91
6.3.3 Root Cuttings 110

6.3.4 Layering 111

6.3.4.1 Tip layering 111

6.3.4.2 Mound layering 112

6.3.4.3 Air layering 112

6.3.5 Division 113

6.3.6 Grafting 114

6.3.7 Micropropagation 115

6.3.7.1 Organogenesis (microcuttings) 115

6.3.7.2 Somatic embryogenesis (SE) 117

6.3.8 Summary 119

6.3.9 Literature Cited 120

Contents

6.3.1 Introduction 93

6.3.1 .1 Objectives and resources affect propagation 

method 93

6.3.1.2 Advantages and disadvantages of vegetative 

propagation 95

6.3.1.3 Basic concepts and terminology 95

Juvenility 96

Adventitious rooting 97

6.3.1.4 Vegetative propagation systems 98

6.3.2 Stem Cuttings 99

6.3.2.1 Importance of source 99

6.3.2.2 Types of stem cuttings 100

Hardwood cuttings 100

Semi-hardwood cuttings 102

Softwood cuttings 102

6.3.2.3 Collecting and processing stem cuttings 102

Condition of donor or stock plants 102

Collection equipment and supplies 102

Size and orientation of cuttings 103

Sanitation 103

Rooting hormones 104

Cultural treatments 105

6.3.2.4 Planting stem cuttings 106

Direct sticking 106

Prerooting 106

Propagation environments 106

92
6.3.1 Introduction

Vegetative propagation is defined as the production of new plants 

that contain the exact genetic characteristics of the parent plant. 

This is possible because the nucleus of each living cell contains all 

the genetic information that is necessary to reproduce another 

identical plant, a concept known as "totipotency." Because only one 

parent is required and no genetic recombination is involved, 

vegetative propagation is also known as asexual propagation. By 

avoiding the genetic recombination inherent in sexual reproduction 

and seed development, nursery managers can produce multiple 

"carbon copies" of the parent plant.

6.3.1.1 Objectives and resources affect propagation

method

In nature, some plants spread naturally by vegetative propagation, 

but most plants propagate by seed. This is also the case in forest 

and conservation nurseries, where well over 95% of the species are 

produced from seed. In ornamental nurseries, where producing

many identical plants from specific "cultivars" (cultivated varieties) is 

the objective (figure 6.3.1 A), vegetative propagation is the most 

common method. One technique in particular, rooted cuttings, 

constitutes from 70 to 90% of the total production (Davies 1994). 

Some ornamental plant cultivars have been specifically selected for 

their ability to root easily.

As stressed in chapter 1, the choice of propagation method 

depends on the management objectives and the characteristics of 

the plant species. If the objective is to generate a large number of 

plants that have been geneti cally selected for a particular 

characteristic (such as fast growth for use in plantation forestry, for 

example), then vegetative propagation is a logical choice (figure 

6.3.1 B). On the other hand, if the objective is to produce plants that 

maintain the broad range of genetic diversity that is found in nature, 

then seed propagation makes more sense.

Special circumstances, however, may make vegetative propagation 

the best or only option (table 6.3.1). Some plants just do not 

produce seed in sufficient quantities or frequently enough, and so 

they must be propagated vegetatively. In other situations, pests 

have so damaged the flowers or seeds that propagation by seed is 

not an option. For example, when a spruce budworm epidemic 

destroyed the seed crop of black spruce in Nova Scotia for over 10 

years, nurseries there had to propagate this species vegetatively 

from cuttings (Levy 1983). Many

seeds of forest and conservation species have complex dormancy 

requirements that make vegetative propagation more practical, 

especially when time is critical. This often is the situation for 

restoration projects at high elevations. Rooting cuttings collected 

during the summer were found to have many advantages over seed 

propagation (Scianna and others 1998). Plants that have unique 

genetic properties, such as resistance to an insect or disease, can 

be propagated vegetatively so that the desired genes are not lost in 

sexual recombination (figure 6.3.2A). Vegetative propagation is also 

used to maintain or even increase populations of rare and 

endangered species (figure 6.3.213). One exciting recent 

development is the successful production of American chestnut 

plantlets through micropropagation. This tree was once

93
the most important broadleaved forest tree in the eastern United 

States but was essentially eliminated earlier this century by a fungal 

blight. Scientists believe that they can produce disease-resistant 

chestnuts through genetic engineering and micropropagation 

(Carraway and Merkle 1997). Lastly, tree improvement programs 

can benefit greatly from vegetative propagation techniques. For 

example, it takes about 21 years for a seedling seed orchard to 

become fully productive whereas one established from cuttings can 

produce improved seed in as little as 9 years (Greenwood and 

others 1991).

94

Micropropagation, the newest type of vegetative propagation, is 

often attractive to prospective nursery developers or new customers 

because it is so modern and "high-tech." The idea of being able to 

produce thousands of new plants from a small amount of plant 

tissue is certainly attractive. However, micropropagation has many 

of the same limitations as the other vegetative propagation 

techniques and is by far the most expensive. So again, the choice 

of propagation method should be determined by the objectives and 

resources of the nursery.
6.3.1.2 Advantages and disadvantages of vegetative

propagation

The interest and activity in vegetative propagation has

increased dramatically in recent years because of its

many advantages:

• Rapid multiplication of selected plant material in a

short time

• High degree of crop quality and uniformity (Jones

and others 1996)

• No complex seed dormancy problems

• Ability to overcome low seed availability and

seasonality

• Earlier and more frequent flowering than plants

grown from seed (Jones and others 1996)

On the other hand, vegetative propagation has certain

limitations:

• Cost-vegetatively produced plant material can be

several times as expensive as seedlings (table 6.3.2)

• Propagation environments-sophisticated structures and 

equipment are required, particularly for micropropagation

• Labor intensive-all vegetative propagation methods require 

more hand labor, which often exceeds 80% of total costs 

(Davies 1994)

• Reduced vigor-some vegetatively propagated plants

are less vigorous than those raised from seed

• Loss of genetic diversity -vegetative propagation usu

ally means less natural variation

• Possibility of plagiotropism-vegetatively propagated

plants may lose apical dominance

6.3.1.3 Basic concepts and terminology

A clone is defined as a group of genetically uniform individuals that were originally derived from a single parent

by asexual propagation. Clonal forestry is a term used to

95
describe the silvicultural system based on regeneration with 

'vegetatively propagated plants, which are usually outplanted in 

intensively managed plantations. Horticulturists refer to the parent

as the "mother" plant and the progeny as "daughter" plants 

whereas forest geneticists refer to the original plant as the "ortet" 

and each its progeny as a "ramet" (Hartmann and others 1997). 

Stock plant is another term for the mother plant and will be the

preferred term in this volume. For collections in the wild, cuttings 

are collected from a donor plant.

