1
Contents

2.1.1 Introduction 3

2.1 .1 .1 History of container use in forest nurseries 3

2.1.1.2 Terminology 3

2.1.2 Characteristics of Containers for

Forest Nurseries 5

2.1 .2.1 Characteristics that influence seedling growth 5

Container size 5

Container spacing 9

Design features to control root growth 11

Container properties that affect growing 

medium moisture content 14

Container properties that affect growing 

medium temperature 15

2.1 .2.2 Characteristics that affect nursery and 

outplanting operations 16

Need to match containers to both the nursery and 

outplanting system 16

Cost and availability 6

Durability and reusability 17

Ability to monitor growing medium condition

and root growth 17

Ability to interchange and consolidate

containers (unitization) 18

Handling, shipping, and storage 18

2.1.3 Types of Containers 20

2.1 .3.1 Containers planted with the seedling 20

2.1.3.2 Containers removed before outplanting 26

Individual cells in trays 26

Book and sleeve containers 28

Block containers 29

Miniature containers 31

2.1.4 Chemical Root Pruning 32

2.1.5 Conclusions and Recommendations 35

2.1.6 Literature Cited 36

2
2.1.1 Introduction

2.1.1.1 History of container use in forest nurseries

Although ornamental plants have been grown in containers since 

the early days of human civilization (Matkin and others 1957), the 

production of forest tree seedlings in containers is a relatively 

recent innovation. One of the first large-scale uses of container 

tree seedlings in North America was during the Great Plains 

Forestry Project of the 1930's. A tarpaper pot system was 

developed to produce hardy container seedlings for the harsh 

environmental conditions encountered in shelterbelt plantings 

(Strachan 1974).

The first large-scale production of reforestation seedlings in 

modern plastic containers was in Canada: the "Walters Bullet" in 

British Columbia (Walters 1974) and the "Ontario Tube" in Ontario 

(Reese 1974) (fig. 2.1 .1). Based on these early prototypes, other 

containers were developed and tested in Canada and the United 

States during the 1960's and early 1970's, including some that are 

still popular today: Styrofoam® blocks (Sjoberg 1974), 

Spencer-Lemaire (S/L) Rootrainers® (Spencer 1974), and the Ray 

Leach Single Cell® system (Allison 1974). In addition to these 

North American products, the "Japanese paperpot" was adapted in 

Scandinavia (Rasanen 1982) and subsequently imported to the 

United States and Canada.

Many types of containers have been tested in North American 

forest tree nurseries during the past 20 years (fig. 2.1.2), but the 

perfect container has yet to be developed. In reality, no single 

container will ever fill the needs of every nursery manager 

because of differences in nursery cultural practices or 

outplanting site conditions. Which is the best container for a 

given purpose will depend on the specific objectives of the 

nursery and of the reforestation system.

2.1.1.2 Terminology

Certain terms that are used to describe containers in forest tree 

nurseries need to be defined here. In container nurseries that 

produce ornamental tree seedlings, the individual containers are 

relatively large, single containers that are called pots or cans. 

Reforestation stock, by comparison, is grown in relatively 

small-volume containers. The individual containers are often 

referred to as cells or cavities, and are usually produced in 

aggregates called blocks, trays, or racks (fig. 2.1.3). In the 

common nursery vernacular, however, the term container can 

refer to a single cell or the entire block.

Seedlings grown in containers have been called containerized, or 

container-grown, but they will be known as container seedlings in 

this publication. This term is simple and definitive and is consistent 

with the most current terminology in the ornamental nursery trade.

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5

Container size. The "best" container for a particular seedling crop 

will depend on both biological and economical factors. Biological 

considerations include the size of the seed or cutting, the ultimate 

size of the crop plant, and the environmental conditions on the 

Outplanting site. Economically, the initial cost and availability of the 

container and the amount of available growing space are primary 

considerations.

