one seedlot from Italy. The second 

Italian seedlot was collected within a stand. The 

seedlots from Serbia and Montenegro and the UK 

(2002 and 2003) were from scattered individual 

trees and therefore treated as populations. A total 

of 10 European provenances and two populations 

have been collected (Fig. 2).

Seedlots were collected from 20 provenances 

across 13 States in eastern North America (Fig. 

3): Alabama (1), Illinois (2), Indiana (3), Iowa (1), 

Kentucky (2), Maryland (2), Minnesota (1), North 

Carolina (1), Ohio (1), Pennsylvania (1), Tennessee 

(1), Wisconsin (3), and Missouri (1). The majority 

of the American seedlots were collected from trees 

within stands and the number of trees per stand 

from which seed was collected varied from 10 to 30.

Over 1,600 seedlings were raised in 2003 for 

establishment in the first phase of the provenance 

trials in winter 2003/04. The germination rates for 

the Austria and Czech Republic provenances were 

similar at 74% and 72% respectively; however for 

the UK population only 28% germinated as seed 

from some trees failed to germinate. One of the 

provenances from the Slovak Republic failed to 

germinate whereas the other had a germination 

rate of 39%. Some 20,000 seeds were stratified in 

winter 2003 and sown in spring 2004. The resulting 

seedlings will be planted out in the second phase of 

the provenance trial in winter 2004/05.

Figure 2.—Distribution of European black walnut 

provenances sampled for UK trials.

Figure 3.—Distribution of North American black 

walnut provenances sampled for UK trials.
International Walnut Research 137

Black Walnut in a New Century

CLONAL COLLECTIONS

Between 1997 and 2003, graftwood from superior 

black walnuts was supplied by INRA Bordeaux, 

France, and HTIRC, Indiana, USA, and collected 

by HRI from outstanding mature trees in the UK. 

The graftwood was propagated at HRI-East Malling 

on seedling rootstocks of common walnut (Juglans

regia L.) using a ‘hot pipe’ technique developed 

for the propagation of difficult species (Toogood 

1999). A total of 63 superior trees were successfully 

propagated (26 US, 25 UK, 12 Mainland Europe) 

and planted in the walnut collection at Bradbourne 

House, East Malling (Russell 2002) which also 

includes accessions of common walnut (fruit 

and timber selections) and interspecific timber 

selections. The accessions are being assessed for 

form, vigour, budbreak, and resistance to pests and 

diseases, including anthracnose.

FUTURE WORK

The provenance/progeny trials will be assessed for 

survival, flushing, vigour (height and diameter), 

form, and occurrence of anthracnose. Data will be 

analyzed and superior progenies and individuals 

will be selected to form the basis of the next stage 

of the improvement programme after 10 years. It 

is anticipated that clonal trials of plus trees and/

or of seedling selections will be established in the 

future. In addition, it is hoped that a collaborative 

research programme on the genetic diversity of 

these collections can be initiated with HTIRC with 

the objectives of assessing the extent of diversity 

in black walnut represented in the programme and 

potentially providing information on origin of the 

European material.

ACKNOWLEDGMENTS

The programme has been supported in the UK 

by Jaguar Cars, The National Forest Company, 

Woodland Heritage, the East Malling Trust for 

Horticultural Research, the Northmoor Trust 

for Countryside Conservation, the Department 

for Environment, Food and Rural Affairs and 

the Forestry Commission. Our thanks go to the 

research institutes and individuals in Austria, 

Czech Republic, France, Italy, Slovak Republic, 

Serbia and Montenegro, and North America, 

particularly the HTIRC, who have provided seedlots 

and/or graftwood to date and to Alba Trees plc for 

raising the seedlings.

LITERATURE CITED

Bean, W.J. 1981. Trees and shrubs hardy in the 

British Isles. 8th ed. Revised, Vol. II D-M. 

London: John Murray. 476 p.

Buckley, M. 2002. Revivalist movement. Timber 

Trades Journal, 9th February: 14.

Brinkman, K.A. 1974. Juglans L., walnut. In: Seeds 

of woody plants in the United States. Agric. 

Handb. 450. Washington, DC: U.S. Department 

of Agriculture, Forest Service: 454-459.

Hemery, G.E. 2002. Walnut forestry tour of the 

mid-western United States. Woodland Heritage: 

20-22.

Hemery, G.E.; Russell, K. 2002. Filling walnut’s 

black hole. Forestry and British Timber. 

December: 22-23.

Kerr, G. 1993. Establishment and provenance of 

walnut in Britain. Forestry. 66: 381-393.

Russell, K. 2002. The Walnut collection at HRI-East 

Malling 2002. The Walnut Club Newsletter. 

Issue 3 (July 2002): 13-15.

Toogood, A., ed. 1999. The Royal Horticultural 

Society - propagating plants. London: Dorling 

Kindersley. 109 p.

Turnball, J.W.; Griffin, A.R. 1986. The concept of 

provenance and its relationship to infraspecific 

classification in forest trees. In: Styles, B.T., ed. 

Infraspecific classification of wild and cultivated 

plants. Oxford: Clarendon Press: 157-189.
138 International Walnut Research

Proceedings of the 6th Walnut Council Research Symposium

GENETIC AND SILVICULTURAL RESEARCH PROMOTING COMMON WALNUT

(JUGLANS REGIA) FOR TIMBER PRODUCTION IN THE UNITED KINGDOM

Gabriel E. Hemery1

ABSTRACT—A combination of genetic and silvicultural research is required to improve 

the viability of common walnut for timber production in the UK. A summary of a research 

programme, initiated in 1996, is provided. Establishment of walnut plantations using 

tree shelters indicated positive benefits using 0.75 m shelters but larger shelters (1.20 

m) caused early flushing and increased dieback. Direct seeding trials indicated that pretreatment of seed with gibberellins was highly effective in reducing the need for lengthy 

pre-treatments and promoted 87% germination. Artificial application of nitrogen fertiliser on 

newly established walnut trees indicated that applications up to 500 kg ha-1 had no effect 

on survival or early growth. Assessments of a range-wide collection of walnut, including 

provenances from Kyrgyzstan, point to high variability in flushing and growth between 

provenances and progenies 4 years after establishment. Genotype by site interaction within 

the three provenance trials is evident. Adoption of new, to the UK, silvicultural systems 

incorporating tree and shrub nurses indicated promising benefits from using the nitrogenfixing nurse species autumn olive and Italian alder.

Common walnut (Juglans regia L.) is an ancient 

introduced species to the United Kingdom but 

today there are probably fewer trees than at any 

time since the late sixteenth or early seventeenth 

centuries. Interest in walnut as a timber waned 

with the increasing availability of tropical 

hardwoods from the early nineteenth century 

onwards and it has been infrequently planted in 

the UK since that time, particularly as a forest 

tree. Supplies of hardwoods such as mahogany 

are now, nearly 200 years later, becoming scarce. 

There is also an increasing awareness by European 

consumers that the use of tropical timbers may 

contribute to deforestation, resulting in a reluctance 

to buy them. There are therefore good reasons 

to believe that valuable, decorative, temperate 

hardwoods are likely to be in much greater demand 

as tropical supplies decline.

Common walnut is perhaps the finest of these 

valuable species, and is seen as a tree that could 

regain the place it had centuries ago, as the 

provider of high quality timber on relatively short 

rotations. The wood is used for making quality 

furniture and producing highly figured veneers, 

usually from burrs, which are used for cabinetmaking and decorative panels. At present however, 

common walnut is often overlooked by foresters due 

to its reputation as being a species which is site 

demanding, usually of poor form and which suffers 

badly from the effects of frost.

The tree phenotypes seen in the UK today are 

often of poor form for timber production because 

their likely origin is from European trees originally 

selected for nut production. Phenotypes for 

timber or nut production are generally viewed as 

incompatible because good phenotypes for timber 

(e.g., long and straight stemmed, finely branched) 

have deliberately been selected against. Shortboled, spreading and branched trees were sought 

for high nut productivity and ease of harvesting. 

Additionally, some phenotypes in Britain may 

originate from ancient introductions, taken 

from environments unsuitable for widespread 

introduction to the British climate.

To encourage a revival of interest in walnut in the 

UK and to promote best practise, a number of 

activities were necessary:

1Forestry Research Manager (GEH), Northmoor Trust, Little Wittenham, Oxfordshire, OX14 4RA, UK. Call ++44 (0)1865 407792 or email Gabriel.Hemery@NorthmoorTrust.co.uk. website: www.NorthmoorTrust.co.uk.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.; Coggeshall, M.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. 

Gen. Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p. 
International Walnut Research 139

Black Walnut in a New Century

research directed towards improving initial 

establishment;

desirable, straight stemmed and finely branched 

trees needed to be selected to suit the climatic 

conditions of the UK;

the collected genotypes needed to be tested in a 

range of conditions in the UK;

novel plantation designs needed to be tested to 

improve silvicultural conditions;

research programmes and promotion of walnut 

growing needed to be coordinated.

PLANTATION ESTABLISHMENT

Tree Shelters

Background and Method

Tree shelters are widely used in the UK as a 

means to protect establishing trees from pests and 

additionally to provide protection from herbicides in 

combination with micro-climatic benefits. Costs for 

both shelter and supporting stake vary according 

to size but for an average 0.75 m shelter/stake 

costs approximately $1.6 at current prices, and are 

often more cost effective than fences when the trees 

are spaced widely. An experiment was established 

in 1996 testing the effects of two common sizes of 

shelter (0.75 and 1.2 m) and a control (no shelter) 

on the establishment and early growth of common 

walnut, planted as bare-rooted transplants (Hemery 

and Savill 2001). Observations were made on height 

and stem diameter growth, flushing (leaf burst) and 

dieback over a 4-year period.

Results and Discussion

Treeshelters were beneficial for both tree height 

increment and stem diameter growth however, the 

1.2 m tall shelters promoted earlier flushing than 

the other shelter treatments, resulting in a risk of 

increased frost damage (Hemery and Savill 2001). 

Additionally, a substantial degree of stem dieback 

occurred during one dormant season in the 1.2 m 

shelters when 46% of tree height was lost, whereas 

there were no significant differences for dieback 

between those trees without shelters and those in 

0.75 m shelters (20% and 15%, respectively). Hemery 

and Savill (2001) therefore recommended medium 

sized (0.60 or 0.75 m) shelters where possible for the 

successful establishment of walnut. Medium-sized 

tree shelters were therefore adopted for most of the 

experiments described here and are widely adopted by 

landowners planting walnut in the UK.

1.

2.

3.

4.

5.

Direct Seeding

Background and Method

Walnut plantations can be costly to establish both 

in terms of plant cost (typically equivalent to $1.6 

or more per 60 – 90 cm transplant in the UK) 

and labour due to the large taproot. Walnut also 

has a reputation as being sensitive to transplant 

shock and intolerant of damage to the taproot 

(Popov 1981). Direct seeding was proposed as 

an alternative method to establish a walnut 

plantation although methods of ensuring adequate 

germination and growth needed to be determined.

The effectiveness of four different seed preparation 

techniques in relation to walnut establishment by 

direct seeding was tested in a trial sown in spring 

2001. The trial was a randomized complete block 

design with five nine-tree replicates per treatment. 

In addition to a control (no pre-treatment), seeds 

were stratified according to recommended practise 

(Dirr and Heuser 1987) where seeds were placed 

in damp peat (sedge peat -neutral pH) and stored 

between 0 and 4° C for 4 - 6 weeks. Another 

treatment included water-soaked seeds where these 

were partially opened at the basal joint with a blunt 

knife and soaked for 48 hours in water, immediately 

preceding sowing. The final treatment was GA3 

soaked seeds, where seeds were partially opened at 

the basal joint with a blunt knife but soaked for 48 

hours in a solution of gibberellic acid (GA3 – Sigma 

7645) at 48 ppm2, immediately preceding sowing. 

Gibberellins were successfully used by Hemery 

(2000) in nursery trials of walnut seed germination.

The seeds were sown by placing them at twice their 

width below ground and then covered by sedge 

peat. A tree shelter was placed over each sown 

seed. Germination and tree height growth data 

were assessed and analyzed using Genstat (Lawes 

Agricultural Trust, Rothampstead Experimental 

Station, UK).

Results and Discussion

There were no significant differences for 

germination between different pre-treatments. 

The lowest mean germination rate was for control 

treatments at 76% and highest for GA3-soaked 

seed at 87%. Overall 80% of seeds germinated. 

In the first year following germination, trees that 

had grown from stratified and GA3-soaked seeds 

grew more (p < 0.05) than those water-soaked and 

control treated. By year 2 and onwards however, 

2

Example calculations: 45 seeds of mean volume 14.5 cm3 each 

= 652 cm3 total volume whilst approximate bucket volume = 

2356 cm3. Remaining bucket volume (2356-652) = 1704 cm3 (1.7 

litres). GA3 required at 100 ppm = 0.1g l-1 = 0.17g for 1.7 litres of 

water.
140 International Walnut Research

Proceedings of the 6th Walnut Council Research Symposium

there were no differences for height between the 

treatments.

The high germination rates achieved by the use 

of gibberellins indicate a highly practicable way 

of pre-treating seeds for direct sowing but further 

research is needed to study methods that may 

improve germination in comparison to traditional 

methods. This research does indicate the 

efficiency of establishing walnut by direct seeding, 

particularly in terms of the cost of a seed, which 

is typically only 2% the cost of a transplant. 

Although not tested in an experimental 

framework, it is interesting to note that the 3-

year-old seedlings in the trial averaged 98 cm tall 

compared to ‘control’ common walnut trees in a 

neighboring trial (described below) whose mean 

height was 166 cm after the same three growing 

seasons, being planted at 46 cm tall.

Nitrogen Fertiliser

Background and Method

Visual observation of many young walnut 

plantations on ex-farmland in the UK, where 

leaves often turned yellow and growth slowed 5 to 

8 years after establishment, indicated nitrogen (N) 

deficiencies. Foliar analysis subsequently confirmed 

critical levels of N in these cases 2.0% - 2.5% 

element N in dry weight (Bonneau 1995).

A trial neighboring the direct seeding experiment 

was established in 2001 to test the effects of 

applying artificial nitrogen on the establishment 

and early growth of walnut transplants, both 

common and black (Juglans nigra). The trial was 

planted in ground that had been fallow for 7 

years preceding establishment of the trial. It was 

laid out as a full factorial incorporating the two 

walnut species and six nitrogen treatments in a 

randomized complete block experiment with two 

replicates. In addition to a control (no N applied) 

there were five N application treatments covering 

the range of recommended rates of N in addition 

to low and high extremes (100, 200, 300, 400 and 

500 kg N ha-1). Savill and others (1997) stated that 

the normal range would be 150-250 kg of element 

N ha-1. The two walnut selections were common 

walnut (provenance RA464) and black walnut, both 

sourced from France. The N was applied once at 

planting, in the form of ammonium nitrate (NH4NO3) 

in which N was present at 34.5%, and again at 

the start of the third growing year. Survival and 

tree height data were assessed and analyzed using 

Genstat.

Results and Discussion

After both growing seasons following application of 

nitrogen (years 1 and 3) there were no significant 

differences for height growth between the different 

nitrogen treatments. Over the three growing 

seasons the common walnut grew more in height 

(p > 0.05) than black walnut but was this was 

not attributable to the nitrogen treatments, the 

interaction being non-significant. Trees subjected 

to the highest dose of nitrogen (500 kg N ha-1) 

grew less (80 cm) than all other treatments (range 

96 – 106 cm) but these differences were nonsignificant. Extreme high doses of nitrogen would 

be expected to have a negative effect on growth but 

the results of this experiment indicate than nitrogen 

can be applied at nearly double the recommended 

rate without adversely effecting growth. More 

importantly it indicates that applying nitrogen may 

be a costly exercise, both in terms of unnecessary 

monetary expense and environmental pollution, and 

therefore not recommended.

SELECTION OF GENOTYPES

Natural Range of Common Walnut

The natural range of common walnut is confined 

to the Asian continent extending across 21 modern 

political boundaries: from Turkey, Azerbaijan, 

Armenia, Russia, Georgia, and Iraq (Kurdistan) in 

the west, across the northern lands of Iran and 

Afghanistan and the heart of central Asia in the 

independent states of Kazakhstan, Turkmenistan, 

Uzbekistan, Kyrgyzstan, Tajikistan and their 

giant neighbor China in the Xinjiang Autonomous 

Region, formerly ‘Chinese Turkestan’, extending 

further south in a narrowing range nestling in 

the mountains of Pakistan, northern India and 

Nepal, and finally reaching its eastern extent 

in Bangladesh, Myanmar (Burma), Bhutan and 

southern China (Nekrassowa 1927, Schmucker 

1942, Berg 1950, Browicz 1976, Davis 1982, 

Puri and others 1983, Komarov 1985). Jalas and 

Suominen (1976), and Tutin and others (1993), 

reported that common walnut might be native to 

Greece and elsewhere in the Balkan Peninsula.

Seed Collections

In provenance selection, the movement of seed 

from a source of origin that is very different to 

the introduced location can be problematic if 

high altitude or high latitude sources are moved 

to low altitudes or low latitudes, or indeed the 
International Walnut Research 141

Black Walnut in a New Century

reverse (Zobel and Talbert 1984). However, high 

altitude sources from low latitudes can often be 

introduced successfully to low altitude locations at 

high latitudes (Zobel and Talbert 1984). The prime 

area for concentrating sampling was Kyrgyzstan 

which, although situated 10° south of the UK, it is 

situated at the northern limit of the natural range 

of the species (Schmucker 1942, McGranahan and 

Leslie 1991). Therefore the seed sources from this 

mountainous country, where the average elevation 

of collections made were 1,460 m above sea level 

(a.s.l.), provided a possible source for successful 

introduction due to the altitude difference between 

the two locations (UK trial sites ranged from 15 to 

245 m a.s.l.).

The genetic resources of forest trees are usually 

located in primeval or ancient forests (Frankel and 

others 1995). Kyrgyzstan, containing 25,600 ha 

of natural walnut forest (Musuraliev 1998), was 

therefore the main focus for seed collecting with 

the dual purposes of sampling genetic variation 

(Hemery and Popov 1998) and identifying superior 

genotypes for timber production. 

Eleven provenance collections, incorporating 253 

half-sib progenies, were sampled from Kyrgyzstan 

in September 1997 during a 3-week expedition 

(Hemery 1998). On average, 23 trees were sampled 

from each provenance location, each containing a 

wide variety of tree phenotypes, ranging in height 

from 7 to 34 m and in DBH from 13 to 128 cm 

(Hemery 2000). A wide array of tree characteristics 

were assessed including height to first branch, 

branch angle, crown diameter, stem straightness, 

presence/absence of burrs and many seed 

parameters. Provenance sites were surveyed for 

accurate location, altitude, aspect, slope, soil type, 

forest species diversity and basal area.

A further 13 provenance collections (and 122 

progenies) were amassed through collaboration 

with research institutes and individuals across 

the natural and introduced ranges of the species 

covering 12 countries. Overall 18 provenances 

contained progeny-level data.

PROVENANCE AND PROGENY 

PERFORMANCE

Method

The 375 common walnut genotypes were established 

across three provenance/progeny trials in southern 

England during winter 1998. The three provenance 

trials are arranged as randomized complete block 

designs with unbalanced multiple-tree plots. The 

largest of these (Oxfordshire) was a combined 

provenance and progeny trial containing 18 

provenances and 199 half-sib progenies. The progeny 

trial in a randomized incomplete block design with 

single-tree plots. Tree heights and stem diameters 

were measured annually and flushing (leaf-burst) 

assessed using a scoring system developed for this 

research, on two days in spring 1999, and analyzed 

using the Kruskal-Wallis non-parametric test. 

Survival percentages were arcsin transformed for 

analyses. All statistical analyses were conducted on 

plot means using Genstat.

Results and Discussion

Across all three field trial sites survival has been 

impressive at 98.4% after four growing seasons. 

Within the 18 provenances with progeny-level data, 

mortality was only observed within the Kyrgyz 

provenances (mean 97.4% survival).

At the end of the fourth growing season (2002) 

mean tree height was 122 cm but variable between 

sites (p < 0.001) and provenances (p < 0.001). 