Juvenility. The concept of juvenility, which is critical to successful 

vegetative propagation of woody plants, is difficult for many people 

to understand because it differs between plants and animals 

(Geneve 1995). Each organism goes through a normal 

developmental and aging process that begins with an embryo, 

continues through juvenility, and then develops into a mature stage 

in which it is capable of sexual reproduction. The difference 

between animals and plants is that chronological age does not 

equal biological age in plants like it does for animals, because 

different parts of a plant can be at different stages of maturity 

at the same time. The biologically youngest (most juvenile) but 

chronologically oldest part of a tree is located at the junction 

between the root and shoot (figure 6.3.3 A-E). The juvenile phase is 

characterized by the inability to produce flowers under conducive 

environmental conditions and often can be identified by specific 

morphological and physiological traits including leaf shape, 

thorniness, vigor, and disease resistance. Of interest to 

propagators, however, is the fact that cuttings taken from juvenile 

plant tissue regenerate roots easily. For example, black spruce 

cuttings taken from the bottom third of the crown of 9-year-old 

saplings rooted almost twice as easily as those collected from the 

top third (Tousignant and others 1995). 

96

On the other hand, cuttings or buds collected from the upper parts 

of the crown are sexually mature, which is advantageous when the 

object of propagation is to collect mature grafting material that has 

the ability to flower and produce seeds. These "scions" are then 

grafted on rootstocks (figure 6.3.4). and outplanted in clonal seed 

orchards.

Growers can manipulate plants, either culturally or 

chemically, to retain juvenility for a longer period.

• By repeatedly cutting a plant back to the base, it is possible to 

produce stump sprouts (figure 6.3.3E), which are then 

maintained in the nursery to provide a steady supply of cuttings. 

Pruning the stock plant all the way back to the root collar often 

is most effective. For example, the rootability of cuttings from 

American elm stump sprouts increased from 38% near the top 

to 83% at the base (Schriebner and Kawase 1975).
97

Adventitious rooting. The definition of an adventitious plant 

structure is one that does not develop from a normal meristem or 

bud (Hartmann and others 1997). Adventi tious shoots can develop 

in burl-like structures called sphaeroblasts on the stem (figure 

6.3.3D), or adventitious roots can be induced in juvenile plant material. Because rooted cuttings are the most common vegetative 

propagation technique, the ability to culturally induce adventitious 

roots is of major interest in horticulture. Adventitious rooting is 

genetically controlled and the responsible genes are active during 

juvenility but, as the plant tissue ages, they gradually are switched 

off.

• Hedging consists of regularly cutting stock plants back to a 

predetermined height and is particularly useful for species 

that do not produce stump sprouts. Hedging is an efficient 

means of increasing the yields of cuttings from a limited 

number of stock plants.

• The maturity process can be chemically retarded through 

the use of hormones. juvenile plant material typically 

produces roots naturally, but more mature cuttings often 

can be stimulated to produce roots with hormone 

treatments. Cuttings that fail to root, even when treated with 

hormones, are termed recalcitrant (Geneve 1995).
New biotechnology techniques may soon be able to manipulate the 

juvenility genes to increase their sensitivity to hormones or even 

switch them back on and reverse the maturity process (Davies 

1994). At the present time, however, growers still use cultural 

techniques to induce adventitious rooting.

There are two types of adventitious roots: preformed roots, and

wound roots. The former develop naturally at the base of the stem 

and the latter form only after the cutting is made. Some woody 

plants, such as poplars, willows, and gooseberries, have latent root 

bud primordia that are preformed in their inner bark. These buds lie 

dormant as long as the stems remain attached to the stock plants

but develop into roots after cuttings are taken. Wound roots form 

out of the mass of callus cells that develops as part of the 

wounding response (figure 6.3.5). Although it varies with the type of 

plant, adventi tious roots generally develop around the central core 

of

vascular tissue. There is a general belief that callus tissue is a 

precursor to the development of adventitious roots, but some 

recalcitrant species form callus without subsequent root formation 

(Hartmann and others 1997).

6.3.1.4 Vegetative propagation systems

Once the decision has been made to propagate a plant 

vegetatively, the best method to use depends on several factors, 

including: species characteristics, the type of propagation 

environment, and the skill of the propagator. Several different 

vegetative propagation methods are being used in forest and 

conservation nurseries (table 6.3.3). The following sections discuss 

the basics of each technique. For a more detailed discussion on 

basic vegetative propagation concepts and procedures, the reader 

is referred to horticultural texts including Dirr and Heuser (1987), 

Hartmann and others (1997), and Macdonald (1986).

98
99

6.3.2 Stem Cuttings

Rooted stem cuttings—steckling—are the most widely used 

vegetative propagation technique for forest and conservation 

species. Some of these species can also be produced from seed, 

but growers turn to vegetative propagation for certain specific 

reasons: • Ease of propagation

--Poplars and willows have traditionally 

been produced from hardwood stem cuttings because their very 

small seeds are hard to handle, do not store well, and are 

covered with fine hairs that resist water imbibition. Most poplars 

and willows root well without hormones, although some 

recalcitrant species and clones require them. For example, 

hardwood cuttings of Scouler willow rooted and survived 

significantly better when treated with 0.3% IBA (indole-3-butyric 

acid) (Edson and others 1995).

• Lack of seed--Rooted cuttings have been the principal method 

of reforesting Alaska-cedar in British Columbia for the past 15 

years, and currently over a half-million stecklings are produced 

annually. The original purpose of this program was to 

supplement seed that was in short supply, but it has recently 

been expanded to include genetic improvement, with over 1,000 

clones being evaluated in a cutting orchard (Russell 1993).

• Genetic improvement--Rooted cuttings are becoming 

increasingly popular in plantation forestry for the bulking or

cloning of genetically improved plant material (Ritchie 1996). 

Bulking is done at the family level and involves making a 

relatively few copies of a large number of genotypes, the 

offspring (progeny) of which have been tested to verify the 

genetic gain. Bulking is most common with conifers and is 

particularly popular for expanding the number of propagules 

from control-pollinated seed orchards (figure 6.3.6). Cloning is 

primarily done with broadleafed trees and shrubs and consists 

of making a relatively large number of copies of a few elite 

genotypes. Although clones have been phenotypically selected, 

sometimes over several generations, their offspring have not 

been progeny-tested.

6.3.2.1 Importance of source

For forest and conservation purposes, knowledge of the origin of 

cuttings is just as important as with seeds to insure that the nursery 

stock is well-adapted to its outplanting environment. For restoration 

projects, cuttings should always be collected from donor plants on 

or near the outplanting site. One Intermountain nursery collects 

from at least 50 different plants from a specific project
100

outplantings in the Great Plains and they are maintained as 

stooling beds in regional nurseries (USDA SCS 1991). Most 

government nurseries have established stooling beds or stock 

plants of the species and ecotypes that are adapted to their local 

area and thus can be a potential source of cutting material for 

private growers.

6.3.2.2 Types of stem cuttings

In horticulture, cuttings are divided into categories depending 

on the type of and maturity of tissue (Hartmann and others 

1997; Dirr and Heuser 1987).

Hardwood cuttings. Hardwood cuttings come from either 

broadleaf or conifer species, consist of mature woody tissue of 

last season's growth, and typically are collected during the 

dormant period (figure 6.3.8A).

area to ensure adequate genetic diversity (Atthowe 1993). A 

cutting carries the exact genetic information as the donor 

plant, so the best recommendation for maintaining maximum 

biodiversity is to collect cuttings from as many donor plants as 

is practical and economical.