Although in the current forest nursery vernacular container size 

means volume, the concept of size includes all dimensional 

aspects, including volume, height, diameter, and shape. The 

volume of the cavity is one of the most obvious and important 

characteristics of a container because, in general, the larger the 

container, the larger the seedling that can be produced (Kinghorn 

1974). North American container tree nurseries currently use 

containers ranging in volume from a minimum of 40 cm3; (2.5 cubic 

inches) to a maximum of 492 cm3; (30.0 cubic inches) (table 2.1 .l).

When tree seedlings are grown in a series of different container 

types, seedling size generally increases with the rooting volume of 

the container (fig. 2.1.4A) (Alm and others 1982). Container volume 

has a significant effect on the size and growth rate of lodgepole 

pine (fig. 2.1 .4B & C) (Endean and Carlson 1975) and white spruce 

(Carlson and Endean 1976) seedlings when they are grown in a 

variety of different container sizes. Root, shoot, and total dry weight 

as well as shoot length increased significantly with increasing 

container volume, whereas the shoot-root ratio was unaffected 

(table 2.1 .2). Seedling growth comparisons between different 

container types must consider both container volume and growing 

density (the spacing between containers), however, because 

containers with the same volume can have different growing 

densities. This important relationship between container capacity 

and growing density is discussed in the following section.

2.1.2 Characteristics of Containers for Forest

Nurseries

The properties of the ideal container for raising forest tree 

seedlings have been debated for many years. Although 

containers can be compared in several different ways, the most 

appropriate is the functional approach. The primary function of 

any container is to hold a discrete supply of growing medium, 

which in turn supplies the seedling roots with water, air, mineral 

nutrients, and physical support while the seedling is still in the 

nursery.

Forest tree seedling containers must perform certain other 

functions, however, that reflect the special requirements of 

conservation or reforestation plantings. Some of these container 

characteristics shape seedling growth in the nursery, such as 

design features to prohibit root spiraling. Other operational 

characteristics of containers are related to economic and 

management considerations both at the nursery and on the 

outplanting site. These characteristics are listed and discussed in 

detail in the following sections.

2.1.2.1 Characteristics that influence seedling growth

Tree seedlings differ from most ornamental container crops 

because conservation and reforestation seedlings are essentially a 

root crop. Most ornamental container crops are grown for their 

flowers or foliage, but the quality of a tree seedling is determined 

by its outplanting performance-both initial survival and subsequent 

growth. Tree seedling survival and growth are directly related to the 

ability of the root system to promptly regenerate new roots (known 

as root growth potential, or RGP) and grow out into the 

surrounding soil (Ritchie 1984). For this reason, many tree seedling 

container features are designed to encourage the seedling to form 

a good root system in the nursery and to protect these roots until 

the seedling is outplanted. The relative health and vigor of the root 

system is also reflected in the morphology and growth of the 

seedling shoot, and for this reason many of the following container 

characteristics were designed to enhance this root-shoot 

relationship.
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The major constraint on container volume is economical, not 

biological, because (a) larger containers take up more growing 

space, (b) seedlings grown in large containers require longer 

growing periods for the seedling root system to occupy the 

container completely, and (c) large containers are bulkier to handle 

during shipping and outplanting. Nursery managers should select a 

container that will produce an acceptable seedling at the highest 

practical growing density, in the shortest rotation time, and one that 

is suited to the conditions of the outplanting site (the effect of 

container volume on outplanting success is very important and is 

discussed in more detail in section 2.1 .2.2).

Optimum container size varies according to many different factors, 

including growing density, seedling species, size of seedling 

desired, type of growing medium, environmental conditions, and 

length of the growing season. There does appear to be a minimum 

container size for conservation and reforestation stock, however. 