Provenance x site interaction was also present 

(p < 0.001) with all main 18 provenances being 

tallest at the Somerset site and shortest at the 

Gloucestershire site (Fig. 1). Notably, provenance T1 

(Trabzon region, Turkey) was twice the height (p < 

0.001) at Somerset than at Gloucestershire.

Flushing was highly variable for provenances 

(p < 0.001) on Julian day 85 (March 26th 1999), 

between the earliest flushing provenances E1 

(Spain), T1 and T2 (both Turkey) and the latest 

flushing provenance J1 (Tajikistan). Within 

the Kyrgyz provenances (K1-11) there was no 

significant variation for flushing at day 85. On 

day 97 (April 7th 1999) provenances K1 and J1 

were the only provenances (p < 0.001) not to have 

flushed.

This long-term research programme continues with 

the ultimate aim of identifying genotypes suitable 

for producing timber in the UK. Crown closure 

is predicted by year 15 (2013) by which time 

thinning will become necessary. There is therefore 

a window of opportunity in the intervening years 

in which to conduct thorough research, both 

phenotypic characterisation and genetic variation 

studies, before some genotypes will be lost due to 

silvicultural needs within the trials.
142 International Walnut Research

Proceedings of the 6th Walnut Council Research Symposium

PLANTATION DESIGN

Walnuts in Britain are usually grown in pure 

stands or as individual trees, rather than in mixed 

plantations. Research conducted in the USA and 

Italy has indicated that walnuts can benefit from 

being planted in mixed stands, particularly with 

nitrogen-fixing species. Advantages may include 

increased growth rates and improvement of tree 

form, combined with protection from frost damage 

and reduced weed competition.

Background and Method

Common walnut is a light-demanding species and 

has the largest crown diameter, in relation to any 

given stem diameter, of the main timber-producing 

species used in British forestry (Hemery 2000). 

Traditionally in Europe it is usually recommended 

that common walnut is planted at very wide 

spacing, due to its intolerance to shading, unless 

the accompanying nurse species is very carefully 

maintained (MacDonald and others 1957, Evans 

1984). Becquey (1997) recommended planting at 10 

to 12 m for the common walnut but importantly, 

emphasized the importance of the genotype at such 

low densities.

There is however, increasing evidence that 

walnut may benefit when planted with suitable 

nurse or companion trees and shrubs. In the 

USA, Schlesinger and Williams (1984) tested 

black walnut in mixtures with black locust 

(Robinia pseudoacacia), autumn-olive (Elaeagnus

umbellata) and alder (Alnus glutinosa). These 

species were chosen for their nitrogen-fixing (Nfixing) capabilities because many hardwood trees, 

including black walnut, have shown improved 

growth when grown with N-fixing species (Finn 

1953). Autumn olive is a fast growing, multistemmed shrub reaching 6 to 9 m in height and 

wide-spreading (3.6 to 5.5 m), which is tolerant of 

exposure and will grow in most soil types in the 

UK (Bean 1950), and the most hardy of the Genus 

(Crawford 1998). Schlesinger and Williams (1984) 

demonstrated that walnut height growth improved 

with certain nurse species. Walnuts interplanted 

with autumn olive resulted in height gains of 56 

to 351% over non-nursed walnuts. Campbell and 

Dawson (1989) calculated projections of growth 

that showed average DBH of 28 cm in 31 years for 

Figure 1.—Mean tree heights for provenances 1 – 18 in 2002, based on plot means within each of the field trial sites; 

Oxfordshire, Gloucestershire and Somerset. Provenance K stands for Kyrgyzstan (1 – 3 Ak-Terek, 4 Sharap, 5 

Yaradar, 6 Shaidan, 7 Kyzyl-Ungur, 8 Katar-Yangak, 9 Kyok-Sarau, 10 Kyr-Sai, 11 Ters-Kolt), E1 Catalonia - Spain, 

J1 Tajikistan, P1 Karaj - Iran, R1 Romania, S1 Slovakia, T for Turkey (1 Trabzon, 2 Anatolia). Error bars show 95% 

confidence limits.

0

50

100

150

200

K1 K2 K3 K4 K5 K6 K7 K8 K9 K10 K11 E1 J1 P1 R1 S1 T1 T2

Provenance

Mean tree height (cm).

Oxfordshire

Gloucestershire

Somerset
International Walnut Research 143

Black Walnut in a New Century

walnut inter-planted with the autumn olive. They 

projected that 40 years of growth would be required 

for the walnut to achieve 28 cm DBH with an alder 

nurse, or 80 years for those planted without a 

nurse.

In central Italy, mixed plantations with black 

walnut established in 1985, have also demonstrated 

impressive growth (Buresti and Frattegiani 1994). 

The first plantations with common walnut were 

established more recently, using nurses of cherry 

(Prunus avium), oleaster (Elaeagnus angustifolia), 

Italian alder (Alnus cordata) and robinia (Buresti 

1995). Six year results indicated that walnut 

increased in height by 48% and stem diameter 

by 36% when planted with cherry (non N-fixing) 

compared to those walnuts with no nurse (Buresti 

1995). However, walnuts with N-fixing nurses were 

76% taller and average DBH was 42% greater than 

for pure-grown walnuts (Buresti 1995).

Research was initiated in 2000 with the aim of 

investigating planting mixtures that promote 

the growth of common walnut in UK, in terms of 

stem quality and vigor, leading to a reduction in 

rotation time (Hemery 2001). Several field trials 

were established in southern England, three with 

common walnut during spring 2000, and a large 

replicated trial with common, black and hybrid 

walnuts in 2002, to test the effects of growing these 

species with a number of companion species.

The trials include all combinations of the nurses 

(17): tree nurses only (4), tree + shrub nurses 

(12) and a walnut-only treatment (1). The trials 

were a randomized complete block (RCB) design, 

comprising 17 plots in each block (replicate), and 

two blocks per site. The walnuts were planted in a 

triangular pattern at approximately 5 x 5 m. At this 

spacing, based on a predicted crown diameter: stem 

diameter relationship for walnut (Hemery 2000), 

and a stem diameter increment of 1 cm year-1, the 

onset of crown competition between walnuts would 

begin 15 to 20 years after planting. The tree nurse 

was planted at similar spacing between the walnuts 

in an alternative triangular pattern. Shrub nurses, 

where included, were planted in alternate rows, 

effectively surrounding the walnuts.

Results and Discussion

At the end of the second growing season walnut 

survival was high at 99.85%, with mortality only 

recorded at one site. No differences were recorded 

for height between sites or treatments at the end of 

the second of third growing seasons. At this early 

stage no effect from the nurse treatments would be 

expected although the observed impressive growth 

of the autumn olive and alder is predicted to have a 

positive effect on the walnut from the fifth growing 

season onwards.

DISCUSSION

The research programme described aims to build 

on the wealth of experience already gathered across 

the world in walnut growing for timber production. 

Walnut forestry in the UK is undergoing a revival 

of interest fuelled by the research programmes 

initiated in the country. Variation in performance 

has been demonstrated within existing widely 

planted genotypes and between those sampled 

within the novel research programme including 

Kyrgyz provenances. Several key common 

misconceptions relating to the difficulty of 

establishing walnut successfully have been laid 

to rest by this research, not least by the excellent 

survival reported in all the walnut trials described 

above. Phenology remains an important research 

area for the future, where the importance of 

selecting late-flushing phenotypes is paramount 

for successful walnut establishment, given the 

unpredictable spring climate in the UK. Future 

research must also address predicted climate 

change that might favour the silviculture of walnut 

species in the UK. Strategic consideration needs 

to be directed towards supplying the predicted 

increased demand for tested and quality-assured 

walnut planting stock for use in the UK. It is 

hoped that the combined silvicultural and genetic 

improvement research programmes will continue to 

encourage and foster a revival of interest in growing 

walnut in the UK.

ACKNOWLEDGMENTS

Woodland Heritage for supporting the establishment 

of the walnut nitrogen and companion planting trials 

and for sponsoring several overseas walnut field 

trips. Collaborative partners within the British & Irish 

Hardwoods Improvement Programme and overseas, 

including Istituto Sperimentale per la Selvicoltura 

(Italy) and Institute of Forest and Walnut Breeding 

(Kyrgyzstan). Direct support from The Royal Forestry 

Society, The National Forest Company, Nicholson 

Nurseries and landowners hosting external trials. 

Finally the trustees and patrons of the Northmoor 

Trust for their continued support.

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Bean, W.J. 1950. Trees and shrubs hardy in the 

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Murray.
144 International Walnut Research

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Becquey, J., ed. 1997. Les noyers a bois, 3rd ed. 

Paris: Institut pour le developpement forestier

Berg, L.S. 1950. Natural regions of the U.S.S.R. 

New York, NY: Macmillan Company. 

Bonneau. (1995). Fertilisation des forêts dans les 

pays tempérés. Nancy Cedex, France: ENGREF.

Browicz, K. 1976. Juglandaceae. In: Flora Iranica. 

Rechinger, K.H., ed. Austria: Akademische 

Druck-u Verlagsanstalt. 121: 1-5.

Buresti, E. 1995. Walnut trees in mixed stands with 

shrubs and trees. In: European development 

of walnut trees for wood and fruit production 

as an alternative and extensive system to 

agricultural crops. Workshop proceedings 

of E.U. AIR/walnut project; March 27-30; 

Thessaloniki, Greece: 72-81.

Buresti, E.; Frattegiani, M. 1994. First results 

of mixed plantation with high quality timber 

broadleaves and N-fixing trees: mixed stands. 

In: Proceedings from IUFRO sysmposium, 1994 

April 25-29. Portugal: Louse/Coimbra.

Campbell, G.E.; Dawson, J.O. 1989. Growth, 

yield, and value projections for black walnut 

interplantings with black alder and autumn olive. 

Northern Journal of Applied Forestry. 6: 129-132.

Crawford, M. 1998. Nitrogen-fixing plants for 

temperate climates, 2nd ed. Devon: Agroforestry 

Research Trust.

Davis, P.H., ed. 1982. Flora of Turkey and the east 

Aegean islands. Vol. 7. Edinburgh: Edinburgh 

University Press. 654 p.

Dirr, M.A.; Heuser, C.W. 1987. The reference 

manual of woody plant propagation: from seed 

to tissue culture. Georgia: Varsity Press Inc.

Evans, J. 1984. Silviculture of broadleaved 

woodland. For. Comm. Bull. 62. London: 

H.M.S.O.

Finn, R.F. 1953. Foliar nitrogen and growth of 

certain mixed pure forest plantings. Journal of 

Forestry. 51: 31-33.

Frankel, O.H.; Brown, A.H.D.; Burdon, J.J. 

1995. The conservation of plant biodiversity. 

Cambridge: Cambridge University Press.

Hemery, G.E. 1998. Walnut (Juglans regia) seedcollecting expedition to Kyrgyzstan in Central 

Asia. Quarterly Journal of Forestry. 92: 153-157.

Hemery, G.E. 2000. Juglans regia L.: genetic 

variation and provenance performance. Oxford: 

Department of Plant Sciences, University of 

Oxford. 176 p. D.Phil. thesis.

Hemery, G.E. 2001. Growing walnut in mixed stands. 

Quarterly Journal of Forestry. 95: 31-36.

Hemery, G.E.; Popov, S.I. 1998. The walnut 

(Juglans regia L.) forests of Kyrgyzstan and 

their importance as a genetic resource. 

Commonwealth Forestry Review. 77: 272-276.

Hemery, G.E.; Savill, P.S. 2001. The use of 

treeshelters and application of stumping in the 

establishment of walnut Juglans regia. Forestry. 

74: 479-489.

Jalas, J.; Suominen, J., ed. 1976. Atlas Florae 

Europaeae, Vol. 3. The committee for mapping 

the flora of Europe, Helsinki.

Komarov, V.L. ed. 1985. Flora of the U.S.S.R, Vol. 

5. Germany: Koeltz Scientific Books: 197-199.

MacDonald, J.; Wood, R.F.; Edwards, M.V.; 

Aldhous, J.R. 1957. Exotic forest trees in Great 

Britain. For. Comm. Bull. 30. London: H.M.S.O.

McGranahan, G.; Leslie, C. 1991. Walnuts (Juglans

L.). In: Genetic resources of temperate fruit 

and nut crops. Moore, J.N.; Ballington, J.R., 

eds.Wageningen: International Society for 

Horticultural Science: 907-951.

Musuraliev, T.M. 1998. The current condition of 

walnut-fruit forests of the Kyrgyz Republic. 

In: Blaser, J.; Carter, J.; Gilmour, D. Gland, 

eds. Biodiversity and sustainable use of 

Kyrgyzstan’s walnut-fruit forests. Switzerland 

and Cambridge, UK and INTERCOOPERATION, 

Bern, Switzerland: IUCN: 3-18.

Nekrassowa, V.L. 1927. The genus Juglans in 

Turkestan. Bulletin of Applied Botany, of 

Genetics and Plant-Breeding. 18: 303-360.

Popov, S. 1981. Morphological peculiarities and 

dynamics of the growth of the European 

walnut root system depending on the methods 

of planting and growing of the plantations. 

Goroskoistspanska Nauka. 18: 25-33.

Puri, G.S.; Meher-Homjii; Gupta, R.K.; Puri, S. 

1983. Forest ecology, Vol. 1: Phytogeography 

and forest conservation. New Delhi: Oxford and 

IBH Publishing Co.
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Savill, P.; Evans, J.; Auclair, D.; Falck, J. 1997. 

Plantation silviculture in Europe. Oxford: 

Oxford University Press.

Schlesinger, R.C.; Williams, R.D. 1984. Growth 

response of black walnut to interplanted trees. 

Forest Ecology and Management. 9: 235-243.

Schmucker, T. 1942. The tree species of the 

northern temperate zone and their distribution. 

Berlin: Silvae Orbis 4. Berlin-Wannsee.

Tutin, T.G.; Burges, N.A.; Chater, A.O.; 

Edmondson, J.R.; Heywood, V.H.; Moore, 

D.M.; Valentine, D.H.; Walters, S.M.; Webb, 

D.A. 1993. Juglandaceae. In: Moore, D.M., ed. 

Flora Europaea, 2nd ed. Vol. 1. Cambridge: 

Cambridge University Press.

Zobel, B.; Talbert, J. 1984. Applied forest tree 

improvement. New York, NY: John Wiley and 

Sons.
146 International Walnut Research

Proceedings of the 6th Walnut Council Research Symposium

WALNUT TISSUE CULTURE: RESEARCH AND FIELD APPLICATIONS

Jose M. Lopez1

ABSTRACT—Vitrotech Biotecnologia Vegetal began researching propagating Juglans 

regia (English walnut) and various Juglans hybrids by tissue culture in 1993 and has 

operated on a commercial scale since 1996. Since this time, more than one and a half 

million walnuts of different species have been propagated and field planted. Tissue 

cultured walnuts on their own roots have proven to be more vigorous and productive 

than conventionally propagated walnuts. Also, tissue culture is an efficient tool to help 

breeders select and test possible superior genotypes. 

WALNUT: A DIFFICULT TREE TO PROPAGATE

Most of the species of the genus Juglans are 

difficult to propagate by conventional vegetative 

propagation using rooted cuttings, budding, and 

grafting (Coggeshall and Beineke 1997). Walnut 

trees are propagated by commercial nurseries by 

grafting onto seedling rootstocks both for J. regia 

(English walnut) nut production as well as clonal 

selections of J. nigra (black walnut for timber 

production. Variability with different conditions 

(climatic, type of plant material, and technique) 

also affects the success of budding and grafting. 

Results may be quite variable from year to year. 

Budding and grafting are still widely used and 

there are a large number of commercial nurseries 

around the world that are propagating and selling 

walnuts with this procedure.

Cuttings have been tried many times with variable 

success (Coggeshall and Beineke 1997). In fact, 

the most important limitation to commercial 

propagation has been the variability of the 

results. McKenna (1997) got good results with 

rooted cuttings, but the trees were not able to be 

commercially deployed. Some investigators indicate 

that viability of propagated cuttings has been worse 

than seedlings.

Walnut has proven to be a genus that is also hard 

to propagate by tissue culture (Rodriquez and 

others 1989, Preece and others 1989). Attempts 

have been made by many different researchers 

(Driver and Kuniyuki 1984; McGranahan and 

Leslie 1988; Rodríguez and others 1989; Gruselle 

and Boxus 1990; Gautman and Chalupa 1990; 

McGranahan 1992; Jay-Allemand and others 1993; 

Bourrain and Navatel 1994, 1995). The difficulties 

have been related to: internal contamination 

of plant material and susceptibility of explants 

to damage caused by disinfectant substances, 

oxidation of the explants, maladaptability to the 

culture media, poor proliferation, poor elongation 

and rooting rates, difficult acclimatization (stage IV) 

of the rooted propagules and complications when 

growing small plantlets in the nursery. Despite this, 

Vitrotech Biotecnologia Vegetal began the research 

in this field in 1993 and walnut commercial 

propagation by tissue culture in 1996. Since then 

this company has propagated around one and a 

half million trees of different species of this genus.

WALNUT PROPAGATION: 

WHAT TO GET AND HOW?

Commercial tree propagation usually starts with 

the best method to get the maximum profit from 

the plant material. This consists of grafting the 

selected clones/varieties/cultivars onto selected 

clonal rootstocks using scions that have desirable 

traits for yield, fruit or timber quality, resistance to 

diseases and/or adverse climatic conditions. The 

investigator can also use rootstocks selected for 

good soil adaptability, vigour or resistance to soil 

pathogens. In the case of walnuts, this has not been 

available, because the lack of a propagation method 

for rootstocks, or when selections have been made, 

no scientific tests could be performed because no 

replicates of the selected clone were available.

1

General Manager (JML), Vitrotech Biotecnologia Vegetal, N-340 Km 631, 30840 Alhama de Murcia (Spain). JML is corresponding 

autor; to contact call +34 968 633231 or e-mail at jose.m.lopez@vitrotech.es

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
International Walnut Research 147

Black Walnut in a New Century

When Vitrotech started commercial propagation 

of J. regia by tissue culture, we found that no 

rootstocks were available for their propagation, or 

the ones accessible were not tested thoroughly. 

Under such circumstances we decided to try the 

propagation of the varieties on their own roots. We 

started with walnut tissue cultures obtained from 

other researchers around the world (UC Davis, 

Burchell Nursery, INRA, and CTIFL) and propagated 

some varieties/clones on their own roots.

Most of the initial production of our company was 

based on English walnut nut cultivars, though 

some clones of J. hindsii, paradox (J. hindsii x J. 

regia), and other hybrids (J. major x J. regia, J. 

nigra x J. regia) were also propagated. Throughout 

Europe and most of the world (except California) J. 

regia cultivars were grafted onto J. regia seedlings. 

Our company thought that a tissue cultured 

walnut tree on its own roots should not have less 

productivity than a grafted tree. In addition, most of 

the walnut cultivars come from mother trees grown 

on their own roots (seedlings), that could show us 

that those cultivars can be good root systems (or, at 

least, not worse than the average of seedlings).

AGRONOMIC FEATURES OF 

MICROPROPAGATED WALNUTS

We were surprised to find that the performance 

of self-rooted walnut cultivars was better than 

expected. In most cases, they were superior to 

performance with grafted trees. Prunet and Ginibre 

(2000) described that self-rooted J. regia ‘Lara’ 

were more vigorous than trees of the same cultivar 

grafted onto seedling rootstocks of J. regia and

J. nigra x regia hybrids, as well as clonal J. regia

rootstocks that had been selected for their excellent 

vigour and propagated by tissue culture.

Hasey and others (2001) found that self-rooted J. 

regia ‘Chandler’ was much more vigorous than 

Chandler grafted onto Paradox rootstock (Table 1). 