If there will be sufficient long-term demand for a particular ecotype, 

then an easily accessible supply of cuttings can be obtained from 

outdoor stooling beds (figure 6.3.7A1, hedging orchards (figure 

6.3.713), or container stock plants (figure 6.3.7C). All of these 

options reduce collection costs and maintain control over the source 

and quality of cuttings. The USDA Natural Resources Conservation 

Service has established stool beds of selected cultivars of willows 

and other riparian species at their plant materials centers (Carlson 

1992). Several hybrid poplar clones have proven successful for 

conservation
101
Condition of donor or stock plants. Because the physiological 

condition of donor or stock plants affects rooting success, cuttings 

typically are collected from healthy donor plants that are growing in 

the full sun or from well-fertilized stock plants in the nursery. It is a 

good idea to visit collection sites a year in advance if possible to 

locate and label suitable donor plants (Scianna and others 1998). 

To minimize moisture stress, wild collections should always be 

made as early in the day as possible and on days with little or no 

wind. Cloudy or foggy days would be ideal. Container stock plants 

should be irrigated before cuttings are taken and moved to a shady 

area for several days prior to collection if possible. Moe and 

Andersen (1988) present a comprehensive discussion of the 

maintenance of stock plants and cultural treatments that increase 

rootability of cuttings.

Collection equipment and supplies. Proper equipment and 

supplies greatly increase ease of collection and rooting success. 

High-quality pruning shears and a sharp knife are essential. Two 

types of plastic bags will keep cuttings together and prevent

desiccation: smaller plastic zipper-lock bags for cuttings from 

individual plants and larger white trash bags for bulking collections. 

For wild collections, a list of supplies should include spray bottles 

with extra water, permanent labels and marking pens, work gloves, 

and white portable coolers with blue ice to keep the cuttings cool 

and protected (Scianna and others 1998).

with the season of the year, and the best season for collection also 

varies by species and even genotype. With red ironbark 

eucalyptus, the rootability of two genotypes varied during the year, 

whereas another was completely recalcitrant to rooting (table 

6.3.4).

Hardwood cuttings are easy to prepare, store, ship and handle, 

and sometimes can be rooted under normal irrigation without 

relatively few special cultural treatments.

Semi-hardwood cuttings. Semi-hardwood cuttings are collected 

from the semi-lignified tissue of actively growing plants (figure 

6.3.813). Because they consist of partially mature tissue, 

semihardwood cuttings will bend but not break when flexed. They 

cannot be stored for more than a few days and require specialized 

propagation environments, such as a greenhouse with mist or a 

rooting chamber.

Softwood cuttings. Softwood cuttings are collected from the soft 

succulent new shoots during the growing season, and suitable 

cuttings will snap or break cleanly when flexed. They cannot be 

stored and must be carefully handled. Because softwood cuttings 

are very sensitive to desiccation, a mist bed or rooting chamber and

treatment with hormones (figure 6.3.8C-E) usually are necessary.

This traditional nomenclature works well when collecting from stock 

plants or stooling beds in the nursery. Propagating plants for many 

conservation and restoration projects, however, requires collecting 

cutting material on remote project sites where access to donor 

plants is a severe limitation. For example, hardwood cuttings may 

need to be collected from remote, high-elevation donor plants during 

the summer or fall-most subalpine and alpine plants are low in 

stature and buried by snow during the winter and early spring. 

Repeated browsing often eliminates the type of plant tissue that can 

be collected at field locations, so it may be useful to distinguish 

between first-year tissue and second-year hardwood tissue. For 

example, cuttings from first-year hardwood of gooseberries showed 

61 % rooting compared with 21 from second-year wood. Therefore, 

a more practical terminology for cuttings from forest and 

conservation plants would include the season of collection 

along with the type and dormancy status of the tissue (for 

example, late-fall dormant hardwood, summer softwood) 

(Scianna and others 1998).

6.3.2.3 Collecting and processing stem cuttings

The best type of cutting will depend on the species, genotype, and 

season of collection. For example, hardwood cuttings of willows are 

generally best, but there are some exceptions. Scouler willow does 

not root easily, so softwood cuttings were more successful (Edson 

and others 1994). Rootability of stem cuttings also varies greatly

102
Sanitation. The wounding that is inherent in collecting cuttings 

leaves them vulnerable to decay, making sanita tion critical. The 

epidermis or bark of a plant functions like human skin in preventing 

infection by fungi or bacteria, but this protection is lost when 

cuttings are made. For example, cottonwoods and willows are very 

susceptible to a canker fungus (Cytospora spp.) that can normally 

be found on the outside of branches. Because of the frequent 

wounding, this fungus builds up in stooling beds to the extent that 

the beds must be periodically ripped out and replaced with new 

healthy plant material.

To prevent the spread of decay when collecting cuttings, pruning 

shears and other tools should be kept clean and

103

Size and orientation of cuttings. The best size of cutting to collect 

depends on species, type of plant tissue available, and size of the 

growth container. In nursery stooling beds, willow and poplar are 

collected as long whips that are then cut into the proper length. If 

collected by hand, the basal cut is typically made just below a node 

where roots form more readily. When large numbers of cuttings 

have to be made, then bundles of whips are cut with a band saw. 

Hardwood poplar cuttings are collected relatively large, ranging from 

10 to 25 cm (4 to 10 in.) in length and 0.5 to 2 cm (0.2 to 0.8 in.) in 

diameter. The bundles of cuttings are then secured with a rubber 

band and stored under refrigeration at 0 to 4.5 °C (32 to 40 °F) to 

keep them dormant until they are needed.

Semi-hardwood and softwood cuttings are generally smaller than 

hardwood cuttings. Semi-hardwood and softwood cuttings from a 

variety of native donor plants should be from 7.6 to 15.2 cm in 

length (3 to 6 in.) and around 6.4 mm (0.25 in.) in diameter 

(Scianna and oth ers 1998). Softwood cuttings of Alaska- cedar 

range from 3 to 8 crn (1 .1 to 3.2 in.) in length (Russell and 

Ferguson 1990). Pruning shears, especially the anvil type, crush 

the softer tissues of semi-hardwood or softwood cuttings so each 

cutting should be recut at an angle with a sharp knife when back at 

the nursery. This removes any damaged tissue and improves 

uptake of water and rooting hormones (Scianna and others 1998).

Stem cuttings have an inherent polarity and will always produce 

shoots at the distal end (nearest the bud) and roots at the proximal 

end (nearest the main stem or root system). 'To distinguish between 

the top and bottom of hardwood cuttings, the bottoms are cut at an 

angle, which not only ensures that the cuttings are planted right 

side up but makes them easier to stick (Hartmann and others 

1997). Cuttings from lateral branches of conifers may exhibit 

plagiotropism, which is the term for a horizontal growth habit that 

some lateral cuttings maintain after they are rooted (figure 6.3.9). 