Scarratt (1972) found that the growth of white spruce seedlings 

varied significantly in containers of three different diameters (12, 

19, and 31 mm), but that only seedlings in the larger container (31 

mm) achieved acceptable growth during a normal production 

period. Barnett and Brissette (1986) found that species that are 

intolerant of crowding, such as longleaf pine, grew bigger in 

large-volume containers with a low growing density. Other tolerant 

pine species, such as loblolly

pine, could be produced in small-volume containers with a high 

growing density. Hardwood species generally require larger 

volume containers, with their concomitant lower growing density, 

than do conifer seedlings because the large leaves of hardwood 

species intercept more water and nutrients and generate more 

shade.

Other aspects of container size are also important. One of the 

most biologically and culturally important container dimensions is 

height, because of its effect on the water-holding properties of the 

growing medium (see section on container properties that affect 

growing medium moisture content). Carlson and Endean (1976) 

found that the height-diameter ratio had a significant effect on the 

growth of white spruce seedlings: a container with a 1 :1 

height-diameter ratio produced heavier seedlings than containers 

with 3:1 or 6:1 configurations. This effect was apparently 

species-specific, however, because lodgepole pine did not show 

any growth differences when grown in the same three container 

sizes (Endean and Carlson 1975). Boudoux (1970) studied root 

system growth in relation to container dimensions and concluded 

that, to increase root density, diameter is more important than 

height.

8
Forest tree seedling containers are produced in a variety of shapes: 

round, rectangular, hexagonal, or square in cross- section, and most 

are tapered from top to bottom. Although useful for seedling 

extraction, an extreme taper may be biologically detrimental, 

however, because normally the majority of roots are produced in 

the bottom of a container (Tinus 1974). The actual shape of the root 

plug is probably not operationally significant unless the seedling is 

going to be planted with a planting tool, such as a dibble, that has a 

specific size and shape. Container seedlings that are to be 

transplanted into bareroot nursery beds as plug-plus-one seedlings 

or outplanted in a transplanting machine must have a root plug that 

can be handled efficiently by the planting equipment.

Container spacing. The distance between the individual cells in 

the block generates seedling growing density, one of the most 

important container characteristics affecting seedling growth. The 

spatial arrangement of cells within the block also has economical 

implications, however. Tree seedlings require a certain minimum 

amount of growing space, which varies with species and age. 

Nursery managers, on the other hand, need to produce the 

maximum number of seedlings per unit area of growing space.

In general, container seedling quality increases with a 

corresponding decrease in growing density. Tanaka and Timmis 

(1974) studied the effect of growing density on seedling 

characteristics and concluded that Douglas-fir seedlings produced 

at lower densities had physical and physiological properties that 

lead to improved outplanting performance, including greater dry 

weight and smaller height-diameter and shoot-root ratios.

When comparing tree seedlings produced in different container 

types, seedling growing density should be as important a 

consideration as cell volume. Direct comparisons are often difficult 

to interpret because there is a definite interaction between these 

two factors (Barnett and Brissette 1986). Most published 

comparisons of containers of the same volume do not consider 

the effects of seedling density and therefore their conclusions 

should be interpreted accordingly. Timmis and Tanaka (1976) 

reported the results of one of the few properly designed container 

comparison studies that considers the interaction between cell 

volume and cell density. They grew Douglas-fir seedlings at 

different growing densities in containers with the same volume, 

and found that seedling morphology and weight varied between 

the different seedling spacings (table 2.1.3). Shoot height 

increased with increasing density, probably as a result of greater 

competition for light between the seedlings. Stem diameter, shoot 

weight, and root weight, however, decreased with closer spacing 

(which is also reflected by the larger shoot-root ratio at the greater 

densities).

Seedlings produced at closer spacings grow taller and have 

smaller stem diameters (caliper) and lower biomass (dry weight) 

than those grown further apart. Scarratt (1972) grew white spruce 

seedlings in three different volume containers at three different 

growing densities and found that shoot height, stem caliper, and 

dry weight increased with both container volume or spacing (fig. 

2.1.5). For this species, however, container volume was more 

important than seedling growing density, and the author 

concluded that the use of a larger container was both more 

biologically effective and cost efficient than using smaller 

containers at wider spacings. In trials

9
with extruded peat containers, Hocking and Mitchell (1975) found 

that all growth characteristics of lodgepole pine, white spruce, and 

Douglas-fir seedlings increased when containers were larger or 

the space between cells was wider.