This was a not expected, as Paradox hybrid is more 

vigorous than conventional J. regia rootstocks of 

which Chandler is considered as a medium-vigor

cultivar (Table 1). Cumulative yield over 5 years on 

self-rooted Chandler reached close to three times 

that of conventional grafted Chandler onto Paradox 

(Table 2). In addition, self-rooted Chandler trees 

were supporting a higher nematode population on 

their roots than grafted trees (causing nematode 

root lesions) and some of them died back. Hasey 

and others (1999) mentioned that it was striking 

to observe the higher vigor of those self-rooted 

Chandler, even with a high nematode population on 

their roots. They explained that a similar situation 

was found when vigorous Paradox rootstocks 

showed more tolerance to nematodes than J. hindsii

rootstocks. One more interesting observation 

was that several grafted Chandler onto Paradox 

exhibited crown galls (Agrobacterium tumefaciens), 

while none of the self-rooted Chandler did.

With our own experience, we have found that selfrooted walnuts of different species and cultivars 

have shown outstanding vigor, even with very 

difficult growing conditions (Fig. 1). On a farm 

located in Murcia province, Spain, soil pH ranged 

from 8.2 to 9.3. Active calcium content reaches 

between 23% and 30%. The soil was extremely 

rocky, and soil preparation was very poor. Climatic 

conditions were good (no late or early frost, 

Table 1.—Trunk circumference (cm) and trunk cross sectional area measured at 60 cm from 1998-2001 

for trees planted in 1991 (Hasey and others 2001).

Circumference (cm) Trunk Cross Section Area

Treatment 1998 1999 2000 2001 1998 1999 2000 2001

Own-rooted 64.3 a 68.1 a 71.4 a 74.6 a 340 a 381 a 419 a 457 a

On Paradox 41.4 b 45.1 b 49.4 b 51.7 b 146 b 173 b 208 b 227 b

Means followed by the same letter in a column are not significantly different (LSD P < 0.05) 

Table 2.—Yield in kgs/tree from 1995-2001 (Hasey and others 2001).

Treatment 1995 1996 1997 1998 1999 2000 2001

Own-rooted 10.8 a 23.4 a 43.2 a 29.1 a 41.9 a 33.2 a 46.4 a

On Paradox 4.2 b 7.9 b 14.2 b 10.1 b 19.8 b 19.7 b 31.6 a

Means followed by the same letter in a column are not significantly different (LSD P < 0.05) 
148 International Walnut Research

Proceedings of the 6th Walnut Council Research Symposium

Figure 1.—Summer 2001 photograph of a self-rooted walnut tree (Juglans regia) planted on a farm 

located in Murcia (Spain) in 1997.
International Walnut Research 149

Black Walnut in a New Century

maximum temperatures around 35-40o C and 

minimum around -5o C), with a long vegetative 

season (from mid-March to the end of October). 

Water quality was excellent. The height of the 

plants planted on the farm ranged from 4 to 20 

cm (Fig. 2). The main planting was done between 

February and April, 1997. At the second year, trees 

of some of the more vigorous cultivars (J. regia 

‘Sunland’ and ‘Serr’) reached more than 3.5 m. 

Table 3 shows average diameters per cultivar found 

in September 2001. Fruit bearing was also very 

precocious. Some trees produced their first female 

flowers during their second leaf in the field. Female 

flower production increased since then, and is still 

increasing. However catkin production was very 

delayed compared to conventional grafted trees. 

First catkins were seen at the sixth leaf, depending 

on the cultivar (Frutos 2003). Yield has not been 

recorded yet, as all the original trees on this farm, 

including pollinators, were self rooted, so most of 

the possible production has been lost because the 

lack of pollen.

A further and clear advantage of the use of selfrooted cultivars over grafted trees onto seedling 

rootstocks is the higher homogeneity of the 

plantations due to using clonal rootstocks of the 

same cultivar. Some of the stronger defenders of the 

use of rootstocks could say that by using self-rooted 

cultivars an unknown clonal root system is being 

utilized, which could be very risky. However, our 

evidence shows that more than 20 different clones 

on their own roots have shown a better performance 

than the original mother tree, and no further 

problems have been found regarding adaptability 

to different soils than those already present on 

Figure 2.—Typical in-leaf tissue-cultured walnut plants when planted at an irrigated farm located 

in Murcia (Spain).

Table 3.—Trunk diameters (cm) of the different treatments ± SE. Different letters in the same row indicate 

differences at 95% level (Lopez 2001).

Age/cultivar Sunland Serr Chandler Sorrento Vina

2nd leaf 10.94±0.50a 9.88±0.28a --- 8.01±0.25b 7.01±0.26b

3rd leaf 18.68±0.33a 17.88±0.20a 13.85±0.22b 12.67±0.47b ---
150 International Walnut Research

Proceedings of the 6th Walnut Council Research Symposium

the different species/clones. In some cases this 

adaptability has been clearly better when using selfrooted trees with many different clones.

What is the reason for such improved behavior? 

Several possible explanations can be considered:

Root architecture of self-rooted walnut trees 

is completely different than walnut seedlings. 

Instead of the usual main taproot, self-rooted 

trees exhibit a fibrous root structure. We have 

counted more than 20 primary roots with all of 

them coming directly from the crown (Fig. 3). 

Anchorage is very good and no guides are used 

with most of our trees. Such root structure 

may provide for excellent water and nutrient 

absorption. At the Murcian farm mentioned 

before, we observed that, in the same 

conditions, grafted or seedling trees showed 

leaf chlorosis (due to the limiting soil conditions 

relative to pH and calcium content) much easier 

than self-rooted trees. Also, vigor of self-rooted 

trees was much higher than the vigor of grafted 

trees.

The lack of a graft union seems also to improve 

the growth of the trees. Olson and Walton 

(2001) found that self-rooted Chandler (SRCh) 

walnut trees are more vigorous than SRCh 

scions grafted onto SRCh rootstocks with 

1.

2.

similar root architecture. In fact, SRCh were 

the most vigorous of any of the treatments 

when Chandler scions were grafted to Paradox, 

J. regia seedlings, or clonal Paradox. Thus, it 

was concluded that graft unions limit the vigor 

of the walnut trees. Prunet and Ginibre (1999) 

also reported that self-rooted Lara trees were 

more vigorous than Lara grafted onto very 

vigorous clonal J. regia rootstocks propagated 

by tissue culture. These differences cannot be 

explained by the root architecture (similar on 

the treatments), but by the presence/absence of 

the graft union.

Homogeneity of the plant material in terms of 

genetics. Root systems have a strong influence 

on the vigor of the tree. Vigorous rootstocks are 

commonly used to increase productivity or to 

decrease it (dwarfing rootstocks). This feature is 

especially important when dealing with trees, as 

most of the research in breeding with walnuts 

has been looking for very vigorous scions 

that must be grafted on seedling rootstocks 

of unknown and heterogeneous growth 

capabilities. Once selected scions are shown to 

be very vigorous and productive on their own 

roots, it can be assumed that micropropagated 

selections of walnuts must result in more 

homogeneous and vigorous trees than the same 

selections grafted onto seedling rootstocks.

3.

Figure 3.—Root architecture of a 3-year-old walnut tree (J. regia cv. Vina). 
International Walnut Research 151

Black Walnut in a New Century

Disease tolerance/resistance. No evidence has 

shown that tissue cultured walnuts of any 

clone/cultivar are resistant to any soil disease. 

As stated before, Hasey and others (2001) said 

that nematode root lesion had affected SRCh 

trees less than expected, and they assumed 

that was due to their superior vigor. The same 

authors found, while in the same block, some 

grafted trees on Paradox rootstock were affected 

by crown gall and not one of the self-rooted 

trees showed this disease. The reason was 

that soil diseases on grafted trees grown in a 

nursery row were much more difficult to control 

than when the trees are grown in pots (case of 

the tissue cultured self-rooted). Also, digging 

usually causes injuries to the roots that can be 

the entry points for such pathogens. 

OUTLOOK OF THE USE OF 

TISSUE CULTURED WALNUTS

Commercial walnut propagation by tissue culture 

has been successfully achieved by our company. 

This does not mean that all the walnut plant 

material is ready to be propagated. 

Walnut tissue culture requires research to be 

performed on almost every new clone/variety to 

be propagated. Differences with tissue culture 

behavior between species are greater than with 

other vegetative propagation methods. J. nigra has 

shown to have very different requirements than J. 

regia, and its performance in vitro is more difficult. 

This is just one illustration, but shows the trend of 

the entire genus.

Other limitations to walnut mass propagation by 

tissue culture are the need of more and longer 

tissue culture stages, which involves larger facilities 

and more staff to produce the same amount of 

plants than for other species. The great influence 

on the process of supposedly less important factors, 

which with walnuts can limit the production to 

levels sometimes below what can be considered to 

be commercially viable, also affect production costs.

In the case of J. regia cultivars used for nut production, 

the use of self-rooted cultivars is of high interest 

particularly on those situations where no specific 

rootstock needs are required. These are mostly on sites 

where J. regia seedlings are used as rootstock. There 

is some advantage in price, but the main benefits are 

found with the performance of the trees.

When a specific rootstock is needed because 

the presence of particular soil condition, it is 

4. desirable to use clonal material coming from 

superior selections. Under those circumstances, 

performance of the trees will be better than those 

grafted onto seedling rootstocks. Tissue culture is 

also a determinant tool to make a good rootstock 

selection, because there is a reliable method for the 

necessary propagation of the original selected trees 

in order to get enough repetitions to perform field 

tests in different conditions. This is the case of most 

of the California walnut orchards that commonly 

require the use of Paradox rootstocks because 

they are more tolerant than J. regia to Phytophtora

species, nematodes, and water logging. Paradox 

rootstocks in California are also more vigorous 

when compared to other species of the genus, but 

in this case, the results of the trials performed to 

compare self-rooted trees with trees grafted on 

Paradox have shown that self-rooted trees are more 

vigorous.

The use of clonal rootstock is justified by a specific 

requirement (mostly related with adaptability to soil 

conditions or tolerance/resistance to soil diseases), 

and when such requirement cannot be obtained by 

the use of self-rooted trees. The cost of a grafted 

tree onto a clonal rootstock propagated by tissue 

culture will be clearly higher than the one of a 

self-rooted tree. If the grafted tree is not providing 

a commercial advantage then there is no reason to 

use more expensive walnut trees.

The use of tissue-cultured walnuts for timber 

production seems to be as favorable as for nut 

production, or probably more, because vigor 

is usually a more desirable feature for timber 

production. Selected clones can improve both 

productivity and timber quality, and can allow the 

utilization of plant material adapted to specific 

soil or climatic conditions, or resistant/tolerance 

to either soil or air borne pathogens. Uniformity 

of plantations will be greater, and phytosanitary 

treatments can be applied more effectively. In the 

case of self-rooted trees, the lack of graft union 

may avoid the presence of defects to the wood that 

could reduce the price of the timber, although 

selected clonal rootstocks can be used if necessary. 

Particularly, J. nigra, native to the eastern USA, 

where soil diseases are not a problem, and most 

of the clonal selections are based on the features 

of the scion, propagation of the same by tissue 

culture to produce self-rooted clonal trees is a 

feasible possibility when looking for increased 

productivity, shortened rotations and higher quality 

timber. In the same way as for nut producing trees, 

when clonal rootstocks provide desirable features, 

tissue culture is the only propagation method 

available for them.
152 International Walnut Research

Proceedings of the 6th Walnut Council Research Symposium

LITERATURE CITED

Bourrain, L.; Navatel, J.C. 1994. Micropropagation 

of the walnut tree Juglans regia L. Part 1: In 

vitro production. Infos-Ctifl. 98: 40-46. 

Bourrain, L.; Navatel, J.C. 1995. Micropropagation 

of the walnut tree Juglans regia L. Part 2: 

Acclimatization in the greenhouse and nursery 

performance. Infos-Ctifl. 112: 28-33.

Coggeshall, M.V.; Beineke, W.F. 1997. Black walnut 

vegetative propagation: the challenge continues. 

In: Van Sambeek, J.W., ed. Knowledge for 

the future of black walnut: 5th Black walnut 

symposium; 1996 July 28-31; Springfield, MO. 

Gen. Tech. Rep. NC-191. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, North 

Central Forest Experiment Station: 70-77.

Driver, J.A.; Kuniyuki, A.H. 1984. In vitro 

propagation of Paradox walnut rootstock. 

HortScience. 19(4): 507-509. 

Frutos Tomas, D. 2003. Diferenciacion floral en 

vergel de los cultivares de nogal (Juglans regia, 

L.) enraizados in vitro. Actas de Horticultura no 

39. X Congreso Nacional de Ciencias Horticolas, 

Pontevedra, España: 271-273.

Gautam, R.; Chauhan, J.S. 1990. A physiological 

analysis of rooting in cuttings of juvenile 

walnut (Juglans regia L.). Acta Horticulturae. 

284: 33-41.

Gruselle, R.; Boxus, P. 1990. Walnut 

micropropagation. Acta Horticulturae. 284: 45-

52.

Hasey, J.; Westerdahl, B.; Micke, W.; Ramos, D.; 

Yeager, J. 2001. Yeild performance of ownrooted ‘chandler’ walnut versus ‘chandler’ 

walnut on paradox rootstock. In: Germain, E.; 

Calvi, D., eds. Proceedings, 4th International 

walnut symposium; Bordeaux, France. Acta 

Horticulturae. 544: 482-489.

Hasey J.; Westerdahl, B.B.; Lampinen, B.D. 1999. 

Long-term performance of own rooted ‘chandler’ 

vs ‘chandler’ on paradox rootstock: the 

walnut research reports annual proceedings. 

Sacramento, CA: The Walnut Marketing Board 

of California: 87-92.

Hasey J.; Westerdahl, B.B.; Lampinen, B.D. 2001 

Long-term performance of own rooted ‘chandler’ 

vs ‘chandler’ on paradox rootstock: the 

walnut research reports annual proceedings. 

Sacramento, CA: The Walnut Marketing Board 

of California. 87-92.

Jay-Allemand, C.; Peng S.; Capelli, P.; Cornu, 

D. 1993. Micropropagation of hybrid walnut 

trees: some factors involved in rooting. Acta 

Horticulturae. 311: 117-124.

Lopez, J.M. 2001. Field behaviour of self-rooted 

walnut trees of different cultivars produced by 

tissue culture and planted in Murcia (Spain). 

In: Germain, E.; Calvi, D., eds., Proceedings, 

4th International walnut symposium. Bordeaux, 

France: Acta Horticulturae. 544: 543-546.

McKenna J.; Sutter, E. 1997. Root formation in 

walnut hybrid rootstoocks (Juglans hindsii X J. 

regia). In: Altman, A.; Waisel, Y., eds. Biology 

of root formation and development. Basic Life 

Sciences. 65: 85-90.

McGranahan, G. 1992. Micropropagation of persian 

walnut (Juglans regia L.). In: Bajaj, Y.P.S., 

ed. Biotechnology in agriculture and forestry, 

high-tech and micropropagation II. Berlin, 

Heidelberg: Springer-Verlag. 5(18): 136-150.

McGranahan, G.; Leslie, C.A. 1988. In vitro 

propagation of mature persian walnut cultivars. 

HortScience. 23(1): 220.

Olson, B.; McKenna, J.; McGranahan, G.; Walton, 

J.; Bertagna, N. 2001. Walnut rootstock trial 

2001. The walnut research reports annual 

proceedings, Sacramento, CA: The Walnut 

Marketing Board of California: 99-100.

Preece, J.E.; Van Sambeek, J.W.; Huetteman, 

C.A.; Gaffney, G.R. 1989. Biotechnology: In 

vitro studies with walnut (Juglans) species. 

In: Phelps, J.E., ed. The continuing quest for 

quality: 4th Black walnut symposium; 1989 July 

30-August 2; Carbondale, IL. Indianapolis, IN: 

Walnut Council: 159-180.

Prunet, J.P.; Ginibre T. 2000. Comportement 

agronomique des nouveaux portegreffes 

CTIFL Juglans regia. Travaux de Recherche et 

d’expérimentation. Station Expérimentale de 

Creysse.

Rodríguez R. Revilla A.; Albuerne M.; Pérez C. 

1989. Walnut (Juglans spp.). In: Bajaj, Y.P.S., 

ed. Biotechnology in agriculture and forestry. 

Berlin, Heidelberg: Springer-Verlag: Trees II. 5: 

99-126.
International Walnut Research 153

Black Walnut in a New Century

CURRENT RESEARCH IN SPAIN ON WALNUT FOR WOOD PRODUCTION

Neus Aletà1

ABSTRACT—The Department of Mediterranean Trees at the Institut de Recerca i 

Tecnología Agroalimentàries (IRTA) in Spain initiated a research program in 1993 to 

examine the variability among walnut species for wood production and to establish 

orchards with improved selections. The main objective of the programme is to obtain 

superior Persian walnut (Juglans regia L.) selections, half-sib progenies or clones, for 

wood production. Another important goal is to use controlled crosses among different 

walnut species to produce new hybrids, especially Persian x black (J. nigra L.) for wood 

production adapted to Spain’s Mediterranean climate. 

Fossil evidence indicates that Persian walnut (J. 

regia) was present in Europe as long ago as the 

end of the Pliocene and later widely spread by the 

Greeks and Romans through the Mediterranean 

basin, including the Iberian Peninsula (Manchester 

1989, Rivera and others 1997, Leslie and 

McGranahan 1998). Wild walnut trees in large 

numbers grew throughout Spain for centuries. 

During the 1940s and 1950s, the dire social 

situation in Spain after the civil war led to the 

removal and sale of many outstanding trees. There 

was little regeneration, and as a consequence the 

number of wild walnut trees in the countryside 

decreased drastically (Ministerio de Agricultura 

Pesca y Alimentación 2001). 

In Western Europe, the demand for quality timber 

is leading to farmers to plant deciduous hardwoods, 

particularly Juglans spp. This trend started few 

years ago in France, Italy, and some countries of 

central Europe. However, in Spain the interest 

in timber production is recent; new plantations 

in fields previously designated for horticulture 

appeared only 10 years ago. In the 1990s, the EU 

started programs for reforestation and plantations 

for timber production. Several large enterprises 

decided to invest in the production of quality wood 

using intensive management practices, and many 

farmers started to show an interest in shifting from 

horticultural orchards to woodlands.

Farmers and companies wondered what they 

should plant. No Persian walnut timber selections 

were available. Biotypes or land-races such as 

‘Lozeronne’ or ‘Charente in France or ‘Bleggiana’, 

‘Feltrina’ or ‘Sorrento’ in Italy were all that 

1

Research Genetist (NA), Mas Bové Center, Departament of Mediterranean Trees, Institut de Recerca i Tecnología Agroalimentàries, 

P.O. Box 415, 43280 Reus (Spain). NA is corresponding author; to contact, call (34) 977 328424 or e-mail neus.aleta@irta.es

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.

was available. On the other hand, the French 

hybrid progenies, ‘Mj209xRa’ and ‘Ng23xRa’ (J. x 

intermedia) were outstanding for their vigor and 

growth habit (Aletà and others 2003, Fady and 

others 2003). In Spain, farmers decided to plant 

these hybrids since the climate in Spain is suitable 

for timber production in intensively managed 

orchards. At the end of 2003, nearly 5,000 acres 

of Juglans, mainly hybrids, were growing in Spain 

under intensive management.

Starting in 1993, the Institut de Recerca I 

Tecnologia Agroalimentàries (IRTA) initiated a 

research programs concentration on the selection 

of walnut for wood production and development of 

improved practices for woodland management. The 

main aims of the IRTA programme are:

Evaluate variability within remaining Persian 

walnut populations.

Selection of Persian walnut progenies for wood 

production.

Selection of genitors for breeding new hybrid 

progenies.

Clonal selection.

Evaluation of training systems for wood 

production.

Development of irrigation schedules to optimize 

supply of water.

Design mixed planting systems for timber 

production.

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154 International Walnut Research

Proceedings of the 6th Walnut Council Research Symposium

VARIABILITY WITHIN SPAIN’S WALNUT 

POPULATIONS

Morphological and isoenzymatic variability are 

being studied in different populations of wild 

Persian walnut. Fifty individuals from nine 

populations have been analyzed using six enzymatic 

systems. Morphological characteristics of trees 

in field/germplasm collections are also being 

evaluated on representative clones that originated 

in different parts of Spain.