Plagiotropism is much more troublesome in some species than in 

others. About one-third of western larch stecklings grown in greenhouses exhibited plagiotropism during the first growing season. This 

and other growth abnormalities makes the greenhouse production 

of larch stecklings unpractical for the present time (Edson and 

others 1996). Propagators have developed innovative cultural 

procedures to overcome plagiotropism. For example, some 

nurseries have found that transplanting their Douglas-fir rooted 

cuttings in bareroot beds as plug+one transplants helps eliminate 

plagiotropism (Ritchie 1996).
104

ingredients found in commercial rooting compounds at rates 

ranging from 1,000 to 20,000 parts per million (ppm) (Van Dellen 

1998). Rooting compounds contain varying concentrations of IBA 

and NAA as well as other supplemental materials such as 

fungicides and vitamins (table 6.3.6). This can be confusing to the 

novice because there is no standard nomenclature for the contents 

or their concentration. For example, the numbering of the Hormex® 

line of products indicates the concentration whereas that of 

Hormodin® compounds does not. Van Dellen (1998) provides an 

excellent discussion of the commercial rooting compounds that are 

currently on the market.

Powder formulations are easier to use because the hormone 

concentration is preset and the powder sticks to the cut surface. 

Liquid products are typically formulated with ethyl or isopropyl 

alcohols, and some must be diluted to the proper strength (Dirr and 

Heuser 1987). Some growers prefer liquids because they penetrate 

the plant tissues better than powders (Van Dellen 1998). Care 

should be taken during dilution, as this process provides opportunity 

for error. For nurseries without easy access to laboratory chemicals, 

radiator antifreeze (~ 95% propylene glycol) and windshield washer 

fluid (~50% methanol) proved to be safe and effective dilutants 

(Chong and Hamersma 1995). As with any new practice, it would 

be wise to test these products before using them on an operational 

scale.

disinfected regularly. Several different disinfectants have been used 

in cutting preparation (table 6.3.5). Household bleach (sodium 

hypochlorite) is the most common disinfectant because it is cheap 

and readily available but may pose environmental risks. Bleach 

contains hypochlorite ions that react to form very stable 

organochlorine compounds that accumulate in animal tissue and 

may cause health problems. Although they are more expensive, 

benzalkonium chlorides and hydrogen peroxide are just as effective 

and have no potentially damaging breakdown products (McClelland 

and Smith 1994).

Cuttings can also be treated at the nursery. Some nurseries soak 

their willow and cottonwood cuttings in a surface sterilant or 

fungicide to retard storage molds and decay of the cut surfaces. 

Cuttings from donor plants in the wild can be soaked in a 

broad-spectrum fungicide such as thiram and the rooting medium 

drenched with fungicide (Scianna and others 1998).

Rooting hormones. Treating stem cuttings with hormones greatly 

increases the speed and uniformity of root development for many 

species, especially those that are hard to root. Many different 

rooting hormones are commercially available (figure 6.3.8C). IAA 

(indole-3-acetic acid) was the first natural auxin discovered but is 

not used commercially nowadays because it is relatively unstable. 

Two synthetic auxins-IBA (indole-3-butryic acid) and NAA 

(naphthaleneacetic acid)-are the active
Rooting hormones are applied to cuttings in three ways (Blazich 

1988):

• Dipping in hormone powder--This method is quick and easy 

for single cuttings but it is difficult to control the application rate 

because the amount that adheres to the cutting depends on 

size, surface texture, and moisture content (figure 6.3.8D).

• Dipping in liquid formulations for a few seconds ("quick 

dip")--This technique is simple and fast and provides uniform 

results when cuttings are bundled during treatment. It is 

recommended for more concentrated alcohol-based products. 

The duration of the dip should be kept constant to ensure a 

uniform application rate and minimize the possibility of 

phytotoxicity.

• Soaking in liquid formulations for 2 to 24 hours--This 

method is slower but provides a more uniform application rate 

per cutting. Best for water-based formulations of lower hormone 

concentration.

Cultural treatments. Growers use several other cultural 

techniques at the nursery to improve rooting.

Warm-temperature callusing is achieved by treating hardwood 

cuttings with rooting hormones and storing them under relatively 

warm, moist conditions (18 to 21 °C (65 to 70 °F) for 3 to 5 weeks. 

Bottom heat is becoming a standard practice but is particularly 

valuable for

difficult-to-root species. Nurseries that process many cuttings have 

special rooting benches with bottom heat, but less expensive 

heating mats are commercially available. The temperature 

difference between the roots and shoots needs only to be a few 

degrees, as excessive temperatures or callusing time can weaken 

stored photosynthate reserves and lower survival after planting 

(Hartmann and others 1997). Bundles of willow or cottonwood 

cutting can be stored upright in moist sawdust or wood shavings 

while in the cooler, which keeps the bottoms warmer than the tops 

thus promoting callusing and speeding-up root initiation.

Basal wounding involves removing the outer bark or making 

incisions at the bottom of the cutting (Hartmann and others 1997). 

This not only exposes the cambial tissue to the rooting hormone but 

encourages the formation of callus tissue that is often the precursor 

of root initials. Removing approximately 2 to 5 cm (1 to 2 in.) of bark 

tissue was found to be beneficial for encouraging root development 

of native plants (Scianna and others 1998).

Clipping, which includes removal of shoot tips or basal leaves, has 

been shown to improve rooting success with semi-hardwood or 

softwood cuttings (Hartmann and oth ers 1997). To minimize 

transpiration, plants with large leaves should be pruned to remove 

25 to 50% of the surface area of each leaf (Scianna and others 

1998).

105
prerooted cuttings must be carefully planted to avoid root injury or 

the formation of a "J-root." Making a hole in the medium in the 

growth container with a pointed dibble protects the tender roots and 

the medium can be pressed around the stem. Some growers use a 

forked tool to position the roots. Transplanted cuttings should be 

irrigated frequently in the weeks following transplanting, and 

fertilization can begin as soon as the plants have rooted in the 

growth container. Established cuttings are treated like normal 

seedlings and hardened-off before outplanting.

The height of the rooting trays and the moisture-holding and 

aeration characteristics of the medium are critical to rooting 

success. Just as with growth containers, taller trays will provide 

better drainage than shallower ones. The rooting medium should 

provide four functions: prevent disease, hold the cutting in place, 

supply water as fast as it is lost through transpiration, and permit 

easy penetration of air to the bottom of the cuttings. Cuttings are 

particularly susceptible to rot fungi and therefore the growing 

medium should be sterilized. Rooting trays are prepared with a 

sterile but well-aerated medium that will hold the cuttings upright 

yet drain easily. Fine sand or mixtures of perlite, vermiculite, or coir 

have been used (Hartmann and others 1997). The choice of 

medium affects both the speed of rooting and the type of roots 

produced. Some species produce very fine, fragile roots in a 

well-drained medium, such as sand, and these roots often break-off 

during transplanting.

Propagation environments. Cuttings must be carefully cultured in 

special propagation environments until they can form new roots. 

Some growers construct relatively simple rooting beds (figure 6.3.11 

A) or chambers (figure 6.3.11 B) that feature bottom heat and 

humidity controls. Nurseries that produce a large number of cuttings 

use sophisticated propagation facilities that have completely 

controlled environments including sophisticated fogging systems. 