Container spacing also has some other biological and cultural 

implications on seedling growth. Timmis and Tanaka (1976) 

reported that seedlings grown at lower densities received ten times 

more photosynthetically active radiation in their lower crowns and 

had lower water potential than seedlings grown at closer spacings. 

The temperature of the growing medium was also higher in closely 

spaced containers. It is also more difficult for irrigation water and 

liquid fertilizers to penetrate dense patches of seedling foliage. 

Foliar diseases, such as grey mold, are more of a problem in 

densely grown seedlings because the fungus is able to invade the 

weaker, senescent foliage in the lower part of the seedling crown. 

Grey mold is also encouraged by the higher relative humidity and 

lower light in dense groups of seedlings. A Styrofoam block 

modified with vents between the cells reduced grey mold incidence 

on Douglas-fir seedlings due to better air circulation (Peterson and 

Sutherland 1989). Growing density apparently also affects 

seedling hardiness because seedlings grown at higher densities 

suffered more cambial frost damage than those grown at greater 

spacings (Timmis and Tanaka 1976).

10

The effect of seedling growing density is further complicated by the 

length of the growing season. Barnett and Brissette (1986) 

reported that, in southern pines grown for only 10 weeks, the 

effects of seedling density were not critical. However, when the 

growing season was extended to 12 or 14 weeks, seedling dry 

weight decreased with increasing growing densities (fig. 2.1.6A); 

the effects of growing density and age on seedling height were 

less pronounced, however (fig. 2.1 .6B). This size difference also 

had a carry-over effect: seedlings grown al the lower densities had 

higher survival rates and more height growth when measured 2.5 

years after outplanting than did seedlings grown at higher 

densities. Based on these results, the authors recommend that 

southern pines should not be produced in containers with densities 

greater than 1,075 seedlings/m2 (100 per square foot).

Seedling species also respond differently to the effects of 

crowding and, theoretically at least, broadleaved species and 

shade-intolerant conifers should be produced at lower growing 

densities than conifers or more-shade-tolerant species. Although 

many studies that tested the relationship between species and 

container type have been published, few have attempted to 

separate the effects of container volume from growing density 

(see previous section). As an example, Stauder and Lowe (1984) 

reported that container density did not affect the growth or field 

survival of baldcypress seedlings, although the containers used in 

their study were relatively
large Dee-pots®, which produce a very low growing density 

of 215 cells/m= (20 per square foot).

Design features to control root growth. One of the most serious 

problems in container tree seedling culture is the tendency of tree 

seedling roots to spiral around the inside of the container. 

Seedling roots grow geotropically, but if they do not meet any 

physical obstruction, they may tend to grow laterally around the 

side of the container.

Root spiraling will not adversely affect growth while the seedling 

remains in the nursery, but it can seriously reduce seedling quality 

after outplanting. Spiral roots prevent the seedling from becoming 

properly established in the surrounding soil, which can result in 

frost-heaving, toppling, or even strangulation (Burdett 1979).

Although it can occur in almost any container type, root spiraling is 

most serious in round, smooth-walled plastic containers. Girouard 

(1982) grew 4 species of conifer seedlings in 3 different types of 

containers and found that the only one in which root spiraling 

occurred was the round Quebec tube. Paper containers present 

two root control problems: root spiraling in polyethylene

coated paper containers and root growth between cavities in 

untreated paper containers (bong and Burden 1986).

The problem of root spiraling has been at least partially solved by 

designing containers with vertically oriented ridges, ribs, or 

grooves (fig. 2.1.7A) that protrude into the growing medium and 

present an obstacle to spiral root growth; Kinghorn (1974) 

recommended ribs about 2 mm (0.08 inch) high on the inner cavity 

wall. These ribs intercept spiraling roots and force the developing 

roots to grow downward to the drainage hole (fig. 2.1 .7B), where 

they stop growing because of the low humidity and become 

airpruned. Most types of containers used in forest tree nurseries 

have some sort of anti-spiraling rib design, and one container 

manufacturer has even incorporated this feature into its brand 

name, the Spencer-Lemaire Rootrainer® (fig. 2.1 .7A).