In 2003, we collected seed from Persian walnut 

from throughout Spain. In 2004, seed from more 

than 50 wild populations will be sowed in three 

progeny tests planted in different environments 

near Galicia (northwest of Spain), Zaragoza (north 

central of Spain), Granada (southeast of Spain) and 

Catalonia (northeast of Spain). Progeny testing will 

allow for the preservation of germplasm and the 

study of the variability present in existing Persian 

walnut populations, especially variability in traits 

important for timber production.

We already have several publications on the variability 

within wild clones/populations of Persian walnut 

(Aletà and Ninot 1995, Malvolti and others 1998, 

Ninot and Aletà 2003). Our recent work shows 

large genetic differences exist between the northern 

populations and southern populations in Spain.

SELECTING FOR WOOD PRODUCTION

Many surveys in different areas of Spain have been 

made since 1982 looking for wild Persian walnut 

trees with exceptional traits for wood production 

(Germain and others 1997). Selected trees have 

been grafted and established in field collection to 

produce seeds. Seed is collected and planted in 

the nursery for 2 consecutive years. Progenies with 

seedling collar diameters less than 12 mm and 

survival less than 60% are rejected. Progeny tests of 

the juvenile growth of more than 40 half-sib families 

in the nursery have resulted in the identification of 

28 families for further field-testing. Sixteen of these 

families are already under evaluation for further 

selection (Aletà and others 2004).

Seedlings of the best progenies are being planted 

in field progeny tests with different ecologies. In 

addition to evaluating wood production, progenies 

are tested for resistance to the soil diseases 

including Armillaria root and crown rot (A. mellea) 

and Phytophthora.

From superior half-sib progenies that have been 

identified using data from nursery and field trials, 

genitor clones are grafted. Grafts are established 

in field to be used as seed producing trees. In 

parallel studies, seed clonal orchards have been 

prepared using some pre-selected individuals to 

obtain controlled seeds of Persian walnut for timber 

production. Currently this material comes from five 

pre-selected IRTA-clones: ‘MBT-122’, ‘MBT-218’, 

‘MBT-231’, MBSB-13’ and ‘MBPo-6’. 

BREEDING NEW HYBRID PROGENIES

To increase the variability in our breeding 

germplasm, based mainly on Persian walnut, 

we have introduced black walnut (J. nigra L.), 

Argentine walnut (J. australis Grisebach), California 

black walnut (J. hindsii Jeps.), and Arizona walnut 

(J. major Torr.) Heller) materials in field collections. 

A genitor is evaluated both by the percentage of 

successful crosses and the growth characteristics 

of its progenies. For several years we have been 

doing controlled crosses between Persian walnut 

clones and some black walnut trees. The aim is to 

produce Persian x black walnut hybrids suitable 

for woodland plantations. In 2004, we will be 

evaluating 15 full-sib progenies from controlled 

crosses. The programme of controlled crosses is 

going to continue for 3 more years to increase the 

number of individuals per progeny.

CLONAL SELECTION

The decision as to which individual to clone is 

based on the characteristics of both the mother 

plant and the environment where the tree grows. 

Trees must show a suitable growth habit and high 

vigour plus some resistance to soil diseases or to 

drought to be cloned. We are testing some selected 

clones, together with pre-selected progenies, 

for Armillaria root and crown rot soil disease. 

Outstanding trees are micropropagated and studied 

in field trials together with commercial progenies, 

‘Lozeronne’ provenance as J. regia reference or 

‘Mj209xRa’ and ‘Ng23xRa’ as references for hybrid 

materials. In field trials clones will be evaluated 

for at least 10 years. At present an IRTA clone, 

IRTA X-80, is being made available commercially 

as micropropagated plants. IRTA X-80 is a hybrid 

walnut of unknown origin having excellent growth 

and stem form that is well adapted to areas with a 

long growing season.

TRAINING SYSTEMS FOR WOOD 

PRODUCTION

Persian walnut and Juglans x intermedia are not 

forest species and training in juvenile orchard 
International Walnut Research 155

Black Walnut in a New Century

stage must be done to get timber quality. 

Two training systems are under evaluation 

involving these to species: ‘dynamic pruning’ 

and ‘systematic pruning’. Trees grown under 

‘dynamic training’ are pruned, either in summer 

or in winter or in both seasons, looking for a 

clear leader dominance, eliminating all branches 

which compete with the leader; lateral shoots 

measuring more than 1 inch in basal diameter 

are cut. In contrast, ‘systematic training’ involves 

removing all lateral shoots during the summer 

so that there are no branches on the log below 3 

m height. Under this kind of pruning trees need 

a high tutor. All results are still preliminary but 

it has been observed that half-sib progeny of 

‘Ng23xRa’, are well adapted to ‘dynamic training’. 

This system allows reaching logs taller than using 

‘systematic pruning’ in which secondary growth 

is faster too. However, ‘dynamic training’ could 

cause bigger wood internal defects.

IRRIGATION PROGRAM DEVELOPMENT

In the Mediterranean basin, walnut trees need to 

be irrigated to produce regular trunk growth. We 

have calculated water demand based on potential 

evapotranspiration corrected by Persian walnut crop 

coefficients (Kc) established for fruit production. To 

optomize water use in relation to tree growth, two 

schedules of water supply using drip irrigation, in 

Persian walnut and ‘Ng23xRa’, are being evaluated. 

The first irrigation programme irrigates during the 

vegetative period and for the second programme, 

the hydric support finishes when first growth stops. 

Diameter contractions during the vegetative period 

are measured continuously by dendrometers placed 

on the trunk. Until the fourth growing season, there 

are no differences in juvenile growth between the 

two irrigation calendars.

MIXED PLANTING SYSTEMS FOR 

TIMBER PRODUCTION

Starting in 2000, the effects of inter-row planting 

of shrubs to naturally train walnut for timber 

production are under evaluation. Progeny of the 

hybrid ‘Ng23xRa’ was established, at 6 m x 6 m 

spacing, with European hazel (Corylus avellana

L.), at 2 m from the walnut trees, and autumn 

olive (Eleagnus umbellata Thunb.), at 1 m from 

the walnut trees. After three growing seasons, 

some natural stem training and reductions in 

branching with European hazel have been already 

observed. 

LITERATURE CITED

Aletà, N.; Ninot, A. 1995. Field evaluation of J. regia

selected clones from seedling populations of 

Mediterranean and Atlantic Spanish coasts. 

Acta Horticulturae. 442: 63-67.

Aletà, N.; Ninot, A.; Voltes, J. 2003. Caracterización 

del comportamiento Agroforestal de 12 

genotipos de nogal (Juglans sp) en dos 

localidades de Cataluña. Investigación Agraria: 

Sistemas de Recursos Forestales. 12(1): 39-50. 

Aletà, N.; Ninot, A.; Voltes, J. 2004. Retrospective 

evaluation of parental selection in Juglans 

regia L. using a mixed model analysis. Sylvae 

Genetica. (in press). 

Fady, B.; Fulvio, D.; Aletà, N.; Becquey, J.; Díaz, 

R.; Fernández, J.; Jay-Allemand, C.; Lefèvre, F.; 

Ninot, A.; Panetsos, K.; Paris, P.; Pisanelli, A.; 

Rumph, H. 2003. Walnut demonstrates strong 

genetic variability for adaptive and wood quality 

traits in a network of juvenile field tests across 

Europe. New Forest. 25: 211-225.

Germain, E.; Aletà, N.; Rouskas, D.; GomesPereira, J.; Monastra, F.; Limongelli, F.; 

Ninot, A.; Zakinthinos, G. 1997. Prospections 

réalisées dans les populations de semis de 

noyer d’Espagne, de Grèce, d’Italie et du 

Portugal: caractérisation des populations 

et description des présélections issues de 

ces prospections. CIHEAM-CE/DG, ed. 

Options Mediterranéennes. Série B: Études et 

recherches. 6(16): 9-40. 

Leslie, C.; Granahan, G.H. 1998. The origin of the 

walnut. In: Ramos, D.E., ed. Walnut production 

manual. Publ. 3373. Oakland, CA: University of 

California: 3-7.

Malvolti, M.E.; Aletà, N.; Ninot, A.; Spada, M. 1998. 

Walnut (J. regia) genetic resources in Europe. 

Nucis FAO-Newsletter. 7: 20-23.

Manchester, S.R. 1989. Early history of the 

Juglandaceae. Plant Systematics and Evolution. 

162: 231-250.

Ministerio de Agricultura Pesca y Alimentación, 

2001. Anuario de estadística agroalimentaria. 

Subdirección General de Estadística. Madrid, 

Spain.

Ninot, A.; Aletà, N. 2003. Identification and genetic 

relationship of Persian walnut genotypes using 

isozyme markers. Journal of the American 

Pomological Society. 57(3): 106-114.

Rivera, D.; Obón de Castro, C.; Ríos, S.; Selma, 

C.; Méndez, F.; Verde, A.; Cano, F. 1997. 

Juglandáceas. In: Las variedades tradicionales 

de frutales de la cuenca del río Segura. Catálogo 

Etnobotánico. Ed. Univ. de Murcia- Spain: 44-55. 
156 Economics of Walnut Production

Proceedings of the 6th Walnut Council Research Symposium

BEYOND THE WILD NUT: 

MOVING TOWARD PROFITABLE BLACK WALNUT NUT CROPS

Brian Hammons, Felix Ponder, Jr., and John Rickman1

ABSTRACT—Currently, about 2 million pounds of black walnut nutmeats are consumed 

annually, requiring about 26 million pounds of wild in-shell nuts (hulled, wet weight). 

Walnuts from wild trees are variable in quality, yield, and moisture, reducing the 

amount of good, salable nutmeats produced. Consequently, the price that can be paid 

to the harvester/producer is limited. Improved varieties of black walnut trees differ from 

wild trees in that they are typically planted in orchards, produce nuts more consistently 

and the nuts have higher percent kernel yield and quality. Thus, the price on such 

improved nuts can be higher. The black walnut industry (Hammons Products Company) 

has developed quality guidelines whereby growers of improved varieties can receive more 

for nuts with higher nutmeat yields and desirable characteristics. High yield of good 

quality nutmeats is the key to profitable nut crops.

QUALITY

Over the years, many hours of work have gone into 

evaluating black walnut nutmeat quality. In 1977, 

the Nebraska Nut Growers Association decided to 

hold a contest to find the largest black walnut nut 

(Bish 1999). However, soon, it was decided that 

big was not always better. In that test, the largest 

nut, Rowher variety, weighed 24.3 g, but yielded 

only 27.3% nutmeat compared to nuts of the 

Emma K variety, which weighed 17.2 g and had a 

yield of 35.9% nutmeat (Table 1). By comparison, 

kernel percent from wild trees averaged only 23%. 

Nut quality evaluation of the species must be an 

ongoing process to select high-producing, quality 

varieties that require fewer resources to produce.

There is evidence to suggest that the performance 

of nut varieties may be affected by climate or 

geographic area (Hanson 1999, Rink and others 

1994). Earlier, Bey (1973) demonstrated that the 

growth performance of black walnut was affected 

by geographic origin in provenance tests. In test 

plantings, black walnut trees originating well to the 

south consistently outgrew those of local origin. 

The geographic region for nut varieties may be even 

narrower than for timber trees (Hanson 1999). 

There is an increasing need for a greater, more 

stable supply of black walnuts to support long-term 

consumption growth. One way to help increase the 

supply is through the use of more nuts from trees of 

improved varieties. Virtually all the available supply 

of black walnuts for the commercial market is now 

harvested from wild trees in a few states. The wild 

nuts are variable in quality, yield, and moisture, 

which reduces the amount of good, salable 

nutmeats produced. The average yield, based on 

wet drying weight, is only about 7.5% (Hammons 

2001). This low yield, plus increasing hauling costs 

and freight costs, keeps the purchase price low at 

the buying stations. Also, because yields on nuts 

from eastern states are much less than nuts from 

the central states, the purchase price there is even 

lower.

The price for black walnuts must be higher in 

order to encourage growers to invest in orchards 

and harvest more nuts. In addition to volume, 

production of high quality black walnut nuts also 

will require a much higher price. The following 

sections review evaluations of “Quality” and then 

discuss one mechanism for determining a higher 

price based upon high quality.

1

President (BH), Hammons Products Company, P.O. Box 140, Stockton, MO 65758; Research Soil Scientist (FPJr), USDA Forest Service, 

North Central Research Station 208 Foster Hall, Lincoln University, Jefferson City, MO 65102; and Plant Manager (JR), Hammons 

Products Company, P.O. Box 140, Stockton, MO 65758; FPJr is the corresponding author: to contact, call 573-681-5575 or 573-875-

5341, ext. 234 or e-mail at fponder@fs.fed.us.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
Economics of Walnut Production 157

Black Walnut in a New Century

For nut varieties, this region should be bounded 

at about 50 miles north and south and about 100 

miles east and west of a variety’s viable location. 

This does not mean trees will not perform well 

outside the area. It does mean that if one is going to 

invest in nuts for commercial purposes, one should 

be conservative on large-scale selections. Trees 

should be tested by planting small tests of varieties 

outside the geographic region and expand after 

proven performance. It also appears that if a variety 

is viable within a region, the source of scion wood 

has little or no impact on that cultivar’s viability 

(Hanson 1999).

Sustainable, commercial black walnut nut 

production in orchards will be impossible 

without substantial yearly inputs of nitrogen (N) 

and perhaps other nutrients. Although specific 

recommendations are still in the developmental 

stage, all nut tree literature supports the need for 

N for producing trees. In general, to satisfy N needs 

for nut production, a split application of 90 to 100 

lbs of actual N per acre should be applied with 

about two-thirds applied in March to early April 

and the remainder in early August. Phosphorus 

and potassium (K) should be applied based on 

soil and analyses. Both P and K, when needed, do 

not exceed a rate of 60 to 100 pounds of actual 

ingredient and should be applied in the spring. In 

cases of severe deficiency larger quantities can be 

applied. To conserve moisture and improve nutrient 

uptake when both nutrients and moisture are 

adequate, mulching trees may be applicable.

Most producers plant more than one variety, 

primarily for adequate pollination, because varieties 

behave differently to natural environmental 

occurrences and to cultural treatments. Aside from 

nut maturity (early, mid season, and late), planting 

several varieties will likely minimize risks of weather 

insects, and disease; and for growers with limited 

labor and equipment, spread out the harvest 

workload. Spreading out the commercial harvest 

would allow more nuts to be harvested before juices 

produced by decaying husk darken nutmeats and 

degrades nut quality. 

With all this information for the grower, “Quality” 

for black walnuts is ultimately defined by the 

consumer (and therefore the industry). It is a 

nutmeat that is light brown to tan in color, is fullmeated, with a rich, naturally pungent flavor. It 

must also be firm, but not too crunchy in texture, 

and relatively large in size, over 0.25 inches (~6.3 

mm). To provide these nutmeats, the industry 

needs in-shell nuts that produce a high percentage 

of high quality kernels. And the growers will need a 

higher price. 

PRICING MODEL

If growers can produce and deliver nuts with higher 

nutmeat yields that meet the desired characteristics 

of quality and flavor, Hammons Products Company, 

currently the only commercial processor of eastern 

black walnuts, can pay producers a much higher 

price based upon a hand-tested sample of each 

delivery. For example, a load of nuts of a single 

variety, hulled and in-shell, and dry, delivered to 

the shelling plant, with a hand test yield of 28%, 

could be worth at least $0.42 per pound. In some 

years, such nuts with particularly good flavor and 

color could be worth between $0.50 and $0.60. 

Nuts with 30% nutmeats could bring $0.45, or 

perhaps over $0.60 in some years. The seller gets 

the best price when varieties and orchard locations 

are kept separate because poor quality nuts will 

affect the overall price of the load. Also, if varieties 

are commingled, larger testing samples may be 

needed to accurately measure average yield, and 

the price may be less. Other key variables affecting 

growers’ profit will be moisture, harvesting, hulling, 

drying, and freight costs.

Guidelines developed by Hammons Products 

Company provide a mechanism for industry and 

growers to use in planning and trading eastern 

black walnuts from improved varieties (Table 2). 

These guidelines show that the price for nuts 

Table 1.—Black walnut nut characteristics for variety 

and plantation grown wild trees.

Tree Type

Nut 

Weight

Kernel 

Weight 

Percent 

Kernel

Variety1 g g

Emma K 17.181 6.17 35.91

McGinnis 17.83 5.66 31.24

Surprise 20.22 6.61 32.71

Sauber 14.70 4.93 33.55

Hay #1 20.90 6.77 32.10

Sparks 147 16.90 5.77 33.42

Rowher 24.26 6.37 27.34

Sparks 127 15.01 5.11 33.69

Myers 15.52 5.03 32.89

Krause 18.36 4.77 28.00

Wild2

Area 17 12.44 2.86 22.14

Area 22 14.14 2.66 23.95

1

Data for variety trees adapted from Bish (1999).

2

Each value is the mean from 100 nuts. Wild trees are on Hammons 

Products Company SHONEF Plantation, Stockton, MO.
158 Economics of Walnut Production

Proceedings of the 6th Walnut Council Research Symposium

increases as nutmeat yield increases. This will 

result in more dollars to the grower for better 

quality nuts. The price quoted per percent yield may 

vary upward in some years, as noted above.

It is important to note that in order to sell improved 

varieties of nuts using these guidelines the amount 

of hulled nut delivered should weigh at least 1,000 

pounds and be clean and dry (Table 2). (Smaller 

deliveries have been handled in order to test the 

concept, but ideally in the future purchases would 

be in full truckload lots of 35,000 – 40,000 pounds.) 

An adjustment in nut weight is necessary to 

compensate for the moisture that must be removed 

before processing (Table 3). In this example, the 

in-shell delivered weight was reduced by 72 pounds 

(8.0% moisture – 4.5% moisture = 3.5% moisture 

x 1.25% of weight for each 1% above 4.5 = 4.375% 

x 1635 pounds = 71.53 or 72 lbs). In order to hull 

and clean the nuts, the buyer may provide that 

service and charge separately, or the buyer may 

lease the hulling machine to the seller. The buyer, 

a third party, or a grower cooperative may also 

provide drying service. If the buyer provides hulling 

or drying, a fee for that could be stated separately 

and deducted.

Price per pound of delivered nuts is based upon 

nutmeat grade and yield. The price per percent 

yield is based on a buyer’s assessment of crop 

availability, market conditions, and nutmeat 

desirability. For example, in Table 3, the quoted 

price of $0.018 per 1% yield of fancy kernel, with 

$0.0075% yield of standard kernel is not static. 

Thus, if a delivery tested 30% average hand-test 

yield, all fancy grade, and if all nuts were cleaned 

with moisture less than 4.5%, the grower could 

be paid $0.45 per pound. If the nut quality is 

particularly desirable, especially in flavor, aroma, 

and color, the quoted price could be higher, 

perhaps $0.02 or more per 1% yield resulting in 

$0.60 per pound or more. Compared to $509.54 

that was paid to the seller in the example in Table 3 

for nuts from trees of an improved variety, the seller 

would have been paid only $164.00 at $0.10 per 

pound for the same amount (1,635 lbs) of nuts from 

wild trees. 

Equipment modifications for processing nuts from 

trees of improved varieties are made necessary 

because of shell thickness. Generally, nuts from 

trees of improved varieties crack more easily. Also, 

processing time could be reduced because nuts of 

improved varieties tend to have fewer small kernel 

parts.

The vision for the future of the black walnut nut 

industry includes new ways of buying nuts and 

paying much higher prices for improved varieties 

harvested from managed orchards. It is reasonable 

to anticipate that harvesting, hulling and drying 

nuts will be done differently as more managed 

plantations come into production. Most importantly 

for the grower is, how much will nuts be worth and 

how can the price be higher? Our long-term goal is 

to assure that both industry and consumers have 

a growing supply of black walnuts at a reasonable 

cost. Increasing the purchase price of improved 

varieties of orchard grown black walnuts will help 

the species to become a more popular and viable 

agricultural crop.

Table 2.—Purchase guidelines for improved varieties of eastern black walnuts.

Delivery—Minimum delivery quantity of 1,000 pounds from one orchard or contiguous orchards planted 

predominately in one variety. All nuts are delivered clean and dry. 