The Quebec Ministry of Natural Resources has developed an 

innovative rack propagation system that consists of enclosed 

shelves that hold 4 "mini-greenhouses" equipped with fluorescent 

lights and boom irrigation systems (figure 6.3.11C/D). The 24 

mini-greenhouses can hold up to 51,000 cuttings each, for a total 

capacity of over 1.1 million cuttings per propagation structure. The 

facilities can be used year-round for up to 5 production cycles. The 

mini-greenhouses must be installed in an air-conditioned room to 

prevent heat build-up. The temperature within the mini-greenhouses 

is maintained at 20 °C (68 °F), the relative humidity at 90 to 95%, 

and the light

6.3.2.4 Planting stem cuttings

Cuttings can be stuck or set into containers in two ways that differ 

not only in technique but also between types of cuttings and the 

necessary propagation facilities.

Direct sticking. This technique involves planting cuttings directly 

into the growth containers and then moving them immediately to the 

propagation area, where they will grow to shippable size (figure 

6.3.8A-E). Direct sticking is commonly used with hardwood cuttings 

of willows and cottonwoods, which root easily, but also can be done 

with semi-hardwood or softwood cuttings of species that require 

treatment with rooting hormones. Hardwood cuttings stick easily into 

the filled containers but less-woody cuttings need to have the 

container prepared with a hole made by a dibble or a template.

Direct-stick cuttings have relatively simple cultural needs, requiring 

only frequent irrigation or misting to minimize transpiration demand. 

The height of the container is very important to good drainage 

because the perched water table that exists at the bottom of all containers will be proportionately less in taller containers (see section 

4.2.3.3 in volume four of this series for more specifics). Growing 

media for direct sticking should be porous enough to prevent 

waterlogging while promoting good aeration around the developing 

roots. This is especially critical under mist irrigation, where growers 

include high concentrations of perlite or pumice into a standard peat 

moss-vermiculite mix. Coir is a relatively new media component that 

is made from coconut husks and coir-based media have been 

shown to greatly improve rooting success with some species.

Prerooting. This is recommended for semi-hardwood and softwood 

cuttings that are more difficult to root, and therefore treatment with 

rooting hormones and a special propagation environment are 

essential (figure 6.3.10 A-D). Prerooting consists of planting treated 

cuttings into a shallow tray or small container until they begin to 

form roots. Depending on the species, rooting can take from several 

weeks to many months. Root development can be determined by 

gently tugging the cutting-any resistance means that roots have 

formed and the cutting can be transplanted. Rooted cuttings are 

transplanted into growth containers where they are grown to 

shippable size. Newly formed roots are very tender and susceptible 

to breakage, and so rooted cuttings must be carefully removed from 

trays and transplanted into their final growth containers. Just as with 

other types of transplants,

106
at 20 μmol/m2/s for a 18- to 20-hour photoperiod 

(Tousignant and others 1996).

Growers should pay special attention to the following potentially 

limiting factors in the propagation environment. The reader is 

referred to Hartmann and others (1997), Macdonald (1986), and 

the other general references for a more detailed discussion.

• Irrigation--Water loss is the single most important limiting 

factor because newly harvested cuttings do not have any roots 

to replenish moisture lost through transpiration. Some 

nurseries have specially constructed enclosures that generate 

a periodic mist or fog that keeps the relative humidity at almost 

100% (figure 6.3.1 OC). Many automated misting systems 

require sophisticated electrical controls, which can

be quite expensive. However, less expensive systems can 

be constructed using time clocks. Some species root more 

easily in subirrigated trays, although success varies greatly 

between species (Regan and Henderson 1996). The 

quality of irrigation water is critically important with mi st 

nozzles or fog systems, as water high in dissolved salts 

(high electrical conductivity) will leave damaging and 

unsightly deposits on the cuttings (figure 6.3.1 OD). "Hard" 

irrigation water is particularly troublesome in this regard 

(Read 1995). In addition, all irrigation sources must be filtered to remove sediments which may plug nozzles. (See 

section 4.2.4 in volume four of this series for more 

information on irrigation water quality.)

• Temperature--Both air and rooting medium temperatures 

are very important to successful rooting. The relationship 

between day and night temperatures is

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109

be beneficial in finishing cuttings after roots have formed. The 

influence of light quality on the rooting of forest and 

conservation crops has not received much study although root 

initiation, like many other growth processes, is probably 

regulated by the phytochrome process. Crop lighting that gives 

more red than far-red light appears to stimulate rooting in many 

greenhouse crops (Moe and Andersen 1988).

With some species, stock plant etiolation (figure 6.3.12) has 

greatly improved rooting success (Maynard and Bassuk 1988). 

The process consists of forcing stock plants to produce new shoot 

growth under heavy shade or complete darkness and then using 

that portion of the stem as the cutting. Banding or wrapping the 

bottom of stems with Velcro® strips to exclude light is a practical 

application of the technique. Etiolation and banding have been 

used successfully with many woody plants, including ma ples, 

oaks, and pines (Maynard and Bassuk 1988).

Fertilization--Most species do not generally require mineral 

nutrients until they are transplanted, and there is no reason to 

fertilize cuttings until they have rooted. Frequent misting, however, 

can leach nutrients directly from leafy cuttings and from the growing 

medium. Some growers incorporate slow-release fertilizer into the 

growing medium, but research has shown that this is quickly 

depleted. A weekly fertigation of 300 to 400 ppm nitrogen may be a 

better option during root formation (Argo and others 1995).

critical. For most species, daytime air temperatures of 18 to 

32 °C (64 to 90 °F) with nighttime temperatures about 5 °C 

(10 °F ) lower are recommended (Preece 1995). Actually, 

the temperature of the rooting medium is even more critical. 

Root initiation and development have different optima, 

however, and so growers should lower root zone 

temperatures after roots emerge. Although it must be 

determined for each species, a rooting medium target of 30 

°C (86 °F) for root initiation and 20 °C (68 °F) for root 

development is a good place to start. Bottom heat is even 

more important under mist irrigation because the frequent 

irrigation keeps the temperature of the growing medum low 

through evaporative cooling (Read 1995).

• Light--Effects of light on the rooting of cuttings is very 

complex, as some species respond to different daylengths 

(photoperiods), intensity of light, and light quality. 

Traditionally, leafy cuttings have been placed under 50% 

shadecloth, as much to minimize transpiration as to improve 

rooting (Maynard 1993). Maximum photosynthesis of black 

spruce cuttings occurred at light intensities of 200 to 300 

Nmol/s/m2 which is only about 10 to 15% of full sunlight. 

These studies also revealed a strong relationship between 

light and air temperature, however, as successful rooting at 

30 °C (86 °F) required three times the amount of light as 

those rooted at 10 °C (50 °F )(Yue and Margolis 1993). The 

effect of long days on the rooting of cuttings is less clear: 

many growers believe that anything that stimulates shoot 

growth decreases root growth. On the other hand, 

photoperiod lights would
6.3.3 Root Cuttings

Although not commonly used in forest and conservation nurseries, 

propagation by root cuttings is an ancient practice that was first 

described by the Greek philosopher Theophrastus (371-287 BC). 