Root spiraling occurs in most tree species but has been most 

serious in pines. Girouard (1982) found that all four species of 

conifers grown in Quebec tubes exhibited some degree of root 

spiraling but that it was worse in the pine species (fig. 2.1.8). Even 

with pines, however, there is variation; Barnett and Brissette 

(1986) report that

11
longleaf pine is more prone to root spiraling than are loblolly and 

slash pines. Root spiraling and other types of abnormal root growth 

become more serious the longer the seedlings remain in the 

container (Barnett and Brissette 1986); this tendency was 

particularly significant for jack pine (fig. 2.1.8). (Chemical 

treatments to control root spiraling are discussed in section 2.1 .4.)

When the seedling roots reach the bottom of the container, they 

must be forced to air-prune or they will continue growing downward 

along the support bench (fig. 2.1.9A). This exterior root growth 

makes the seedling difficult to extract at the end of the growing 

season, resulting in damaged root systems. Air-pruning of roots at 

the drainage hole is encouraged by providing a layer of air below 

the container (fig. 2.1 .9B). Armson and Sadreika (1979) reported 

that a 1.25-cm (0.5-inch) air gap beneath the container was most 

effective. Some containers are formed with an external rib to create 

this air space (fig. 2.1.9C). With containers that have no built-in 

support to encourage air-pruning, some growers use benches with 

a mesh top (fig. 2.1.9D) or design their benches to support the 

containers so that an air layer is created. Other growers allow the 

roots to grow out the bottom of the container and then mechanically 

prune the roots before the seedlings are extracted. The biological 

implications of this practice have not been scientifically examined, 

but the authors recommend natural airpruning if at all possible.

12
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Container properties that affect growing medium moisture 

content. Certain features of containers such as container height, 

container wall permeability, and presence of a drainage hole affect 

the moisture relationships of growing media. Bassman and others 

(1989) grew western larch seedlings in 3 different types of 

containers and found a significant interaction between container 

type and water regimes.

The effect of container height is discussed in detail in chapter 2 of 

volume four of this series, but basically the taller the container, the 

greater proportion of well-drained medium that it will contain. All 

containers create a perched water table because they maintain a 

volume of growing medium above a layer of air. Because water 

molecules are attracted to the growing medium in a container, 

water does not freely drain from the medium in the bottom of the 

container, thus creating a perpetually saturated layer of growing 

medium. The depth of this saturated layer is a function of container 

height and the physical properties of the growing medium.

14

One other container characteristic that affects root growth is the 

smoothness of the inside walls of the container cells. Roots of some 

seedlings are very fine and tend to grow into any cracks or seams in 

the walls of the containers. Western red-cedar and Alaska-cedar are 

notable in this respect and, in British Columbia, these species are 

grown in smooth-walled S/L Rootrainers instead of the standard 

Styrofoam blocks, which have rough walls (Matthews 1983). This 

root ingrowth makes these seedlings difficult to remove from the 

container during seedling extraction, and the roots extending from 

the extracted plugs make them difficult to plant (fig. 2.1.10A). Torn 

roots that remain in the containers (fig. 2.1.1 OB) provide an 

excellent substrate for root-rotting fungi and may be a significant 

factor in root disease carryover between successive crops.
Moisture within the growing medium is also affected by the 

properties of the container wall. Containers composed of permeable 

materials such as paper or plastic mesh allow water and dissolved 

salts to move laterally through the container wall and into the 

growing medium in adjacent containers. The moisture relationships 

in a block of adjacent paper containers, such as a tray of paperpots 

(fig. 2.1 .l 1), are similar to a tray of unconfined growing medium, 

because water and dissolved salts can move freely from one 

container to the next. Containers with permeable walls, therefore, 

may require a growing medium with a coarser texture to increase 

porosity and thus prevent waterlogging.