Sampling—Approximately one pound (25-30 nuts) per 1,000 pounds will be drawn from different points within the 

load (in bags or bulk, maximum of ten samples). After mixing all samples together, two one-pound samples will be 

tested.

Testing—Each sample will be hand-cracked using an appropriate cracker before removing and separating all 

nutmeats into 3 grades.

1. Fancy - light brown to tan in color, full-meated;

2. Standard – dark brown to nearly black, but full-meated;

3. Reject – dark, shriveled, light weight material that will not produce salable nutmeats.

All nutmeats from the sample will then be tested for moisture with the results reported on the sample evaluation. 

Moisture—Kernel moisture must be no greater than 4.5% based on oven or validated meter test. For nuts delivered 

with greater moisture, an adjustment of 1.25% of the delivery weight will deducted for each 1% moisture over 4.5% 

in calculating delivery weight for payment.

Payment—The sample evaluation will report the percentage of total in-shell weight for each grade and yield, which 

will be the basis for payment. Moisture is considered in adjusting the purchase weight and not the price. 
Economics of Walnut Production 159

Black Walnut in a New Century

Table 3.—An example of Hammons Products Company black walnut test report.

____________________________________________________________________________________

 

NAME OF GROWER/PRODUCER _____________________________________________________

 12/14/02 1/03/03

DATE OF DELIVERY _________________________ DATE OF TEST ______________________

 Emma K

NUT VARIETY _______________________________________________________________________

 1635

POUNDS IN-SHELL DELIVERED ................................................ _____________________

 - 72 lbs

MOISTURE ADJUSTMENT (less 1.25% of wt. for each 1% above 4.5 ____________________

 1563

 POUNDS IN-SHELL DELIVERED, net of adjustment __________________________

SAMPLE ANALYSIS, from 2 samples of about 1 pound each

 In-shell weight Fancy kernel Choice kernel

 g Weight (g) Percent Weight (g) Percent

Sample #1 388.0 80.9 20.9 1.9 0.49

Sample #2 481.4 70.1 14.6 12.5 2.60

 

TOTAL 869.4 151.0 17.4 14.4 1.66

Moisture test (%) 8.0____ 

 

PRICE CALCULATION:

1. Price quoted for Fancy Kernel $0.018 X Percent Fancy 17.4 = 0.313

2. Price quoted for Choice Kernel $0.0075 X Percent Choice 1.66 = 0.013

3. Total price per pound $0.326

4. Total Pounds Delivered, net of moisture adjustment 1563

5. TOTAL PURCHASE PRICE TO BE PAID $ 509.54

Tester _____________________________ Accepted by ___________________________________ 
160 Economics of Walnut Production

Proceedings of the 6th Walnut Council Research Symposium

LITERATURE CITED

Bey, C.F. 1973. Growth of black walnut trees in 

eight midwestern states—a provenance test. 

Res. Pap. NC-91. St. Paul, MN: U.S. Department 

of Agriculture, Forest Service, North Central 

Forest Experiment Station. 12 p.

Bish, C. 1999. Twenty years of nut evaluation. 

Annual Report Northern Nut Growers 

Association. 90: 67-83.

Hanson, B.L. 1999. Commercial black walnut 

orchard in Iowa (Ben’s black walnuts). Annual 

Report Northern Nut Growers Association. 90: 

119-126.

Hammons, B. 2001. A vision for the black walnut 

industry: more $$$$ with better nuts. Annual 

Report Northern Nut Growers Association. 92: 

57-59.

Rink, G.; Kung, F.H.; Cutter, B.E. 1994. Selection 

effectiveness for black walnut nuts and 

timber. Annual Report Northern Nut Growers 

Association. 85: 44-52.
Economics of Walnut Production 161

Black Walnut in a New Century

1

Purdue University, Department of Forestry and Natural Resources, 175 Marsteller Street, West Lafayette, IN 47907-2033.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.

FACTORS AFFECTING THE QUALITY OF WALNUT LUMBER AND VENEER

Daniel L. Cassens1

ABSTRACT—Walnut is a unique species in both its timber and wood characteristics. 

Although market conditions vary it is generally considered a valuable species. Because 

of these factors, setting quality (value) levels for both lumber and veneer can be involved. 

Lumber grades are quantitative thus straight forward once the system is understood. 

Determining quality in veneer is much more subjective. This paper will discuss 

important factors related to quality in both lumber and veneer. Landowners, foresters, 

researchers, and others concerned with the production of walnut timber for lumber and 

veneer should consider these factors before proceeding with their work.

American black walnut (Juglans nigra L.) has been 

and continues to be a premier species for both the 

lumber and face veneer industries. The heartwood 

color is unique and once finished properly it produces 

an unsurpassed and unique brilliance. The wood is 

easily worked with hand and power tools and it is 

strong for its weight. As a result, it was one of our 

most valuable woods until the early 1970s. At that 

time, its value peaked (Hoover 2003), its popularity 

declined probably due to its cost and availability and 

consumers became attracted in other species. Since 

1997 however, its popularity and thus value has been 

slowly increasing.

LUMBER

Commercial sawmills use the National Hardwood 

Lumber Association (2003) grading rules for 

hardwood lumber to set quality and price levels 

for walnut lumber. These rules are quantitative. 

There is one general set of rules referred to as 

standard. Then, there are adjustments made for 

the characteristics and situations unique to each 

species. As an example, plain sawn oak is graded 

standard with one exception for mineral stain. 

For walnut, several adjustments are made to the 

standard rule. These adjustments allow some 

lumber to be moved into the higher grades which 

would not otherwise qualify under the standard 

rules. Grade for grade, the quality of walnut lumber 

is less than that for those species graded with the 

standard rule.

Factors which determine a board’s grade are its 

width and length, size of clear cuttings which can 

be placed between carefully defined defects, number 

of clear cuttings and the percent of the board in 

clear face cuttings (Cassens 2001). The grade 

separations for walnut are FAS, FIF, Selects, No. 1 

Common, No. 2A Common, No. 2B Common and 

No. 3 Common. The white sapwood is considered 

a defect unless the lumber is steamed to darken 

it. Steaming walnut is a standard procedure for 

any commercial mill producing walnut. The most 

significant differences between the walnut rules and 

the standard rule are as follows: 

1. The minimum clear cutting sizes for FAS walnut 

are 4” x 3’ and 3” x 6’ compared to 4” x 5’ and 3” 

x 7’ for the standard rule.

2. The minimum board width is 5” compared to 6’ 

for the standard rule.

3. The FAS walnut grade will admit 6’ and 7’ long 

pieces as compared to 8’ and longer for the 

standard rule. The short pieces are graded on 

a defect basis. Certain defects are carefully 

defined and then limited by the number 

present and their location. The requirements 

for the size and number of clear cuttings and 

the percent of clear area are dropped for this 

short lumber.

4. An extra cutting is allowed for certain sized 

boards in the FAS grade.
162 Economics of Walnut Production

Proceedings of the 6th Walnut Council Research Symposium

5. The FIF grade will accept 6’ and 7’ lumber as 

compared to 8’ for the standard rule.

6. For FIF lumber the reverse side can be sound 

as defined in sound cutting or not below No. 1 

Common. Sound cuttings admit small defect, 

holes and bird peck. The standard rule requires 

the reverse side of FIF lumber to grade No. 1 

Common. The standard rule requires that the 

cuttings on the No. 1 Common side be clear face 

cuttings so no defects are allowed.

7. For the Selects grade the cutting sizes are 

reduced as for the FAS grade.

8. For No. 1 Common and No. 2A Common walnut 

there is no limit as to the number of cuttings 

allowed. The standard rule limits the number of 

cuttings allowed.

9. For No. 1 Common pieces with just 1’ of 

surface measure must yield 12 cutting units as 

compared to being clear in the standard grade.

10. The revenue side of the cuttings in No. 1 

Common and No. 2A Common only need to be 

sound as compared to clear in the standard 

grade.

11. The minimum size of cutting for No. 2A Common 

is 2 inches or wider containing 72 square inches 

as compared to 3” by 2’ for the standard grade.

In summary, a load of any particular grade of 

walnut (except No. 3 Common) will appear to be 

substantially reduced from that of a species such 

as red oak which is graded standard. Over time, the 

rules for walnut have been eroded in an attempt 

to better utilize the available resource. However, 

within the last couple of years there has been an 

effort to remove the short lumber (6’ and 7’) from 

the upper grades. This effort which originated 

from a few producers was unsuccessful. However, 

buyers can still specify that they will not accept 

this material. The seller will probably ask for a 

premium. Anyone interested in grading walnut 

should refer to the National Hardwood Lumber 

Association (2003) grading rules and also obtain 

training from a qualified instructor.

VENEER

There are several methods used to produce 

veneer (Cassens, 2003). Today, the most common 

and simplest procedure is to saw two faces on a 

debarked log. The log is then cut in half with each 

half being called a flitch. The flitches are heated 

to soften them. After cleaning, the flitches are 

sliced into veneer ranging from about 1/32 to 1/48 

inches in thickness (Fig. 1). Each sheet of veneer 

is kept in order and the stack of sliced veneer is 

also called a flitch. The veneer is then dried and 

clipped. After drying, and if the veneer is destined 

for the domestic market, three sample sheets are 

removed (Fig. 2). These sample sheets are examined 

by a potential customer to determine if he wants to 

purchase the entire flitch. Veneer destined for the 

export market has all of the waney edges clipped 

off and all major defects removed. The veneer from 

several trees is usually packed down on pallets. 

Veneer from the same flitch is kept together. The 

buyer will usually look at all of the veneer.

Figure 1.—A veneer slicing operation showing the flitch 

or half log (upper right) being moved downward 

against a knife. The resulting veneer is being 

conveyed forward and stacked in order.

Figure 2.—Three sample sheets from a very high quality 

walnut flitch showing no defects, a centered 

cathedral pattern, and excellent color. The third 

sheet (right) taken from near the center of the tree is 

beginning to show some discoloration in the center.
Economics of Walnut Production 163

Black Walnut in a New Century

The cost and time required to process a log into 

veneer is much more than simply sawing it for 

lumber. Thus, buyers must be particular in what 

they accept as veneer quality logs. If the log does 

not meet expectations the log and manufacturing 

cost can easily exceed the value of the veneer.

The use of American black walnut for face veneer 

is a significant and high valued use for the species. 

There are several factors which affect the quality 

and thus value of walnut trees or veneer logs. 

Many of these factors are difficult to determine 

in standing trees and to determine value, buyers 

must depend on their previous experience in the 

immediate area as well as the condition and the 

history of the entire stand. Once a tree is cut the log 

ends can be observed and a much better evaluation 

can be made. Value will ultimately be determined 

by the buyer’s judgment of the trees and market 

conditions at the time. Some companies will have 

individuals who specialize in standing timber and 

others who deal with cut logs.

First, any bark surface irregularities such as 

overgrown branch stubs, insect damage, old 

mechanical damage, etc., will likely disqualify the 

log as high valued potential veneer. It is generally 

assumed that no surface indicators of interior 

defects are present in the butt section of a quality 

veneer tree or log.

Veneer or logs should be straight and well rounded. 

Bow and crook in a log creates an aesthetic problem 

by causing the cathedral pattern in flat sliced 

veneer to run in and out of the sheet. Tension wood 

is frequently present in leaning trees and buckle 

can occur when the veneer is dried (Fig. 3). Logs 

which are not well rounded or have an off-center 

pith also result in veneer with less than desirable 

grain pattern and are also likely to result in buckle.

Growth rates and thus ring width should be 

uniform across the entire cross section of the 

log. Thus, thinning to encourage faster growth of 

potential veneer trees may not be desirable. Growth 

rates of six to nine rings per inch are usually 

acceptable. Fast growth or very slow growth rates 

are not preferred. The industry uses the word 

“texture” to define growth rate. Soft texture refers 

to a slow growth rate while hard texture refers to a 

fast growth rate. 

Veneer quality trees should be healthy, well formed 

trees on good well-drained timber sites. A past 

history of grazing and or fire will reduce the quality 

and value of any potential veneer tree.

Most hardwood species grow over a wide geographic 

range. As such, climatic conditions, soil types, 

elevations, insect and disease potential, and other 

factors vary. Within the geographic range of each 

species there are certain specific areas where 

buyers feel the highest quality trees come from. 

Buyers will indicate that high quality trees can 

come from other regions as well but the probability 

is much reduced.

Color and uniformity of color in walnut veneer is an 

extremely important factor in determining value and 

it is a very difficult factor for the novice to judge. 

When trees are first cut or a fresh cut is made on 

the end of a log the heartwood should have a mint 

green color which changes to a uniform mousy 

brown color. Cooking schedules for walnut are 

extended over those of other species with the intent 

of darkening the color and the flitches are allowed 

to set several hours after slicing. This waiting 

period allows the mint green color to change to the 

preferred brown color before drying.

Uniformity of color is also critical (Figs. 2, 4). Like 

most other species, walnut can develop streaks of 

light and dark colors. The wood can also develop 

a dark splotchy appearance and this condition is 

sometimes referred to as “muddy”.

Bird peck, also called worm in the face veneer 

industry, is another very important defect in walnut 

(Fig. 5). Yellow-bellied sapsuckers probably cause 

most of the damage. It is generally believed that 

the bird pecks a hole to cause the flow of sap. Sap 

wet trees in the spring are easily spotted. Insects 

are attracted to the sap and the bird then feeds 

on the insects. The peck marks often circle the 

tree and birds tend to return to the same trees. 

If the cambium is penetrated, a small hole often 

with flagging will be found in the veneer. The peck 

mark is plugged or occluded with bark by the tree 

and after some time, it can become very difficult to 

spot. Like many smaller defects, once detected, the 

buyer will generally assume there are several more 

Figure 3.—Buckle in walnut veneer. Note the label. present which cannot be seen.
164 Economics of Walnut Production

Proceedings of the 6th Walnut Council Research Symposium

Pin knots (Fig. 6 ) like bird peck can be hard to 

recognize in standing walnut trees, especially when 

only a few are present. These defects are the result 

of suppressed dormant buds which persist for 

many years as a bud trace or pin knot. As the name 

implies, the buds may not actually break through 

the bark, so in some instances they cannot be easily 

detected. Sometimes, due to a stimulus such as 

thinning and light, the bud may sprout. The sprout 

may develop into a small limb that often dies, but 

normally the bud trace continues to form. Pin knots 

are best observed on the end of the log after the 

tree is cut or where the bark has peeled loose and 

they appear as sharp spikes. On flat-sliced veneer, 

they appear as pin knots, but on quartered surfaces 

they appear as a streak or “spike” across the sheet 

of veneer. Purchasers of veneered panels will often 

specify no pin knots or limit the actual number of 

pin knots per square foot of veneer in the finished 

panels.

Fast growth trees also tend to have a wide sapwood 

zone (Fig. 7). The sapwood is the light colored wood 

to the outside of the darker heartwood. Sapwood 

is usually discarded in high quality walnut veneer. 

Deeply furrowed bark which is not patchy tends to 

be normal to faster growth and in some situations 

it may have a wide ring of sapwood. Figure 8 shows 

the bark on a normal growth and slow growth tree. 

Also, slow growth trees may not have the preferred 

color for walnut.

Figure 4.—Two sample sheets from a walnut flitch 

showing numerous problems. Knots and irregular 

grain patterns are present throughout. The color is 

not uniform and the sapwood is excessive.

Figure 5.—“Worm” or bird peck on the end of a walnut log.

FIGURED WOOD AND BURLS

Walnut tends to have “flash” or figure in the wood. 

Small amounts of figure are not desirable (Fig. 9) 

for commercial applications. End consumers tend 

to ask questions when one piece of furniture or 

paneling has some figure and the next piece does 

not. Also, office furniture is often bought in units 

and additional pieces purchased at a later date. For 

this application, it is particularly important that the 

grain and color characteristics be consistent.

On the other hand, intense figure in particularly 

large flitches can command a premium. Figure 10 

shows an example of a heavy figured flitch which 

will produce a beautiful and unique end product. 

When the flitches are large the value increases 

because they can now be used as a matched and 

continuous pattern for paneling in large rooms.

Walnut burls are another unique item. Substantial 

burl wood was used for decorative Victorian walnut 

furniture manufactured from the 1870s to about 

1900. These burls were taken from old growth 

American black walnut. Black walnut burls are 

essentially nonexistent today. Figure 11 shows an 

example of a sheet of veneer from a small one. Most 

of the walnut burls used today are taken from old 

California nut orchards where the English walnut 

is grafted onto the American walnut root stock and 

a burl develops. These burls are often sold by the 

pound and at least some of the veneer is used in 

the automobile industry.

Crowns and stump or root wood is also veneered for 

its figure. Large old growth walnut trees developed 

large swellings at the ground level. This material 

was also sliced into veneer and produced some very 

figured and decorative patterns. Figure 12 shows a 

sample of crown wood veneer produced in the 1940s.

Walnut typically has a large tap root and it too has been 

sliced for veneer because of the figure it produces.
Economics of Walnut Production 165

Black Walnut in a New Century

Figure 6.—Pin knots in walnut. (A) shows an easily seen pin knot on a debarked surface of a log. These pin knots are 

hard or impossible to detect unless the bark is removed or the log is cut (B) from the tree. They will appear as a 

spot on flat sliced veneer (C) and as light streaks on a quartered surface (B).

A

C

B

Figure 7.—Wide (left) and narrow sapwood on two 

walnut logs. Sapwood is considered a defect in 

walnut veneer.
166 Economics of Walnut Production

Proceedings of the 6th Walnut Council Research Symposium

Figure 9.—Slight flash or figure in walnut veneer. This 

level of figure is not desirable. Numerous pin knots 

can also be seen.

Figure 10.—Heavy and very desirable figure in a flitch of 

walnut veneer.

Figure 11.—Veneer from a low quality and small natural 

black walnut burl.

Figure 12.—Figured wood from a walnut stump.

Figure 8.—Bark on normal and slow growth walnut trees.
Economics of Walnut Production 167

Black Walnut in a New Century

SUMMARY

Walnut is one of our most unique and valued 

species. In regards to planting, natural 

regeneration, growth characteristics and genetics, 

it is probably the most studied temperate hardwood 

species. Wood quality characteristics at least for 

veneer, are not generally well understood outside 

of the veneer industry. Only a limited number of 

walnut trees have the quality characteristics which 

command exceptional value. These characteristics 

need to be studied and better understood.

LITERATURE CITED

Cassens, D.L. 2001. Quality control in lumber 

purchasing: hardwood lumber grades. FNR130, West Lafayette, IN: Purdue University, 

Department of Forestry and Natural Resources. 

8 p.

Cassens, D.L. 2003. Factors determining the 

suitability of trees and logs for the face 

veneer industry. Purdue University Extension 

Publication in process. 

Hoover, W. 2003. Personal communication on the 

prices of Indiana walnut saw logs adjusted for 

inflation.

National Hardwood Lumber Association. 2003. 

Rules for the measurement and inspection of 

hardwood and cypress. Box 34518, Memphis, 

TN 38184-0518. 136 p.
168 Economics of Walnut Production

Proceedings of the 6th Walnut Council Research Symposium

THE BLACK WALNUT RESOURCE IN THE UNITED STATES

Stephen R. Shifley1

ABSTRACT—Between 1989 and 1999 the total volume of black walnut (Juglans nigra

L.) in the United States increased from 1.6 to 2.4 billion cubic feet. Saw log volume 

(International 1⁄4-inch scale) increased from 4.3 to 7.2 billion board feet. Increases 

occurred in most states; however, in Michigan, Ohio, Virginia, Pennsylvania, and Kansas 

black walnut volumes decreased or remained nearly constant and/or mean tree quality 

appeared to decrease. National forest inventory data is available online and can readily 

be queried to summarize and map characteristics of black walnut and other species 

by individual states, groups of counties, or other geographic areas. The majority of 

black walnut trees occur in natural stands in association with other fine hardwoods. 

On average black walnut comprises about 11% of the total volume in stands where it 

occurs. Thus, opportunities to apply silvicultural treatments to increase walnut volume 

or value in natural stands usually also provide the option to do the same for other 

associated hardwood species. 