Root cuttings are not common today because of the greater ease 

of rooting stem cuttings, especially with hormones, bottom heat, 

and intermittent mist. Nevertheless, several hard-to-grow native 

plants can be propagated by root cuttings, including sumacs, 

blackberries, and hawthorns. Root cuttings can also be used to 

meet a specific management objective: for example, quaking aspen 

can be grown from seed but root cuttings can be used when seed 

is difficult to obtain or to maintain certain genotypes. A complete list 

of trees and shrubs that can be produced with root cuttings can be 

found in Del Tredici (1996), who also does an excellent job of 

summarizing the literature.

Root cuttings have their own terminology. The proximal end of the 

root is the end nearest the main stem, whereas distal end refers to 

the end farthest from the stem. This distinction is important 

because, when root cuttings form buds, they are typically found at 

the proximal end. There are three basic groups of plants that form 

root suckers (Hudson 1956):

• Natural suckering without division--This group includes 

species that naturally produce root suckers near the parent 

plant, often forming dense thickets (for example, hawthorn).

• Natural suckering without division--These species naturally 

produce suckers at greater distance from the parent plant and 

form large, spreading colonies (for example, quaking aspen).

• Induced suckering--Plants in this category only produce 

suckers after traumatic injury to the root system (for example, 

pawpaw).

Propagation of the first two groups is similar. For example, aspen 

spreads naturally by root suckers (figure 6.3.13A). Late fall or early 

winter is the best time to collect roots, and thick pieces proximal to 

the parent stem produce shoots more readily. The roots should be 

cut into sections, put into trays filled with a moisture-holding growing 

medium, and placed in a growth-promoting environment. After 

several weeks, shoots will begin to form on the root sections (figure 

6.3.1313). These shoots are cut from the root sections, treated with 

rooting hormones, and then stuck into standard growth containers, 

where they will root and can be cultured just like stem cuttings 

(Schier 1978). Just recently, aspen has been propagated by sticking 

root cuttings directly in growth

110

containers. The cuttings were collected from container stock plants, 

treated with captan fungicide and stored in damp peat moss. When 

the cuttings were deeply stuck, with the proper polarity and the top 

of the cutting just below the surface of the growing medium, rooting 

success ranged from 42 to 91 % (Dreesen and Harrington 1997). 

Paulownia is propagated similarly: sections of the primary and 

lateral roots of 1- or 2-year-old seedlings are used as propagules 

and planted directly in the growth container. Because these 

succulent root cuttings are subject to rotting, the sections should be 

air-dried for 10 days and then dipped into a fungicide solution 

before planting (Stringer 1994).

Root cuttings from plants in the third category will not produce 

shoots and thus the plant must be propagated in place. In late fall, 

the roots should be severed in the wild with a spade starting about 

15 to 25 cm (6 to 10 in) from the main stem and moving out in 

concentric circles. Leaving the severed roots in the ground 

stimulates shoots to develop with their own root systems. These 

separate plants can then be harvested the following year. In addition to pawpaw, this in situ technique will work for other shrubs and 

small trees, including sumac and aralia. (Del Tredici 1996).
6.3.4 Layering

This traditional technique was developed in bareroot nurseries but 

can be adapted to container culture for species that cannot be 

propagated otherwise. Layering involves forcing a part of the stem 

to form adventitious roots while still attached to the parent plant: the 

method has several biological advantages. Layering is a low- stress 

propagation method because there is minimal physical trauma and 

the donor plant provides a steady supply of water and nutrients 

while roots are being formed. This is especially valuable when 

propagating threatened and endangered plants because there is 

little risk to the donor (figure 6.3.2B). Layering also produces a 

relatively large plant in a short time. Because it is so labor-intensive 

and the multiplication factor is low, layering is very expensive and 

can only be justified for special propagation projects. However, 

layering can be very productive in nurseries with minimal 

propagation facilities. See Hartmann and others (1997) for more 

details on the many types of layering used in horticulture. In forest 

and conservation nurseries, three types have been used.

6.3.4.1 Tip layering

This traditional technique consists of bending a side shoot or branch 

over until it can be held in place and covered with growing medium 

or mulch (figure 6.3.14A). Rooting of the buried section is naturally 

stimulated by

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produces the large-l9- to 76-liter (5- to 20-gal) container plants 

that are needed for riparian restoration projects in this relatively 

stressful environment (figure 6.3.14 D).

6.3.4.3 Air layering

With this technique, the bark is wounded or completely stripped 

from a section of stem or a lateral branch of the donor plant (figure 

6.3.14E). The best success occurs with stems of the previous 

season that still have mature leaves. The wounded section is 

treated with rooting hormone and immediately covered with damp 

peat moss or other moisture-holding material that is secured with a 

clear plastic wrap. The air layer is then covered with aluminum foil 

or other reflective wrapping to exclude light and moderate the inside 

temperature. The protective ,wrapping can be removed periodically 

to check for root development and to rewet the peat moss. The new 

plant is ready to be cut off the parent and transplanted in about 2 to 

3 months; however, difficult-to-propagate species may require up to 

two seasons. Air layering has been used to propagate many 

tropical and subtropical plants, notably Citrus spp., as well as some 

pines for tree improvement purposes.

the interruption of the normal basipetal translocation of 

photosynthates that accumulate near the bend and by the exclusion 

of light. Cultural procedures that encourage rooting in stem cuttings, 

such as the use of hormones and wounding, also hasten the 

formation of roots in the buried stem section (figure 6.3.14B).

6.3.4.2 Mound layering

Another type of layering involves inducing roots to form on stern or 

root sprouts. Mound or "stool" layering consists of planting a rooted 

cutting and allowing it to become established. Before growth starts 

the following spring, the top of the shoot is severed just above the 

ground line, stimulating new sprouts to form. When the sprouts 

reach 8 to 13 cm (3 to 5 in.), sawdust or soil is mounded over them 

and they are kept moist until the end of the season. By this time, 

roots have formed on the sprouts (figure 6.3.14C), which are then 

cut off as close as possible to the base and used as rooted cuttings 

(Hartmann and others 1997). Mound layering has recently been 

modified for large container production of Arizona sycamore and 

other riparian trees in New Mexico (Dreesen and Harrington 1997). 

Although these species can be grown from seed, mound layering 

quickly
6.3.5 Division

Some plants naturally spread laterally by forming new shoots, 

rhizomes (modified stems), or bulbs, and so they can be easily 

propagated by separation (removing naturally detachable 

structures) or division (cutting the plant into sections). Although 

Hartmann and others (1997) distinguish between these two 

methods, we see little operational difference and so will refer to 

them both as division.

In forest and conservation nurseries, division is used primarily for 

propagating wetland species such as sedges, rushes, and 

bulrushes. that are used for restoration or constructed wetland 

projects. The initial collections are made from native stands but 

then stock plants are started in large containers or trays in the 

nursery. When they

have grown large enough, the plants can be divided into sections 

(figure 6.3.15A) and transplanted to new containers. With some 

species, even the smallest section will root but the size of the 

divisions is critical with some species; for example, divisions of soft 

rush must have at least 5 stems or they are not viable (Street 

1994). Division can be repeated several times during the growing 

season and the new transplants are then given normal culture until 

they are ready for harvest (Beagle and Justin 1993). During 

harvesting, new plantets can be collected and used to start the next 

crop (figure 6.3.1513). Like other vegetative propagation 

techniques, division is relatively labor-intensive but provides a quick 

and sure way to produce wetland species.