All containers must have one or more drainage holes at the bottom 

to allow excess irrigation water to drain and to encourage excess 

fertilizer salts to leach out (fig. 2.1 .9C). These drainage holes must 

be as large as possible yet should not permit the growing medium 

to fall through during the filling operation. Because a mass of root 

tips eventually develops around drainage holes, they may 

eventually become plugged and cause drainage problems if they 

are too small (fig. 2.1 .9B). The other function of the drainage hole 

is to force the root system to air-prune when the roots reach the 

bottom of the container (see previous section). The benefits of 

these drainage holes are lost, however, if an air layer is not 

provided beneath the container (fig. 2.1.9D).

Container properties that affect growing medium temperature.

The color and insulating properties of container materials affect the 

temperature of the growing medium and, therefore, root growth. 

These properties are important during the growing season because 

root temperature is affected by the sunlight absorption and the 

insulating properties of the container material. Insulation of the root 

system is also important when seedlings are subjected to freezing 

temperatures during hardening or over-winter storage.

The heat absorption and conduction properties of the container 

can be significant in the high-energy environment of a container 

nursery. High root temperatures can inhibit root growth and may 

even result in seedling mortality (Furuta 1978). Whitcomb (1988), 

in his discussion of the effects of high root temperatures in 

container nurseries, emphasizes that considerable variation exists 

in the heat tolerance of different plant species and even varieties of 

the same species.

Heat absorption is a function of container color, with darker colors 

absorbing more solar insulation than lighter ones (fig. 2.1.9D). 

Containers made of a thicker insulating material, such as 

Styrofoam®, will conduct less heat than thinner plastic materials. 

Bassman and others (1989) studied western larch seedling growth 

in 3 different container types, and concluded that medium in 

containers with walls made of thin plastic may heat up more rapidly 

than medium in thicker walled containers. Seedlings growing in the 

warmer medium had more root growth. However, excessively high 

root temperatures injure seedling roots. Whitcomb (1988) reported 

that the temperature of the growing medium just inside the wall of 

large ornamental seedling containers sometimes reached 48 °C 

(120 °F) in direct sunlight; Barney (1947)

15
found that the root systems of several conifer seedlings were 

killed by only a few hours at these temperatures. Brown (1982) 

investigated the effect of container color on 3 ornamental species 

and found that changing the color of the container from black to 

white reduced growing medium temperatures 7 °C (11 °F) and 

produced plants of significantly higher quality. Seedlings on the 

outside of the bench on the sunny side of the growing area are 

most susceptible to root injury from high root temperatures.

Just as important as insulating seedling root systems against high 

temperatures is protecting them against cold injury. Roots are 

much more sensitive to cold injury than shoots, and container 

seedlings stored outside during the winter can suffer severe injury 

if their roots are not protected. Containers constructed of materials 

with a high insulating value, such as Styrofoam, provide more 

protection than thin-walled containers, although Edwards and 

Huber (1982) report cold damage even in Styrofoam blocks (cold 

injury is discussed in more detail in chapter 1 of volume five of this 

series).

2.1.2.2 Characteristics that affect nursery and

outplanting operations

In addition to the characteristics that influence seedling growth, 

there are other container attributes that affect operational aspects 

of the nursery/outplanting process. Some container characteristics, 

such as size, affect operations both at the nursery and on the 

outplanting site. Because no one container is ideal for all purposes, 

a nursery manager should consider all the different characteristics 

and discuss them with customers during the container selection 

process.