Black walnut (Juglans nigra L.) occurs naturally 

throughout the eastern United States (Williams 

1999). Black walnut trees typically occur as 

scattered individuals or in small clusters; rarely do 

they comprise a majority of the stand basal area 

or volume in natural stands. Walnut growth and 

value increment are closely related to site quality, 

and, natural stands with high quality walnut trees 

often contain other valuable hardwoods such as 

northern red oak (Q. rubra L.), white oak (Quercus 

alba L.), white ash (Fraxinus americana L.), black 

cherry (Prunus serotina Ehrh), or yellow-poplar 

(Liriodendron tulipifera L.).

The vast majority of black walnut occurs in 

natural stands. Walnut plantations only cover 

approximately 13,800 acres in the United States, 

and plantations (walnut and mixed hardwood 

plantations) currently include only about 1% of 

all black walnut cubic foot volume in the United 

States. In natural stands, management (including 

harvesting) of black walnut is often carried out 

as part of prescription applied to a mixture of 

hardwood tree species (Slusher 1997).

Black walnut is highly valued for lumber and 

veneer. This fact, coupled with walnut’s relative 

rarity, has resulted in an ongoing interest in 

inventory data detailing the location, volume, size, 

and quality of black walnut trees and how those 

attributes have changed over time (e.g., Quigley and 

Lindmark 1966, Blythe 1973, Cooper and others 

1973, Schmidt and Kingsley 1997). 

This paper provides a contemporary overview and 

update of the status of the black walnut resource. 

In addition to summarizing current conditions, 

it presents some large-scale trends in the walnut 

resource over the last 10 years based on the latest 

data available from the USDA Forest Service’s 

Forest Inventory and Analysis (FIA) program (Miles 

2001, Miles and others 2001). The latest available 

inventory information (mean inventory year of 1999) 

is compared to information reported for inventory 

year 1989 by Schmidt and Kingsley (1997) at the 

Fifth Black Walnut Symposium (Van Sambeek 

1997). The latest inventory data is further analyzed 

to estimate the relative abundance of hardwood 

species that frequently occur in association with 

black walnut.

METHODS

Forest inventory data for black walnut and 

associated species were derived from data available 

on the USDA Forest Service FIA Web site (http://

www.ncrs2.fs.fed.us/4801/FIADB/index.htm). Most 

data queries and initial summaries were conducted 

using the FIA data base (Miles and others 2001) 

1

Research Forester (SRS), North Central Research Station, USDA Forest Service, 202 Anheuser-Busch Natural Resources Building, 

University of Missouri, Columbia, MO 65211-7260. SRS is corresponding author: To contact, call (573) 875-5341 ext 232 or e-mail at 

sshifley@fs.fed.us.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
Economics of Walnut Production 169

Black Walnut in a New Century

coupled with the on-line FIA Mapmaker software 

(Miles 2001). Additional summaries and analyses 

were performed using spreadsheet and statistical 

software. 

Except where otherwise noted, results are for 

timberland. Timberland is forest land that is 

capable of producing at least 20 cubic feet per acre 

per year of industrial wood crops and that is not 

administratively removed from timber utilization 

(e.g., through park, wilderness, or preserve status). 

In the eastern United States, 94% of forest land is 

classified as timberland (Smith and others 2002). 

Non-timberlands, (e.g., wooded pastures and 

wooded strips) do, in aggregate, contain significant 

quantities of black walnut that contribute to timber 

supply, wildlife values and other benefits. Schmidt 

and Kingsley (1997) indicate that black walnut on 

non-timberland has been estimated to range from 5 

to 25% of that on timberland.

FIA inventory methods are undergoing a change 

from periodic statewide inventories at roughly 10-

year intervals to on-going annual inventories that 

survey approximately 20% of a state’s timberland 

each year (McRoberts 2000). Under the annual 

inventory system it requires approximately 5 years 

to complete a state inventory that is equivalent to 

the prior periodic inventories; thereafter the annual 

inventory system continuously provides current 

inventory information based on the most recent 

5 years of inventory data. Change in some forest 

attributes can be estimated from as little as 1 to 

4 years of annual inventory data, although the 

precision of estimates decreases with decreasing 

sample size. 

States in the eastern United States are in differing 

stages of transition from the periodic to the annual 

inventory methods. The mean year of inventory and 

type of inventory data analyzed used in this paper are 

reported in Table 1. The mean inventory year for all 

states combined was computed as a weighted mean 

based on black walnut volume per state. The mean 

inventory year for the most current available data 

is 1999. This amounts to a mean remeasurement 

interval of 10 years from results reported by Schmidt 

and Kingsley (1997) for the black walnut resource in 

1989. As noted in Table 1, there was no new inventory 

data for Kentucky, Louisiana, Mississippi, New York, 

North Carolina, or Oklahoma since Schmidt and 

Kingsley’s (1997) report.

For reporting purposes some information on 

associated trees species was grouped into the select 

white oak group (primarily white oak; bur oak, 

Quercus macrocarpa Michx.; and chinkapin oak, 

Quercus muehlenbergii Engelm.), the select red oak 

group (primarily northern red oak; and cherrybark 

oak, Quercus falcate var. pagodifolia Ell.), the other 

Table 1.—Year of most recent inventory, type of inventory, 

and remeasurement interval used for analysis. 

Periodic inventories were conducted prior to 2001 and 

are complete inventories of all timberland in a state. 

Annual inventories measure approximately 20% of a 

state’s timberland each year on a continuous basis 

with each year’s selection of plots systematically 

spread across the state. Five years of annual 

inventory are typically needed to complete a full 

state inventory cycle; inventories based on fewer 

than 5 years of data are unbiased but have a higher 

variance than a full state inventory cycle. The number 

of annual measurements is shown in parentheses 

for the annual inventories used in this analysis. 

The remeasurement interval indicates the number 

of years between these most current inventories 

and those reported by Schmidt and Kingsley (1997) 

which are used to evaluate change. NA indicates that 

there is no newer information than that reported by 

Schmidt and Kingsley (1997). 

Remeasurement

State Inventory Type Interval (yrs)

Alabama 2000 periodic 10

Arkansas 2001 annual (2) 13

Delaware 1999 periodic 13

Georgia 1997 periodic 8

Illinois 1998 periodic 13

Indiana 2002 annual (4) 14

Iowa 2001 annual (3) 10

Kansas 2002 annual (2) 20

Kentucky 1988 periodic NA

Louisiana 1991 periodic NA

Maryland 1999 periodic 13

Michigan 2001 annual (2) 8

Minnesota 2002 annual (4) 10

Mississippi 1994 periodic NA

Missouri 2002 annual (4) 11

Nebraska 2002 annual (2) 8

New Jersey 1999 periodic 12

New York 1993 periodic NA

North Carolina 1990 periodic NA

Ohio 2001 annual (1) 10

Oklahoma 1993 periodic NA

Pennsylvania 2002 annual (3) 12

South Carolina 2002 annual (5) 7

Tennessee 1999 periodic 10

Texas 2002 annual (3) 9

Virginia 2001 annual (5) 8

West Virginia 2002 periodic 13

Wisconsin 2001 annual (2) 5

Mean 1999 10
170 Economics of Walnut Production

Proceedings of the 6th Walnut Council Research Symposium

red oak group (primarily black oak, Quercus velutina

Lam.; pin oak, Quercus palustris Muenchh.; and 

scarlet oak, Quercus coccinea Muenchh.), the ash 

group (Fraxinus spp.), the hickory group (Carya spp.) 

and the hard maple group (primarily sugar maple, 

Acer saccharum Marsh.) (Miles and others 2001).

Trees were classified into four grade classes with 

grade 1 representing the best quality. Tree grade 

definitions varied among states, but were most 

often based upon grading rules by Hanks (1976) 

and/or by Rast and others (1973). In general, tree 

grade was determined from the best 12 feet of the 

lowest 16-foot log or the best 12-foot upper section 

of the tree if the butt log did not meet minimum 

grade requirements (e.g., due to rot or defects). 

Typical minimum DBH limits for grades 1, 2, 

and 3 were 16 inches, 13 inches, and 11 inches, 

respectively. Typical minimum scaling diameters 

inside bark for grades 1, 2, and 3 were 13 inches, 

11 inches, and 8 inches, respectively. Additional 

grading criteria took into account the number of 

surface defects on logs, and deductions for cull, 

crook, and/or sweep. The best source of detailed 

information on grading rules is the appendix of FIA 

inventory reports for individual states of interest 

(e.g., Schmidt and others 2000). The most up-todate source of information on inventory reports is 

the FIA webpage (http://www.fia.fs.fed.us/). 

RESULTS

From 1989 to 1999 the total number of black 

walnut trees 5 inches DBH and larger increased 

23% (162 to 200 million trees) (Fig. 1A). On a 

percentage basis the greatest increase was in 

walnut trees > 21 inches DBH; that diameter class 

increased by 132% (1 million trees) during the 

inventory interval. However, that increase was on a 

relatively small initial number of trees. In absolute 

numbers of trees, the greatest increase (14 million 

trees) occurred in the 11-inch DBH class (Fig. 1B).

Black walnut volume increased in the 10 states 

with the greatest current walnut cubic foot volume 

(Fig. 2). Iowa, with a 13% annual rate of increase 

(cubic foot basis), had the largest net gain. Virginia 

and Michigan saw net annual decreases of 3% 

and 7%, respectively, in black walnut cubic foot 

volume. Rankings for board foot volume increase 

and percent volume increase differed slightly from 

those for cubic volume, but Iowa still had the 

greatest increase (15% annually) while Virginia and 

Michigan both declined. The composite category 

representing “other states” also showed large 

annual increases in black walnut volume: 10% 

annual increase for cubic foot volume and 9% 

annual increase in board foot volume over a 6-year 

Figure 1.—(A) Number of black walnut trees by DBH 

class for past (1989) and current (1999) inventories. 

Inventory dates for individual states are listed in 

Table 1. Past inventory values are based on Schmidt 

and Kingsley (1997). Percent increase is shown for 

individual diameter classes. (B) Net increase by 

diameter class. 

remeasurement interval (Fig. 2). The combined 

total black walnut cubic foot volume for all states 

increased from 1.6 to 2.4 billion cubic feet (50% 

or approximately 4% annually) over the 10-year 

remeasurement interval (Table 1). Total board foot 

volume (International 1⁄4-inch scale) increased from 

4.3 to 7.2 billion board feet (67% or approximately 

6% annually) over the same interval. 
Economics of Walnut Production 171

Black Walnut in a New Century

Figure 2.—Volume of black walnut growing stock on 

timberland. Current values are for the inventory 

years listed in Table 1; previous values are based 

on Schmidt and Kingsley (1997). Periodic increase 

per state is indicated by the difference between 

bars. State labels show the annual percent change 

in volume and the remeasurement interval for 

each state; NA indicates that no remeasured data 

were available. States are ordered from greatest 

to least current volume (cubic foot basis). Total 

cubic foot volume increased from 1.6 to 2.4 billion 

cubic feet over the period. Total board foot volume 

(International 1⁄4-inch scale) increased from 4.3 to 

7.2 billion board feet (International 1⁄4-inch) over the 

period. 

Figure 3.—(A) Black walnut board foot volume by 

diameter class for past (1989) and current (1999) 

inventories. Inventory dates for individual states 

are listed in Table 1. Past inventory values are based 

on Schmidt and Kingsley (1997). Percent increase 

is shown for individual diameter classes. (B) Net 

increase by diameter class over the remeasurement 

interval of 10 years. Volume increased in all diameter 

classes with the greatest increases in the 16-inch 

DBH class. 
172 Economics of Walnut Production

Proceedings of the 6th Walnut Council Research Symposium

Across the range of black walnut, the board foot 

volume of growing stock on timberland increased in 

all tree diameter classes (Fig. 3). As was observed for 

the total number of trees, the greatest percentage 

increase in black walnut volume occurred in 

the larger diameter classes. Over the 10-year 

remeasurement period volume increased by more 

than 90% for black walnut trees > 19 inches DBH. 

However, on an absolute basis the most volume (735 

million board feet) accrued in the 16-inch DBH class. 

For all states combined, 16% of black walnut 

volume was classified as tree grade 1 (best) (Table 

2). Tree grades 2 and 3 each included 35% of 

black walnut board foot volume, and tree grade 

4 included the remaining 14%. Michigan and 

Iowa were on the high end of the quality range 

for black walnut; they had at least 20% of black 

walnut volume in tree grade 1. In Michigan, Iowa, 

Tennessee, Pennsylvania, Indiana, Illinois, and the 

“Other” group of states at least 50% of black walnut 

volume was in tree grades 1 and 2. Since the 

previous measurement, the proportion of volume in 

tree grades 1 or 2 declined in Ohio, and Kansas. In 

most other states the proportion of volume in tree 

grades 3 or 4 decreased while volume in tree grades 

1 and 2 increased. 

On inventory plots where black walnut occurred, 

it comprised an average of 11% of the total cubic 

foot volume (Fig. 4). Iowa and Kansas were at the 

high end of the range with black walnut comprising 

23 and 18%, respectively, of the total volume on 

sites where walnut occurred. Tennessee was at the 

low end of the range with an average of 7% black 

walnut volume on sites where walnut occurred. 

Across all states black walnut was most commonly 

found growing in association with select white oaks 

(10%), yellow-poplar (9%), hickory (9%), other red 

oaks (8%), ash (7%), select red oaks (5%), and hard 

maple (4%). Other species comprised the remaining 

35% of the volume on plots where black walnut 

occurred. In most states black walnut comprised 

the plurality of the volume by species or species 

groups (Fig. 4) in plots where it occurred. In 

Missouri and Illinois the total volume of the select 

white oak group exceed the total walnut volume 

on plots where walnut occurred. Similarly the total 

volume of yellow-poplar volume exceeded black 

walnut volume on plots in Kentucky, Tennessee, 

West Virginia, and the group of “other” states. 

DISCUSSION

In the 10 years since the last comprehensive 

inventory and summary of the black walnut 

resource in the eastern United States (Schmidt 

and Kingsley 1997), the number and volume of 

black walnut trees has increased (Figs. 1, 3). This 

continues a trend that goes back to at least 1966 

(Quigley and Lindmark 1966). Thus, across the 

region there is little reason to be concerned that the 

black walnut is being depleted at current rates of 

Table 2.—Black walnut sawtimber volume by state and tree grade, 1999. Percent change from previous inventory 

as reported by Schmidt and Kingsley (1997) is shown in italics. State entries are ordered from best (lowest) 

to worst (highest) average tree grade. Tree grade definitions differ among some states as described in the 

text. Statistics showing change over time within a given state are based on a consistent set of tree grade 

definitions. 

State

Remeas.

Interval

Grade 1 Grade 2 Grade 3 Grade 4

Current Change Current Change Current Change Current Change

MI 8 28 8 41 -5 31 0 0 -3

IL 13 16 3 48 10 33 -8 3 -5

IA 10 20 8 42 8 29 -21 9 5

Others 17 7 42 15 32 -21 9 -1

PA 12 18 15 36 11 34 -12 11 -15

TN 10 19 14 31 13 42 -21 8 -6

IN 14 17 6 36 6 27 -23 19 10

MO 11 15 10 31 12 38 -7 15 -16

WV 13 15 2 32 1 34 -4 19 1

KS 20 5 -19 41 4 44 5 10 10

VA 8 8 4 32 2 41 -14 19 8

OH 10 16 7 15 -7 42 -21 28 21

All states 10 16 6 35 7 35 -16 14 3
Economics of Walnut Production 173

Black Walnut in a New Century

harvest and natural mortality. However, the walnut 

resources merit continued scrutiny in Michigan, 

Virginia, and Pennsylvania where walnut volume is 

flat or decreasing. The shift to annual (continuous) 

FIA inventories (McRoberts 2000) provides an 

excellent mechanism to monitor the black walnut 

resource on a continuing basis. 

The composite reverse-J-shaped diameter 

distribution for black walnut trees in the region 

(Fig. 1A) is expected for any species that has 

experienced decades of successful reproduction 

and growth in the absence of excessive depletion 

by insects, disease, harvesting or other disturbance 

factors. What is puzzling about this pattern is that 

Figure 4.—Proportion of volume by species or species groups for inventory plots that contain black walnut. Proportions are 

based on total cubic feet of growing stock volume on timberland. The total proportion of black walnut volume is listed 

with the state name. Across all states, black walnut comprises 11% of the volume where it occurs.

the greatest absolute increase in walnut abundance 

is in the 11-inch DBH class and not in smaller (7-

inch) DBH class (Fig. 1B). This could be a natural 

consequence of an abundance of well-stocked, 

maturing forests that provide fewer opportunities 

for walnut regeneration than in the past. Over the 

long term this pattern bears watching regionally 

and locally to ascertain whether or not fewer black 

walnut trees will be moving into the 11-inch DBH 

class in coming decades. In the short term the fact 

that the largest percentage increases in the number 

and volume of black walnut trees has occurred in 

the largest diameter classes bodes well for those 

interested in an abundance of large, black walnut 

trees for products or other values. 

0% 20% 40% 60% 80% 100%

All (11%)

TN (7(%)

WV (8%)

KY (9%)

Other (10%)

IL (12%)

OH (12%)

MO (12%)

PA (13%)

IN (13%)

KS (18%)

IA (23%)

Black walnut

Select white oaks

Yellow-poplar

Hickory

Other red oaks

Ash

Select red oaks

Hard maple

Other
174 Economics of Walnut Production

Proceedings of the 6th Walnut Council Research Symposium

Percentage increases in board foot volume were 

high in the larger diameter classes (Fig. 3), but 

the absolute increase in board foot volume peaked 

in the 16-inch diameter class (735 million board 

feet). The overall increase in walnut volume was 

substantial (nearly 3 billion board feet over the 

remeasurement interval), but states did not share 

equally in the increase. Although most states 

registered net increases in black walnut board foot 

volume (2% to 15% annually) (Fig. 2), volume in 

Pennsylvania remained virtually unchanged and 

decreased in Virginia (-1%) and Michigan (-5%). 

This highlights the fact that broad regional trends 

tell only part of the story. Local patterns may differ 

from regional trends due to a whole host of factors. 

The availability of web-based tools for summarizing 

FIA data makes it relatively easy to explore trends 

for individual states, ecoregions, groups of counties, 

or even circular areas around a specific point (e.g., 

the procurement zone around a sawmill) (Miles 

2001, Miles and others 2001). Data presented in 

Table 2 and Figures 1-4 were summarized from 

retrievals made from the FIA web-based retrieval 

system. Individuals with interest in a smaller 

geographic region can summarize similar statistics 

specific to that area. Web-based mapping tools 

provide opportunities to display spatial patterns 

of resource data for counties or larger areas. A 

web browser and internet access are all that is 

required to access these data. Spreadsheet software 

is usually a helpful addition for summarizing and 

graphing results.

Black walnut lumber and veneer value is greatly 

affected by log grade. FIA inventories use tree grade 

as an indicator of quality and value. The proportion 

of black walnut volume in the two best quality 

classes increased in most states. This can partially 

be attributed to the overall increase in the mean 

diameter of black walnut trees in the sawlog size 

class during the remeasurement interval. Other 

factors being equal, tree grade increases when 

trees increase in scaling diameter to exceed specific 

merchantability limits (e.g., 8, 11, or 13 inches). 

Other factors related to tree quality may also be 

involved in the observed increase in tree grade, 

but it is difficult to evaluate them separately. As 

described in the Methods section, grading rules 

differed among some states. However, the percent 

changes in tree grade for an individual state 

(Table 2) for are based on a consistent set of grade 

definitions.

Ohio, Virginia, Michigan, and Kansas stand out 

as states that have (1) modest or negative total 

volume change, (2) decreases in the proportion of 

volume in tree grades 1 and 2, and (3) increases 

in the proportion of volume in tree grades 3 and 4. 

Those states are worthy of closer monitoring as new 

inventories are completed in coming years. 