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6.3.6 Grafting

Grafting is the art of propagating plants that are difficult or virtually 

impossible to raise by other vegetative techniques. There are 

several types of grafts but all involve physically binding two plant 

parts together so that they will bond and grow into a single 

individual. The scion, the upper portion of the graft that will develop 

into the new shoot system, is bonded to the rootstock, which is an 

established plant of the same or a closely related species (figure 

6.3.16A). Although it is commonly used to propagate ornamental 

and fruit trees, grafting is only used for genetic improvement 

purposes in forest and conservation nurseries. Commercial forest 

tree species require many years to produce seed naturally and so 

tree improvement specialists use grafting to establish clonal seed 

orchards that can become productive in only a few year.

Scions are collected from the tops of specially selected "plus trees" 

in the forest and then grafted onto seedling rootstocks in the 

nursery. Because these scions are vegetatively mature (see section 

6.3.1.3), they will flower and produce seed much sooner than would 

trees in a seed orchard produced from seedlings (figure 6.3.5). 

Grafts are subject to desiccation and physical damage so container 

grafting is often preferred because the plants can be protected and 

moisture loss can be controlled. In some nurseries, grafted 

container plants are being raised in greenhouses or other sheltered 

propagation environments to further accelerate seed production 

(figure 6.3.1613). Grafting is a very precise technique requiring 

considerable training and experience, and so the reader is referred 

to Hartmann and others (1997) and Macdonald (1986) for more 

specific information.
6.3.7 Micropropagation

This application of tissue culture techniques to propagation involves 

very small plant parts grown under aseptic laboratory conditions. 

Although there are several different types of micropropagation, they 

all start with excising a small piece of plant tissue, cleansing it of 

microorganisms, and culturing it in an artificial medium in a test tube 

or small laboratory vessel. The excised plant part that serves as the 

initial propagule is known as the explant. By manipulating 

laboratory environment and supplying specific hormones and 

vitamins at the proper stage of development, these explants multiply 

and develop into numerous miniature plantlets. Obviously, the 

micropropagation process is quite complicated and so the reader is 

referred to Dirr and Heuser (1987), Kyte and Kleyn (1996), and 

Hartmann and others (1997) for a complete and thorough 

discussion.

Micropropagation has many advantages (Suttle 1995):

• Ease of propagation--Poor rooting percentages and graft 

incompatibility are common problems that can be overcome 

with micropropagation; some plants, such as redbuds and 

lilacs, are just very difficult to propagate.

• Juvenility--Micropropagated plants retain their juvenility longer 

and so can be used as stock plants for a source of easy-to-root 

cuttings.

• Clean plant material--Micropropagated plantlets are inherently 

free of pathogens, especially viruses that can easily be spread 

by other vegetative propagation techniques.

• Low space requirement--Large stool beds or blocks of 

container stock plants are expensive to maintain, whereas 

cultures of micropropagated stock plants require minimal 

space.

• Year-round production--Because micropropagation is not 

subject to seasonal cycles, plants can be propagated all year, 

which spreads out the workload.

Operational implementation of micropropagation, however, poses 

several potential economic and technical problems (Zimmerman 

1985):

• Unique cultural requirements--Each new species requires 

empirical testing to adapt existing procedures, and it may be 

impossible to economically propagate some plants.

• High labor costs--Production costs will remain high because 

specialized labor accounts for 60 to 80% of the Iota] costs of 

micropropagation.

• Scaling up--Although the ability to produce tens of thousands 

of plants from a single explant is theoretically correct, the costs 

of the facilities and labor required to expand to an economical 

scale are often prohibitive.

• Culture contamination--The control of microorganisms, 

especially bacteria, and mites is a continual problem and 

requires sterile facilities and rigorous sanitation procedures.

• Field performance and phenotypic stability--With forest 

crops, the long time needed to evaluate the performance of 

micropropagated plants is an expensive but necessary process. 

Field testing of micropropagated hardwood trees has shown 

some undesir able phenotypic variation (Charest 1996).

Micropropagation is commercially viable for many ornamental plants 

because of their intrinsic high value. For example, Colorado blue 

spruce trees with deep blue foliage are in great demand for 

landscape plantings, but the best cultivars do not grow true-to-seed. 

Only a very low percentage of the seedlings grown from the seeds 

of a blue parent retain the desirable foliage color, and so vegetative 

propagation is required. Rooted cuttings are expensive to produce 

and can remain plagiotropic (making them misshapen), whereas 

grafting is effective but expensive. Micropropagation, however, has 

tremendous potential for high-volume production of desirable cultivars of this valuable landscape tree (Cervelli and Webster 1995).

There are several different types of micropropagation, but only two 

are currently being used in forest and conservation nurseries.

6.3.7.1 Organogenesis (microcuttings)

Organogenesis, the most popular micropropagation process, 

can be divided into four sequential cultural operations (Dirr 

and Heuser 1987):

• Establishment of an aseptic culture--This step begins with 

surgically excising the desired plant tissue and establishing and 

maintaining the explant on artificial media under sterile 

laboratory conditions (figure 6.3.17A).

• Multiplication of the explant--The objective of this stage is to 

rapidly increase the number of propagules by chemically 

coaxing the stock cultures to divide and elongate into miniature 

shoots that can be harvested as microcuttings.

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• Rooting of the microcuttings-The microcuttings are collected 

under aseptic conditions and transferred to another vessel with 

a different media containing hormones that induce rooting 

(figure 6.3.1713).

• Transplanting and acclimating the plantlets.-When the 

microcuttings have developed enough roots (figure 6.3.17C), 

they can be transplanted from the culture vessels to typical 

growth containers and moved to the normal propagation area. 

They are gradually acclimated in several stages and then grown 

to shippable size by normal cultural operations (figure 6.3.17D).

The exact propagation protocol of hormone types, concentrations, 

and timing of the various stages varies between species and even 

cultivars. One of the real challenges of this technique has been to 

develop routine procedures that can be used for operational 

production.

High production costs make microcuttings impractical for most 

forest and conservation species at the present time. However, the 

Simpson Timber Company in Korbel, California, has an operational 

micropropagation system that produces 300,000 to 500,000 

redwood (figure 6.3.18A) and 500,000 eucalyptus seedlings per 

year (Lehar 1997). Compared to normal seedlings, the micropropagated redwood seedlings had up to a 122% gain in volume 

(figure 6.3.1813).

Microcuttings also have application for the conservation of 

threatened or endangered species that, for whatever reason, 

cannot be propagated by conventional techniques. Showy 

stickseed is a forb that currently is found in only one population of 

fewer than 500 individuals. Shoot tips of 10 collections were 

micropropagated in tissue culture (figure 6.3.17C/D), and the 

resultant plantlets have been outplanted at four different sites 

(Edson and others 1994).