Need to match containers to both the nursery and outplanting 

system. The entire nursery and outplanting system should be 

considered during the container selection process. In new 

nurseries, the physical attributes of the container, such as size and 

spacing, will determine bench design and therefore seedling 

production per growing area. Container volume and shape also 

influence the type of growing medium used as well as the type of 

filling and sowing equipment. Because of container effects on the 

moisture content of the growing medium, fertilization and irrigation 

systems should also be considered during the container selection 

process. Large volume containers with their inherent lower 

seedling growing densities will result in faster growth and therefore 

shorter greenhouse rotations, but large

16

containers also take up more growing space. For container 

seedlings that will be transplanted into a bareroot nursery 

seedbed, the type of container affects the type of transplanting 

machine and the ease of handling at the bareroot nursery.

Container attributes also affect other aspects of the reforestation 

process, from seed collection to outplanting. Seed size must be 

considered: species with large seeds, such as oak acorns, will 

require wider containers than small-seeded species. Seedling 

handling, transport, and storage are also affected by the size and 

weight of containers or extracted plug seedlings. Seedlings in 

large-volume containers are inherently more heavy and bulky to 

handle at each stage of the harvesting, storage, and outplanting 

process. Container size and shape may also influence the type of 

planting tool and other logistical operations on the outplanting site 

because fewer large seedlings can be packed per shipping carton 

or carried per planting bag.

Some types of containers were specifically designed to be part of 

a completely automated sowing and outplanting system. The 

paperpot was one of the first containers for which a complete 

filling, sowing, handling, and outplanting system became 

commercially available (Hoedemaker 1974). The Hiko System®, 

which was developed in Sweden, can fill and sow about 250,000 

cavities in one 8-hour shift, and the containers are mechanically 

handled at every step in the nursery. On the outplanting site, the 

containers are carried in specially designed backpacks (Twetman 

1988). The convenience of a coordinated container system must 

be weighed against the cost and the inflexibility of these highly 

automated systems. Once the container is selected and the 

nursery and reforestation systems are designed around it, it 

becomes increasingly difficult to change container types or any 

other part of the equipment or facilities.

Cost and availability. Although the biological aspects of a specific 

container are important, cost and availability are often the 

controlling factors in container selection. Associated expenses, 

such as shipping and storage costs, must be considered in 

addition to purchase price. Many containers are produced at only 

one location and their shipping costs increase as a direct function 

of distance from the manufacturer; others, such as Styrofoam 

blocks, are produced or distributed from various locations around 

the continent and are therefore widely available. Long-term 

availability must also be considered
in the selection process to insure that ample supplies of the 

container can be secured in the foreseeable future.

When conducting an economic analysis of different types of 

containers, the overall cost of seedling production including 

seedling growing density, amount of growing medium required, 

and value of the seedling produced must be considered.

Durability and reusability. Containers must be durable enough 

to maintain structural integrity and contain root growth during the 

nursery period. The intense heat and ultraviolet rays in container 

nurseries can cause some types of plastics to become brittle (fig. 

2.1.12), although many container plastics now contain ultraviolet 

inhibitors. Container durability is especially important when 

considering biodegradable containers because these containers 

must be durable in the moist, humid conditions of a greenhouse, 

yet biodegrade within a reasonable period after outplanting.

Some containers are designed to be used only once, whereas 

others can be reused for 5 or more crop rotations. Reusability 

must be considered in the container cost analysis because the 

cost of reusable containers can be amortized over their life span, 

after adjusting for the cost of handling, cleaning, and sterilizing of 

the containers between crops.

Ability to monitor growing medium condition and root growth.

Although it is easy to monitor the ambient environment and 

observe seedling shoot growth and phenology, it is more difficult to 

monitor the condition of the growing medium and the degree of root 

activity. In a typical container, it is impossible to directly observe 

the moisture content of the growing medium or root growth without 

disturbing the seedling. Late in the growth cycle, however, 

seedlings become large enough to form a firm root plug and can be 

removed from the container and the condition of the plug 

examined. Containers have been developed that can be opened to 

examine the growing medium and root system. Book-type containers (fig. 2.1.7A) are hinged along the bottom of the 

containers so that they can be opened and re-closed whenever 

necessary. Sleeve-type containers have a similar feature consisting 

of two separate but matched sections that can also be opened to 

expose the growing medium and allow examination of the root 

system (fig. 2.1.13A). One operational drawback of these types of 

containers is that they must be assembled (fig. 2.1.13B) and placed 

in holding trays after they are purchased. When different

17
ated storage and subsequent shipping to the planting site. 