Black walnut rarely occurs in pure or nearly pure 

stands. However it often occurs on good sites in 

association with other valuable hardwoods. Across 

all plots where black walnut occurred, it comprised 

an average of 11% of the board foot volume—more 

than any other individual species. Select white 

oaks, yellow-poplar and hickories had slightly less 

volume, followed by other red oak, ash, select red 

oaks, and hard maples. Other species comprised 

the remainder, an average of 35% of the saw log 

volume. Consequently, black walnut growing in 

natural stands usually provides the opportunity to 

simultaneously manage a suite of hardwood species 

with the potential to produce high-valued timber 

and often to produce abundant mast as well. 

There was considerable variation among states in 

the relative abundance of black walnut sawtimber 

volume on plots where black walnut occurred. For 

individual states black walnut had the plurality of 

board foot volume in only five: Iowa (23%), Kansas 

(18%), Indiana (13%), Pennsylvania (13%), and 

Ohio (12%). In Tennessee, West Virginia, and the 

group of “other” states the volume of yellow-poplar 

exceeded walnut volume on those plots where 

walnut occurred. For Illinois and Missouri the 

volume of select white oaks exceeded the volume of 

walnut on plots where walnut occurred. Thus, black 

walnut management opportunities are most often 

found in mixed hardwood stands where general 

hardwood management guides (e.g., Sander 1977, 

Clark and Hutchinson 1998, Perkey and others 

1993, Johnson and others 2002) apply. For best 

results with black walnut, however, hardwood 

management guides should be modified with 

guidelines specific to black walnut (e.g., Schlesinger 

1977, Burde 1988, Van Sambeek 1997). 

CONCLUSIONS

The number, volume, and general quality of black 

walnut trees in the eastern Unites States has 

increased since conditions were last reported by 

Schmidt and Kingsley (1997) at the Fifth Black 

Walnut Symposium. These increases continue 

a trend dating to 1966 (Quigley and Lindmark 

1966). Across the region increases in black walnut 

abundance were observed for a wide range of 

diameter classes. The number of black walnut trees 

between 5 and 9 inches DBH (the smallest DBH 

class examined) increased, but at a rate less than or 

equal to some larger diameter classes. This pattern 

should be monitored as new inventory information 

becomes available.
Economics of Walnut Production 175

Black Walnut in a New Century

All states did not fair equally in the generally rosy 

outlook for the black walnut resource. Michigan, 

Ohio, Virginia, Pennsylvania, and Kansas stand 

out as states where black walnut volume is low 

or declining and/or where overall tree quality 

appeared to decrease. This too should be monitored 

in subsequent inventories. 

Some excellent tools are available to query FIA 

databases for specific information about forest 

resources in individual states, groups of counties, 

or mill procurement zones. These web-based 

tools can be accessed at http://www.ncrs2.fs.fed.

us/4801/FIADB/index.htm 

The vast majority of black walnut volume is 

found in natural stands in association with 

other hardwoods, many of which are valuable or 

potentially so. In stands where black walnut occurs, 

it is often the species with the greatest volume 

per acre, but on average it comprises only about 

11% of the total volume. Thus, opportunities to 

increase walnut volume or value generally provide 

opportunities to do the same for other hardwood 

species. 

ACKNOWLEDGMENTS

Pat Miles and Gary Brand (North Central Research 

Station, St. Paul, MN) provided technical assistance 

necessary for some of the more complex retrievals 

from the FIA on-line databases. Jerry Van Sambeek 

(North Central Research Station, Columbia, 

MO) and Hank Stelzer (Department of Forestry, 

University of Missouri, Columbia) provided helpful 

comments on an earlier version of this manuscript. 

I thank them all.

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Proceedings of the 6th Walnut Council Research Symposium

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fs.fed.us/pubs/viewpub.asp?key=102.

Schlesinger, R.C.; Funk, D.T. 1977. Manager’s 

handbook for black walnut. Gen. Tech. Rep. 

NC-38. St. Paul, MN: U.S. Department of 

Agriculture, Forest Service, North Central 

Forest Experiment Station. 22 p. http://ncrs.

fs.fed.us/pubs/viewpub.asp?key=103

Schmidt, T.L; Kingsley, N.H. 1997. Status of 

black walnut in the United States. In: Van 

Sambeek, J.W., ed. Knowledge for the future 

of black walnut; proceedings, 5th black walnut 

symposium; 1996 July 28-31; Springfield, MO. 

Gen. Tech. Rep. NC-191. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, 

North Central Forest Experiment Station: 

9-22. hwttp://ncrs.fs.fed.us/pubs/viewpub.

asp?key=255.

Schmidt, T.L.; Hansen, M.H.; Solomakos, J.A. 2000. 

Indiana’s forests in 1998. Resour. Bull. NC–196. 

St. Paul, MN: U.S. Department of Agriculture, 

Forest Service, North Central Research Station. 

139 p. http://ncrs.fs.fed.us/pubs/rb/rb_

nc196.pdf.

Smith, W.B.; Vissage, J.S.; Darr, D.R.; Sheffield, 

R.M. 2002. Forest resources of the United 

States, 1997. Gen. Tech. Rep. NC-219. St. 

Paul, MN: U.S. Department of Agriculture, 

Forest Service, North Central Research Station. 

190 p. http://ncrs.fs.fed.us/pubs/viewpub.

asp?key=845.

Slusher, J.P. 1997. Principles of managing for black 

walnut in mixed hardwood stands. In: Van 

Sambeek, J.W., ed. Knowledge for the future 

of black walnut; proceedings, 5th black walnut 

symposium; 1996 July 28-31; Springfield, MO. 

Gen. Tech. Rep NC-191. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, 

North Central Forest Experiment Station: 

148-154. http://ncrs.fs.fed.us/pubs/viewpub.

asp?key=255.

Van Sambeek, J.W., ed. 1997. Knowledge for 

the future of black walnut; proceedings, 5th 

black walnut symposium; 1996 July 28-31; 

Springfield, MO. Gen. Tech. Rep. NC-191. St. 

Paul, MN: U.S. Department of Agriculture, 

Forest Service, North Central Forest Experiment 

Station. 256 p. http://ncrs.fs.fed.us/pubs/

viewpub.asp?key=255.

Williams, R.D. 1999. Juglans nigra L.—Black walnut. 

In: Burns, R.M.; Honkala, B.M., tech. coords. 

Silvics of North America, Volume 2, Hardwoods. 

Agric. Handb. 654. Washington, DC: U.S. 

Department of Agriculture, Forest Service: 391-

399. http://www.na.fs.fed.us/spfo/pubs/silvics_

manual/table_of_contents.htm.
Short Note 177

Black Walnut in a New Century

PERFORMANCE OF BLACK WALNUT IN THE YELLOW RIVER WATERSHED OF

THE PEOPLE’S REPUBLIC OF CHINA

J. W. Van Sambeek, Sheng-Ke Xi, William A. Gustafson, and Mark V. Coggeshall1

1998, China plans to convert 15 million acres of 

farmland to plantation forests within 10 years for 

soil erosion control in the Yangtze and Yellow River 

watersheds (Zhang and others 2000). Recently, 

Xi (1999) published guidelines in Chinese for the 

introduction, establishment and management of 

black walnut.

In the fall of 1996, a 10-person delegation from 

the Chinese Academy of Forestry and the Henan 

Bureau of Forestry journeyed to Nebraska, 

Kansas, Missouri, and Illinois to collect nuts from 

timber-type black walnut trees. Their objective 

was to establish multiple progeny tests and 

possibly identify the most appropriate areas in 

the United States for future seed purchases. Nuts 

were collected from plus-trees growing at the 

following five locations: the Pleasant Valley seed 

orchard (PVSO) located in Alexander County, IL 

and maintained by the Shawnee National Forest; 

the Tree Improvement Center arboretum (TICA) 

located in Jackson County, IL and maintained 

by Southern Illinois University; a grafted orchard 

(HGO) maintained by Leander Hay and located in 

Saline County, MO; Tuttle Creek Experimental Area 

provenance test (KPT) located in Riley County, KS 

and maintained by the Kansas State University; 

and several natural stands located near the Blue 

Bird Nursery (BBN) in Colfax County, NE. When 

possible, the source of the maternal germplasm 

was determined from plantation records or cultivar 

histories to identify county and state of origin, plant 

hardiness zone (Cathey 1990), and normal annual 

total precipitation and mean length of frost-free 

period (Hicks 1998). Plant hardiness zones are 4a 

for BBN, 5b for KPT and HGO, 6a for TICA, and 6b 

for PVSO.

Nuts from 37 individual trees (half-sib families) 

were cleaned and floated at the University of 

Nebraska. Sound seed was flown to Beijing, 

stratified for 3 months, pre-germinated, and planted 

into containers in a greenhouse. In-leaf seedlings of 

The introduction and evaluation of black walnut 

(Juglans nigra L.) into the People’s Republic of 

China is a relatively recent development (Xi and 

others 1999). Small isolated black walnut plantings 

established by missionaries from the United States 

can be found throughout much of China; however, 

few written records document the growth of the 

surviving black walnut trees. In the early 1980s, 

the Chinese Academy of Forestry in Beijing and 

the University of Nebraska initiated a cooperative 

venture with the Friendship Nursery in Henan 

Province to exchange walnut and pecan germplasm. 

Wang (1996) estimates this venture resulted in 

more than 40 acres of black walnut plantings as 

nine demonstration plantings, nut cultivar trials, or 

scionbank orchards. 

In March 1995, the Forestry Research Institute of 

China, the University of Nebraska, the Northern 

Nut Growers Association, and the Forestry 

Bureau of Luoning County called together nursery 

specialists and research horticulturists from China 

and the United States to evaluate the suitability of 

introducing black walnut and its economic potential 

for plantation culture in the semi-arid regions of 

China. Evaluation of existing plantings indicated 

black walnut is well adapted to the lower and 

middle regions of the Yellow River watershed. 

Wang (1996) report expected annual growth of 4 

to 6 feet for height and 0.6 to 1.0 inches in trunk 

diameter. He also reported walnut to be tolerant 

of droughts and deeply rooted with taproot length 

exceeding height. Because of the rapid growth 

and high value for the wood, the economic value 

of black walnut was estimated at 10-fold that 

for Chinese poplar (Populus spp.) and paulownia 

(Paulownia tomentosa (Thunb.) Sieb. & Zucc.). 

One conference recommendation suggested that 

black walnut should gradually replace both 

poplar and paulownia when reforesting farming 

areas in the Yellow River watershed. Under the 

Natural Forest Conservation Program initiated in 

1

Research Plant Physiologist (JWVS), North Central Research Station, USDA Forest Service, Columbia, MO 65211-7260; Horticulture 

Specialist (S-KX), State Administration for Foreign Experts, Beijing, People’s Republic of China; Horticulturist (WAG), University of 

Nebraska, Lincoln, NE 68583; and Tree Improvement Specialist (MVC), Center for Agroforestry, University of Missouri, 10 Research 

Drive, New Franklin, MO 65274. JWVS is corresponding author: to contact, call 573-875-5341 or e-mail at jvansambeek@fs.fed.us.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
178 Short Note

Proceedings of the 6th Walnut Council Research Symposium

most families were transported and planted at sites 

in plant hardiness zones 6 (Beijing provenance), 

7 (Shanxi provenance), and 8 (Henan provenance) 

(Widrlechner 1997). 

The Beijing progeny test (40.5 N, 116.5 E) is located 

on a first terrace site near the Badaling station of 

the Great Wall (Fig. 1). This planting site has loam 

soils and subject to periodic flooding. The Shanxi 

progeny test is part of the XiaoYi Nursery in the 

TaiYuan Basin (Fig. 1). This site has deep, slightly 

alkaline, dark brown soils and a mean annual 

precipitation of 18 to 20 inches with a 160- to 

180-day growing season. The Henan progeny test 

(34.8 N, 113.5 E) is part of the Friendship Nursery 

near Zhengzhou (Fig. 1). This planting site has a 

deep sandy loam with a 60-foot deep water table, 

mean annual precipitation of 23 to 24 inches falling 

primarily in the fall, and a 220-day growing season.

In-leaf seedlings of 30 or more families were 

planted in June 1997 in each progeny test using 

a randomized block design with row plots within 

each of four blocks. Planting design used four-tree 

plots at Henan and Shanxi tests and five-tree plots 

at Beijing test. Tree spacing is nominally 10 feet 

apart within row and 12 to 15 feet between rows. 

Within row vegetation was controlled for 4 years 

by hand cultivation. Survival, stem height (H) and 

trunk diameter (D) have been determined annually. 

Stem volume was estimated as D2H. For each 

planting site, the performance index of each family 

was calculated as the average of the individual 

family percentile rank for stem height, diameter, 

and volume. Percentile rank within plantings was 

calculated as ((family mean—mean of poorest 

family)/(mean of best family—mean of poorest 

family) for each variable.

After four growing seasons, survival of the walnut 

seedlings has exceeded 95% at both the Henan 

and Shanxi plantings. Survival among families at 

the Beijing progeny test range from 20% to 95%. 

Mortality is the result of a summer flood in 1997 

when 26% of the seedlings died and then failure to 

overwinter when an additional 17% of the seedlings 

died. In the spring of 1999, the ambrosia beetle 

(Xylosandrus germanus) attacked many of the 

surviving saplings causing extensive stem dieback 

and resprouting but little additional mortality. 

The Henan progeny test has had the most rapid 

tree growth with average tree heights ranging from 

16 to 23 feet among families after four growing 

seasons (Table 1). Because of the rapid height 

growth, the trees are exceedingly thin with average 

trunk diameters of only 1.1 to 1.5 inches. In the 

fall 2000, many of the trees showed premature 

defoliation in response to walnut anthracnose 

(Gnomonia leptostyla). Although anthracnose has 

relatively little impact on growth of walnut in the 

Central Hardwood region (Van Sambeek 2002), the 

longer growing season in China will likely result in 

a higher incidence of premature defoliation and a 

greater impact on tree growth. In addition, future 

growth in the Henan progeny test is likely to be 

slowed following the removal of all lateral branches 

during August 2000 and used as scionwood in the 

nursery.

The Beijing progeny test had the most variable 

height growth ranged from 14 to 24 feet among 

families (Table 1). Part of the variation is in 

response to the ambrosia beetle that caused 

extensive stem dieback and subsequent basal 

sprouting in the spring 1999. Weber and 

McPherson (1984) indicate this insect was 

introduced from eastern Asia into the United 

States. One growing season after being attacked, 

most trees had produced basal sprouts that 

were nearly as tall as the pre-attack tree heights. 

Weber (1981) reported similar basal sprouting and 

height growth on walnut saplings attacked by the 

ambrosia beetle in the Central Hardwood region. 

The walnut saplings in the Beijing progeny test are 

also exceedingly thin with average trunk diameters 

of only 0.7 to 1.3 inches. 

Figure 1.—Black walnut progeny tests were established 

within the Yellow River watershed near Beijing, 

in Shanxi provenance near Taiyuan, and in Henan 

Provenance near Zhengzhou. For comparison, maps 

of China and North America are aligned along 18 

and 54 degrees north latitude.
Short Note 179

Black Walnut in a New Century

Table 1.—Black walnut progeny collection number and name; site of nut collection; origin of maternal germplasm 

by state, plant hardiness zone, annual rainfall, and frost-free days; performance index (mean of the percentile 

rankings1

); and mean height of 4-year-old saplings at three locations in Yellow River watershed of central China.

 Maternal Germplasm Variables

Progeny Harvest

Site Origin

P.H.

Zone

RainFallFrostFreePerf.

Index

Walnut Age 4 Height

No. Name2 Henan Shanxi Beijing

-in- -days- -%- -ft- -ft- -ft16 NC-5869 AA-34 TICA IL 6a 41 200 0.90 22.2 14.4 -----

27 NC-6134 IB-07 TICA TN 7a 52 180 0.80 20.4 16.6 -----

20 NC-6616 B#1 PVSO IL 6b 45 210 0.73 23.2 14.4 -----

23 Native Hay HGO MO 5b 40 180 0.72 20.9 13.5 23.0

4 NC-6600 B#4 PVSO TN 7a 50 200 0.71 21.7 ----- 23.3

29 NC-6603 B#3 PVSO TN 7a 50 210 0.68 20.4 13.3 -----

 8 NC-6138 PB-09 TICA IA 5a 32 170 0.67 22.1 16.3 17.4

13 Carb #1 BBN NE 4b 30 160 0.65 19.2 16.2 21.0

15 Cheeks #2 B#3 HGO unk unk unk unk 0.61 19.5 13.3 24.3

21 NC-5872 CC-20 TICA MO 6a 45 190 0.59 21.8 13.3 -----

24 Big Tree Clark BBN NE 4b 30 160 0.56 21.9 12.9 17.7

9 Kwik Krop HGO KS 6a 40 200 0.54 19.9 13.0 18.4

33 NC-6609 B#1 PVSO KY 6b 48 200 0.54 ----- 14.4 18.4

22 NC-6622 B#2 PVSO TN 6b 50 190 0.52 19.4 14.0 -----

3 NC-5869 AA-30 TICA IL 6a 41 200 0.52 ----- 12.3 21.3

6 KS-2707 #1 KPT MO 6a 44 200 0.51 21.2 13.4 20.3

12 NC-5875 EE-20 TICA MI 5b 34 150 0.49 20.7 14.0 19.7

28 Walter Jackson HGO KS 5b 37 190 0.47 17.7 15.2 -----

1 KS-2707 #3 KPT MO 6a 44 200 0.46 17.4 14.2 20.3

36 Clarkson #3-2 BBN NE 4b 30 160 0.45 22.7 ----- 19.0

11 Mixed seedlot HGO MO 5b 40 180 0.36 19.3 14.0 18.0

7 NC-6160 KK-13 TICA IN 5a 38 170 0.36 20.5 13.3 14.4

18 NC-6164 FA-07 TICA IN 5a 38 170 0.34 16.9 14.3 -----

2 NC-6176 MB-09 TICA VA 7a 40 180 0.33 20.6 11.2 17.2

10 NC-6163 OA-09 TICA IN 5a 38 170 0.31 16.0 13.1 20.0

26 NC-6134 IB-05 TICA TN 7a 52 180 0.25 17.7 14.0 -----

25 Carb #2 BBN NE 4b 30 160 0.24 ----- 12.7 15.4

17 NC-5875 EE-20 TICA MI 5b 34 150 0.24 18.0 12.9 -----

37 Clarkson #2 BBN NE 4b 30 160 0.23 17.6 12.5 16.4 

5 Clarkson #3-1 BBN NE 4b 30 160 0.11 16.3 11.7 17.0
180 Short Note

Proceedings of the 6th Walnut Council Research Symposium

Figure 2.—Performance index for open-pollinated 

families in Yellow River watershed and plant 

hardiness zone origin of maternal genes.

Figure 3.—Performance index for open-pollinated 

families in Yellow River watershed and mean annual 

rainfall for region from which maternal genes 

originated.

The Shanxi progeny test had the slowest height 

growth and tree height among families averaged 

between 11 and 17 feet (Table 1). As found in the 

other two progeny test, the trees in the Shanxi 

progeny test were also exceeding thin for their height 

with average trunk diameters of 0.7 to 1.5 inches. 

No families were consistently the best or poorest 

performers across all three plantings (Table 1); 

however, computing a performance index from 

percentile ranks did allow us to rank the families 

across the three plantings. The performance index 

of 0.90 for progeny #16 indicates it was not always 

the top performer for height, diameter, and volume; 

however, it does suggest it was one of the best 

performers in the progeny tests that it was included 

in. Likewise, the performance index of 0.11 for 

progeny #5 indicates it performed poorly but was 

not consistently the poorest performer.

All five locations (PVSO, TICA, HGO, KPT, and 

BBN) from which nuts were collected produced 

families with performance indices above and below 

the mean. All locations, except for the BBN, were 

established with seed or scionwood from the walnut 

range, thus, the source of the paternal genes 

(pollen parent) is largely unknown. We determined 

the original source of the maternal genes for most 

of the open-pollinated families. Presumably by 

analyzing climatic variables from which maternal 

genes originated and the family performance index, 

we could identify the most appropriate provenances 

within the Central Hardwood region for future seed 

collections. 