6.3.7.2 Somatic embryogenesis (SE)

This second type of micropropagation involves chemically inducing 

tissue cultures to form somatic embryos. Two propagation protocols 

have been used: indirect embryogenesis, which requires an 

intermediate step of callus tissue to produce the embryos and direct 

embryogenesis, which does not (Dirr and Heuser 1987). Most SE 

systems for forestry species, including spruces and larches, use 

indirect embryogenesis. In some pines, however, direct 

embryogenesis is used to produce numerous identical plantlets from 

a single immature seed. The process
involves a series of sequential cultural procedures that cause an 

excised immature seed to produce somatic embryos that are then 

propagated to produce many plantlets. With radiata pine, at least 

10,000 embryo initials can be obtained from 1 g of fresh tissue 

(Jones 1990)

One of the most critical steps is selecting the immature parent 

embryo during a relatively short window (as short as 2 weeks) after 

fertilization of the seed. This step is particularly tricky for pines 

because the embryo is not fertilized over a year after pollination. In 

Scots pine, for example, fertilization takes place about 13 months 

after pollination, but the timing still varies with temperature, so that 

degree days are used to schedule the excision of the seed 

(Keinonen-Mettala and others 1996). The SE process has been

used for only a few forest species on an operational scale. Spruces 

are particularly suited for SE because the embryos can be 

harvested from stored seed, compared to the narrow 1- to 2-week 

window for Douglas-fir or pines.

In commercial forestry, the most practical application of SE is to 

bulk-up genetically improved control-pollinated seed. The Canadian 

Forestry Service is using this new biotechnology to accelerate the 

tree-breeding cycle of several species of spruces and larches 

(Charest 1996). One operational SE program for interior spruce has 

been developed in British Columbia by the Forest Biotechnology 

Centre of the British Columbia Research Institute (Grossnickle and 

others 1996). The immature spruce seeds are dissected and the 

zygotic embryo explants are placed on an induction medium that 

contains a mixture of nutrients and hormones. After about 6 weeks, 

the embryonic tissue has proliferated and a small quantity is 

transferred to a maturation medium for anoth er 6 weeks. The 

miniature plants, called emblings, are transferred (figure 6.3.19A) 

and eventually transplanted to standard containers, where they are 

cultured into shippable size plants (figure 6.3.1913). Over the past 5 

years, the survival of somatic interior spruce seedlings has consistently reached 95% after the first year and culling rates have 

decreased to 5 to 8%. Somatic seedlings have also performed 

favorably in a battery of seedling quality tests that were done during 

frozen storage and after outplanting. The 5-year program has 

rapidly expanded from an initial production run of 12,000 somatic 

seedlings to a target of 1,000,000 in 1998.

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ments. Some species are completely recalcitrant and cannot be 

rooted. Others can be propagated from root cuttings, layering, or 

division depending on the characteristics of the plant species. 

Grafting is a specialized technique that is only used to establish in 

tree improvement programs.

Micropropagation, the newest type of vegetative propagation, 

requires specialized equipment and techniques. Although it is 

mainly used for bulking-up improved seed of commercial forest 

tree species, micropropagation can also help preserve valuable 

species or ecotypes. Therefore, at the present time, vegetative 

propagation techniques will remain highly specialized and only 

used for less than 5% of all forest and conservation species.

6.3.8 Summary

Vegetative propagation is defined as the production of new plants 

that contain the exact genetic characteristics of the parent plant. If 

the objective is to generate a large number of plants that have been 

genetically selected for some characteristic like fast growth, then 

vegetative propagation is the preferred method. Several different 

vegetative propagation methods are being used in forest and 

conservation nurseries. The choice will depend on factors such as 

species characteristics, type of propagation environment, and the 

skill of the propagator. Rooted stem cuttings are the most widely 

used technique and are preferred for easy-to-root species like willow 

and cottonwood. Rooting cuttings of other species requires 

experience to determine the best time of collection, treatment with 

rooting hormones, and other cultural require-
6.3.9 Literature Cited

Argo B, Hack K, Biernbaum J, Weesies A, Weesies B. 1995. 

Direct stick mist propagation: part 1 . Greenhouse Grower 

13(8): 40, 42, 44.

Atthowe H. 1993. Propagation of riparian and wetland plants. In: 

Landis TD, tech. coord. Proceedings, Western Forest 

Nursery Association; 1992 Sept. 14--18; Fallen Leaf Lake, 

CA. Gen. Tech. Rep. RM221. Ft. Collins, CO: USDA Forest 

Service, Rocky Mountain Forest and Range Experiment 

Station: 78--81.

Beagle G, Justin J. 1993. Using a constructed wetland to treat 

waste water and propagate wetland species. Tree Planters' 

Notes 44(3): 93-97.

Blazich FA. 1988. Chemicals and formulations used to promote 

adventitious rooting. In: Davis TD, Haissing BE, Sankhla N, eds. 

Adventitious root formation in cuttings. Advances in Plant 

Sciences Series, Volume 2. Portland, OR: Dioscorides Press: 

132-149.

Bonga JM. 1982. Vegetative propagation in relation to juvenility, 

maturity, and rejuvenation. In: Bonga JM, Durzan DJ, eds. 

Tissue culture of forest trees. Amsterdam: Elsevier: 387-412.

Boyle ED, Kuser JE. 1994. Atlantic white-cedar propagation by 

seed and cuttings in New Jersey. Tree Planters' Notes 45(3): 

104-111 .

Burger DW, Lee CI. 1987. Genetic variability in the propagation 

of Eucalyptus sideroxylon by stem cuttings. Journal of 

Environmental Horticulture 5(1): 31--32.

Carlson JR. 1992. Selection, production, and use of riparian plant 

materials for the western United States. In: Landis TD, tech. 

coord. Proceedings, Intermountain Forest Nursery Association; 

1991 August 12-16; Park City, UT. Gen. Tech. Rep. RM-211. Ft. 

Collins, CO: USDA Forest Service, Rocky Mountain Forest and 

Range Experiment Station: 55-67.

Carraway DT, Merkle SA. 1997. Plantlet regeneration from 

somatic embryos of American chestnut. Canadian Journal of 

Forest Research 27: 1805-1812.

Cervelli R, Webster F. 1995. Propagation of ornamental varieties 

of spruce (Picea spp.) through somatic embryogenesis. 

Combined Proceedings of the International Plant Propagators' 

Society (1994) 44: 300-303.

Charest PJ. 1996. Biotechnology in forestry: examples from the 

Canadian Forest Service. Forestry Chronicle 72(1): 37-42.

Chong C, Hamersma B. 1995. Automotive radiator antifreeze and 

windshield washer fluid as IBA carriers for rooting woody 

cuttings. HortScience 30(2): 363-365.

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Davies FT Jr. 1994. What's new in the biology of adventitious root 

formation. Combined Proceedings of the International Plant 

Propagators' Society (1993) 43: 382-384.

Del Tredici P. 1996. Cutting through the confusion. 

American Nurseryman 184(7): 22-24, 26, 28.

Dirr MA, Heuser CW Jr. 1987. The reference manual of woody 

plant propagation: from seed to tissue culture. Athens, GA: 

Varsity Press. 239 p.

Dreesen DR, Harrington JT. 1997. Propagation of native plants for 

restoration projects in the southwestern US -preliminary 

investigations. In: Landis TD, Thompson JR, tech. coords. 

National Proceedings, Forest and Conservation Nursery 

Associations. Gen. Tech. Rep. PNW-GTR-419. Portland, OR: 

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