Maternal genes originated from 10 states and four 

plant hardiness zones (Table 1). Families originating 

from Tennessee (plant hardiness zones 6b and 

7a) frequently have higher performance indices 

than families from Nebraska (plant hardiness 

zone 4b). Statistically, there is a significant linear 

relationship between the plant hardiness zone 

from which the maternal genes originated and the 

mean performance index of the half-sib progeny (R2

= 0.161, p = 0.031, 27 df). This relationship may 

be curvilinear because the performance index for 

families originating from plant hardiness zones 6a 

and 6b have the greatest percentage of families with 

above average performance (Fig. 2). 

There is a highly significant linear relationship 

between mean annual rainfall from the region 

where the maternal genes originated and the mean 

performance index of the half-sib progeny (R2 = 

0.3011, p = 0.002, 27 df). Seven of the eight families 

originating from areas in the United States with 

greater than 40 inches of annual rainfall had above 

average performance indices (Fig. 3). This was 

unexpected considering precipitation averages less 

than 25 inches annually at all three sites in China.

Mean performance Index Mean performance Index

Plant hardiness zone (north to south)

Mean annual rainfall (inches)
Short Note 181

Black Walnut in a New Century

Figure 4.—Performance index of open-pollinated families 

in Yellow River watershed and length of growing 

season where maternal genes originated.

There is also a highly significant linear relationship 

between mean number of frost-free days in region 

from which maternal genes originated and the mean 

performance index of the half-sib progeny (R2 = 

0.2868, p = 0.003, 27 df). Nearly all families where 

maternal genes originated from regions with 190 or 

more frost-free days during the growing season had 

above average performance indices (Fig. 4).

In conclusion, we found families throughout the 

Central Hardwood region that performed well 

in the Yellow River watershed of China. We also 

found that seed collections from certain geographic 

regions were more likely to yield highly successful 

families. Preliminary guidelines suggest seed should 

originate from areas within plant hardiness zones 

6a and 6b that have a mean annual rainfall in 

excess of 40 inches with a growing season of 190 or 

more frost-free days.

ACKNOWLEDGMENTS

The authors express their appreciation to Cyril 

Bish, Leander Hays, Wayne Geyer, Todd Morrisey, 

and Harlan Hamernik for assistance in seed 

collections and Lindsey Van Sambeek for preparing 

illustrations.

LITERATURE CITED

Cathey, H.A. 1990. USDA plant hardiness zone 

map. Misc. Publ. 1475. Washington, DC: 

U.S. Department of Agriculture, Agriculture 

Research Service.

Hicks, R.R., Jr. 1998. The Central Hardwood 

Region. In: Ecology and management of Central 

Hardwood Forests. New York, NY: John Wiley & 

Sons, Inc.: 1-73.

Van Sambeek, J.W. 2002. Legume ground covers 

alter defoliation response of black walnut 

saplings to drought and anthracnose. In: 

Van Sambeek, J.W.; Dawson, J.O.; Ponder, 

F., Jr.; Loewenstein, E.F.; Fralish, J.S., eds. 

Proceedings, 13th Central Hardwood Forest 

conference. Gen. Tech. Rep. NC-234. St. Paul, 

MN: U.S. Department of Agriculture, Forest 

Service, North Central Research Station: 556-

564.

Xi, S.K. 1999. Introduction and cultivation of the 

American eastern black walnut. Beijing, China: 

Chinese Academy of Forestry, Institute of 

Forestry. 100 p.

Xi, S.K.; Wang, Z.L.; Dong, P.; Dong, F.-X. 1999. 

American black walnut importation and 

extension. Unnumbered report. Beijing, China: 

Black Walnut Project Group, Chinese Academy 

of Forestry. 18 p.

Wang, Z.L. 1996. The introduction and extension of 

American black walnut. 11 minute videotape. 

Forestry Bureau of Luoning County, Henan 

Province, People’s Republic of China.

Weber, B.C. 1981. Ambrosia beetles in your black 

walnut plantation – how serious are they? 

Annual Report of the Northern Nut Growers 

Association. 72: 68-74.

Weber, B.C.; McPherson, J.E. 1984. Attack on 

black walnut trees by the ambrosia beetle 

Xylosandrus germanus (Coleoptera: Scolytidae). 

Forest Science. 30: 864-870. 

Widrlechner, M.P. 1997. Hardiness zones in China. 

Unnumbered map. Ames, IA: U.S. Department 

of Agriculture, North Central Region Plant 

Introduction Center, Iowa State University. 1 p.

Zhang, P.C.; Shao, G.-F.; Zha, G.; Le Master, D.C.; 

Parker, G.R.; Dunning, J.B., Jr.; Li, Q.-L. 

2000. China’s forest policy for the 21st Century. 

BioScience. 288: 2135-2136.

Mean performance Index

Mean frost free days
182 Short Note

Proceedings of the 6th Walnut Council Research Symposium
Poster Session 183

Black Walnut in a New Century

FINDING A WALNUT IN A HAYSTACK: HOW MOLECULAR BIOLOGY CAN

HELP REDISCOVER LOST “ELITE” BLACK WALNUTS

Rodney L. Robichaud, James McKenna, and Keith Woeste1

The tree improvement program at Purdue University established seed orchards from grafted “elite” source 

trees (ortets) found in the wild. Several hundred candidate elite trees were sampled from many locations 

over the past 40 years. Several dozen researchers were involved in the process. It is sometimes desirable 

or necessary to resample the ortets, but due to either vague or missing records, the exact location of these 

trees may not be known. We have employed microsatellite DNA markers to unambiguously identify five 

ortets being tested in the black walnut improvement program. Identification of ortets was accomplished by 

fingerprinting candidate trees using six highly polymorphic microsatellites and comparing their genotypes to 

the ramets growing in a black walnut seed orchard. 

1

PhD candidate (RLR), Department of Forestry and Natural Resources, Purdue University, 1159 Forestry Building, West Lafayette, IN 

47907-1159; Operational Tree Breeder (JM), USDA Forest Service, Hardwood Tree Improvement and Regeneration Center, Purdue 

University, 1159 Forestry Building, West Lafayette, IN 47907-1159; and Research Molecular Geneticist (KW), USDA Forest Service, 

Hardwood Tree Improvement and Regeneration Center, Purdue University, 1159 Forestry Building, West Lafayette, IN 47907-1159. RLR 

is corresponding author: to contact, call (765) 496-7685 or e-mail at rrobichaud@fnr.purdue.edu

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
184 Poster Session

Proceedings of the 6th Walnut Council Research Symposium

RELATIONSHIPS BETWEEN TREE SIZE AND NUT PRODUCTION FOR

EASTERN BLACK WALNUTS

David Brauer, Adrian Ares, and Andrew Thomas1

ABSTRACT—Eastern black walnut (Juglans nigra L.) is an important tree species for agroforestry and 

forestry practices. However, the relationships between tree size and nut production are poorly understood. 

We are reporting results from two studies. In thefirst study, we examined data reported by Zarger (1946) 

on average trunk diameter (DBH) and annual nut production from 1941 to 1946 for over 150 trees. The Rsquare values for equations relating nut yield to tree DBH ranged from a low of 0.04 to a high of 0.18 when 

data from individual years were analyzed. Averaging nut yields over 2 consecutive years did not improve Rsquare values. However, the R-square value increased to 0.33 when nut yield data from 4 or 6 consecutive 

years were averaged and then regressed against average DBH. 

In the second set of studies, nut yields per tree in 2002 and 2003 were determined for 14 plots with 10 to 

50 trees per plot. R-square values of 0.25 and 0.34 were obtained for the regression equations between nut 

yield and tree DBH for data collected in 2002 and 2003, respectively. The R-square value increased to 0.45 

when the 2-year averages for DBH and nut yield were regressed. The R-square value was greater for the 

regression equation generated from native tree stands then for improved cultivars (0.57 versus 0.27). These 

results indicate that data from at least 2 consecutive years are needed to assess a tree’s nut yield potential 

and that the relationship between DBH and nut yield may differ between native and improved cultivars.

LITERATURE CITED

Zarger, T.C. 1946. Yield and nut quality of the common black walnut in the Tennessee Valley. Annual 

Report of the Northern Nut Growers Association. 37: 118-124.

1

Research Agronomist (DB) and Research Forester (AA), Dale Bumpers Small Farms Research Center, Agriculture Research Service, 

U.S. Department of Agriculture, Booneville, AR 72927-9214; and Research Associate (AT), Southwest Research Center, University of 

Missouri-Columbia, Mt. Vernon, MO. DB is corresponding author: to contact, call (479) 675-3834 or e-mail at dbrauer@spa.ars.usda.

gov.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 
Poster Session 185

Black Walnut in a New Century

SUGAR-BUSHING BLACK WALNUT

G.G. Naughton, W.A. Geyer, and D.L. Chambers1

Native Americans enriched their diet of wild game, nuts, and berries with the sugary products from the 

native maples (Acer spp.) in eastern United States. As Europeans settled the land, they also tapped the 

maple trees eventually improving on the methods of collection. Today, the syrup enjoyed topping our 

flapjacks is made mostly from the sap of sugar maple (Acer saccharum Marsh.); however, most other 

maples can also be used. Various reports have suggested that sap from other hardwoods could also be 

used including black walnut (Juglans nigra L.). The objective of our study was to determine if sap could be 

collected from pole-sized black walnut trees and boiled down to make syrup.

Twenty black walnut trees were tapped beginning in middle February and the sap was collected 5 weeks. 

Trees were approximately 23 years old and 25 to 30 cm in DBH. Trees were tapped about 1 m above the 

ground on the east side. A carpenter’s brace with a No. 10 bit was used to drill the holes. Holes were 

drilled deep enough to fully penetrate the sapwood. We used polyvinyl double-male hose splicing inserts 

for spiles. Plastic 3.8 liter milk jugs were used as the primary collectors. A small hole was cut with a knife 

at the top of the handle and the spile was inserted into the hole. The caps were left on the jugs to minimize 

contamination. Sap was collected daily when it flowed freely. Later we coordinated the collection with cold 

nights and warm days. A standard brewer’s hydrometer (Balling scale) was used to estimate the sugar 

content in percentage by weight. Then the sap was mixed together for refrigerated storage until processing. 

When a sufficient quantity was collected, it was boiled down to 25% of the original volume. Later, a small 

electric hotplate and a 2 liter cast-iron saucepan were used to concentrate the sap. 

The sap quantity varied by individual trees and days. All trees produced sap, but with great variation. 

Although most trees produced fewer than 2.0 L, the highest producer had 13.21 L and 0.43 L for the 

lowest tree. The average sugar content was 2.08% with a 1:60 sugar:sap ratio. Sap production was highly 

correlated with the thickness of the sapwood. We thought that the finished walnut syrup was dark coffeebrown in color, was sweet to the taste, and had no hint of any typical ‘walnut flavor’.

Subsequently, we submitted the syrup to the Sensory Center at Kansas State University for evaluation 

where over 100 consumers taste-tested our product on pancakes. Comparisons were made to Log Cabin 

syrup, pure maple syrup, and our black walnut product. The Log Cabin product was preferred over the 

walnut syrup because it was sweeter and had a greater aftertaste. The walnut syrup was diluted to 85% and 

50% with little change in appearance. Additional tests with five highly trained tasters indicated that the 50% 

dilution was more desirable having lower color intensity and no woody, nutty or musty/earthy flavor. 

1

Retired Professor (GGN), Professor (WAG), Forestry Department and Professor (DLG), Human Nutrition, Kansas State University, 

Manhattan, KS 66506. WAG is corresponding author: to contact call (785) 532-1409 or e-mail at wgeyer@oznet.ksu.edu.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
186 Poster Session

Proceedings of the 6th Walnut Council Research Symposium

ARTHROPOD COMMUNITIES IN MONOCROPPED AND

BLACK WALNUT-INTERCROPPED FORAGES

W. Terrell Stamps, Terry L. Woods, Robert L. McGraw, and Marc J. Linit1

ABSTRACT—Alley cropping holds promise for increasing insect diversity and reducing pest problems by 

improving natural enemy complexes and adding competition to pest species. In a series of experiments, 

we have examined the effects of forages on growth, nut production, and arthropod communities of alley 

cropped eastern black walnut (Juglans nigra L.). An initial experiment compared alley cropped and 

monocropped smooth bromegrass (Bromus inermis Leyss.), alfalfa (Medicago sativa L.), and vegetation-free 

alleys. There were no differences in tree growth among alleyway treatments. The first season’s nut yield 

was greater from trees with vegetation-free alleyways; otherwise nut production did not differ among the 

treatments. Arthropods were more numerous and diverse in alley cropped alfalfa than in alley cropped 

bromegrass or in the vegetation-free controls. Arthropod diversity in the tree canopies did not differ among 

treatments. Alley cropped forages supported a more diverse and even arthropod fauna than did adjacent 

monocropped forages. 

A second, larger, experiment comparing experimental plots of traditionally-grown alfalfa and plots of alfalfa 

intercropped between rows of black walnut trees has yielded similar results. Herbivore numbers were 

significantly lower and beneficial arthropod numbers were significantly higher in the more structurally 

complex alley cropped alfalfa plots than in the monoculture alfalfa plots. On the other hand, alfalfa yields 

were very poor for alley cropped alfalfa, although fiber values for alley cropped alfalfa indicated higher 

quality forage. We are addressing the yield problem in a new study incorporating wider alley ways in the 

hopes of improving alfalfa yields while retaining the insect community benefits of an agroforestry practice.

1

Postdoctoral fellow (WTS), Research Specialist (TLW), and Professor of Entomology (MJL), Department of Entomology, 1-87 

Agriculture Bldg.; and Associate Professor of Agronomy (RLM), Department of Agronomy, 210 Waters Hall, University of Missouri, 

Columbia, MO 65211. WTS is corresponding author: to contact, call (573) 882-0193 or e-mail at stampst@missouri.edu.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
Poster Session 187

Black Walnut in a New Century

CLIMATIC FACTORS AND BLACK WALNUT NUT PRODUCTION

Neal Sullivan, Felix Ponder, Jr., and Viniece Jennings1

Black walnut (Juglans nigra L.) nut yield varies both temporally and spatially. That is, production varies 

among years and for any given year, and production varies across the range for the species. Unpredictable 

annual yields restrict both market expansion and management of black walnut for nut production. New 

knowledge on the bearing habits of wild and plantation grown trees is needed. We have begun to examine 

the relationship between climatic variables and walnut production over a 10-year period for trees grown 

in the wild. Hammons Products Company, located in Stockton, Missouri, provided walnut production 

data (metric tons per station per year) from their network of buying stations for the years between 

1986 and 1996. This network ranges across 18 states from Kansas east to New York and Tennessee to 

southern Wisconsin. Because buying stations may have only been open for part of the study period, the 

data was summarized by dividing the study area into regions defined by latitude and longitude (1o

 by 1o) 

and summing walnut production for the region. This data was converted to an index of production for 

each region-year combination where the index represents the annual production as a proportion of the 

mean production for that region. In this way, differences among regions in walnut collection intensity are 

standardized and differences among years are emphasized. Climate data summaries for the 94 regions in 

which buying stations occurred were extracted from National Climate Data Center records. The climate 

variables of interest were: early frost for the year preceding nut production; freezing temperatures during 

the expected pollination period (EPP) for the region; duration of cool temperatures during EPP; relative 

humidity during the EPP; and cumulative growing season rainfall. Regression analysis is being used to 

determine if these climatic variables can be used to predict walnut production and the reliability of these 

predictions. 

1

Forest Ecologist (NS), North Central Research Station, 202 ABNR Bldg. University of MO, Columbia, MO 65211, Research Soil 

Scientist (FPJr), North Central Research Station, 208 Foster Hall, Lincoln University, Jefferson City, MO 65102, and 1890 Scholar (VJ), 

Delaware State University, Dept. of Agriculture & Natural Resources, 1200 North Dupont Hwy., Dover, DE 19901. NS is corresponding 

author: to contact, call (573) 875-5341, Ext. 235 or e-mail at nsullivan@fs.fed.us.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
188 Poster Session

Proceedings of the 6th Walnut Council Research Symposium

HOW MUCH REGENERATION IS NEEDED TO MINIMIZE

GENETIC EROSION VIA TIMBER HARVEST?

Jeffrey C. Glaubitz, Rodney Robichaud, Keith Woeste, and Olin E. Rhodes, Jr.1

There is an urgent need for a coordinated and systematic approach to the in situ conservation of the 

genetic resources of commercially important forest tree species in the Central Hardwoods. Effective in situ

management of genetic resources would benefit from clear guidelines for how many adult trees can be 

harvested with minimal impact on allelic diversity, given the amount of regeneration that is expected to 

survive to reproductive maturity. We are constructing a computer simulation model for this purpose, and 

present preliminary results based upon replicate harvests of a virtual forest stand consisting of 200 adult 

trees. Our model explores how much regeneration is needed so that there is no more than a 10% risk of 

retaining less than 90% of the original allelic diversity. In the absence of regeneration, up to 55% of the 

adult trees can be harvested without exceeding the 10% risk level. At higher harvest intensities, locallyderived regeneration is needed to replace the alleles removed from the adult population. When all 200 adult 

trees are harvested, the 10% risk level is not exceeded if there are at least 116 regenerants, provided that 

these are derived from pre-harvest random mating among the adults. In the presence of substantial pollen 

flow from a genetically differentiated outside pollen source (e.g., 10-20% pollen flow), the minimum amount 

of regeneration needed is reduced. This indicates that outside pollen can be more efficient, relative to pollen 

from within the stand, at replacing alleles lost from the adult population. This higher efficiency of pollen 

from the outside pollen source likely stems from a subset of alleles that are rare in the pre-harvest adult 

population—and thus prone to loss—yet present at higher frequencies in the outside pollen source. We hope 

that ongoing refinements of our current model will make it a useful tool to aid the effective management of 

the genetic resources of valuable Central Hardwood tree species such as black walnut (Juglans nigra L.).

1

Geneticist (JG), Hardwood Tree Improvement & Regeneration Center, Dept. of Forestry & Natural Resources, Purdue University, West 

Lafayette, IN 47907; Molecular Geneticist (RR), Department of Forestry and Natural Resources, Purdue University, West Lafayette, 

IN 47907; Molecular Geneticist (KW), USDA Forest Service, Hardwood Tree Improvement and Regeneration Center, Department of 

Forestry and Natural Resources, Purdue University, West Lafayette, IN 47907; Wildlife Geneticist (OR), Dept. of Forestry & Natural 

Resources, Purdue University, West Lafayette, IN 47907.

Citation for proceedings: Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., 

eds. 2004. Black walnut in a new century, proceedings of the 6th Walnut Council research symposium; 2004 July 25-28; Lafayette, IN. Gen. 

Tech. Rep. NC-243. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 188 p.
Michler, C.H.; Pijut, P.M.; Van Sambeek, J.W.; Coggeshall, M.V.; 

Seifert, J.; Woeste, K.; Overton, R.; Ponder, F., Jr., eds. 

2004. Black walnut in a new century, proceedings of the 

6th Walnut Council research symposium; 2004 July 25-28; 

Lafayette, IN. Gen. Tech. Rep. NC-243. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, North Central 

Research Station. 188 p. 

 Presents papers and abstracts relating to genetic 

improvement, nursery production, plantation establishment, 

natural stand management, pest management, agroforestry 

and economics of black walnut and related Juglans species.

 KEY WORDS: Juglans, genetic improvement, plantation 

culture, agroforestry.

Send requests for publications to:

USDA Forest Service

North Central Research Station

Distribution Center

One Gifford Pinchot Drive

Madison, WI 53705

OR

Walnut Council

Wright Forestry Center

1011 N 725 W

West Lafayette, IN 47906-9431

Tel: [765] 583-3501
Mission Statement

We believe the good life has its roots in clean air, sparkling water, rich soil, 

healthy economies and a diverse living landscape. Maintaining the good life 

for generations to come begins with everyday choices about natural resources. 

The North Central Research Station provides the knowledge and the tools to 

help people make informed choices. That’s how the science we do enhances the 

quality of people’s lives.

For further information contact:

North Central 

Research Station

USDA Forest Service

 1992 Folwell Avenue 

 St. Paul, MN 55108

Or visit our web site:

www.ncrs.fs.fed.us
Black Walnut in a New Century GTR-NC-243