Different types of containers require different handling 

equipment, and these factors may have a significant influence on 

the best type of container for a given nursery and associated 

planting system.

container types are being evaluated, this added handling cost must 

be compared to the benefits of being able to monitor the condition 

of the growing medium and roots.

Ability to interchange and consolidate containers (unitization).

One operational feature that has several management implications 

is unitization, in which individual containers fit interchangeably into 

a tray system. A typical example is the Ray Leach Single Cell"' 

container (fig. 2.1.14A-B). One of the significant operational 

benefits of a unitized container system is that unwanted individual 

cells can be removed from the tray and replaced with others. This 

is particularly useful during thinning, when empty cells can be 

replaced with cells containing a germinant, and during rogueing, 

when diseased or otherwise undesirable seedlings can be replaced 

with cells containing healthy seedlings. Such consolidation can 

save a considerable amount of growing space in the greenhouse 

(fig 2.1.14C), and during storage and shipping. Unitized containers 

are also commonly used for tree improvement crops where each 

seedling needs to be individually handled and labeled. One 

drawback of the unitized container design is the additional handling 

required to reposition the individual cells in the tray if the seedlings 

are shipped to the outplanting site in the cells.

Handling, shipping, and storage. Containers must be handled 

repeatedly from initial shipping, through the growing season, to 

storage, shipping, and outplanting. Collapsible containers, such 

as the paperpot, are purchased in a compressed form and have 

lower shipping and presowing storage costs (Hoedemaker 

1974); however, these same containers must be expanded 

before filling and sowing and thus require additional handling. 

The size and filled weight of a container will also affect ease of 

handling. Containers must be sturdy enough to withstand 

repeated handling. Automated handling systems also place 

mechanical stress on the containers and racks.

The type of shipping and storage system also needs consideration 

during container selection. If the seedlings are to remain in the 

container, then some sort of shipping box must be used to protect 

them during shipping and interim storage. Many container 

nurseries are extracting the plug seedlings from their growth 

containers and wrapping them in plastic bags or film for refriger18
19
2.1.2 Types of Containers

Container tree seedlings have been grown in many different types 

of containers over the years. Initially, standard horticultural 

containers were used, but nursery managers soon realized that 

woody plant seedlings required containers with special features, 

such as those discussed in the previous section. Many different 

types of containers have been tried but, after 2 decades of testing, 

only a relatively small percentage (28%) of the containers listed in 

"How to grow tree seedlings in containers in greenhouses" (Tinus 

and McDonald 1979) are still in use. New types of containers are 

still being designed and tested at the present time; this effort to 

develop the ideal container will undoubtedly continue because 

there is no single type of container that is perfect for all 

applications.

Although several different systems for categorizing containers 

have been used, the most practical system divides containers into 

two functional categories: those that are planted with the tree 

seedling, and those that are removed before the seedling is 

planted (Tinus and McDonald 1979).

2.1.3.1 Containers planted with the seedling

Much of the original work on developing containers for forest tree 

seedlings centered on designing a container that not only would 

grow an acceptable seedling in the nursery, but also could be 

directly outplanted in the field. Two different types of these 

containers were developed. Containers of the first type are made of 

a biodegradable material, such as molded peat moss or wood fiber 

(for example, peat sticks or fiber pots), that decomposes after 

outplanting. Seeds are sown on the top and the seedling roots 

penetrate through the container. The major problem with these 

biodegradable containers is that they lack a solid wall with antispiraling features so that seedling roots grow randomly, often into 

adjacent containers. Another drawback is that they often become 

covered with algae and mold in the nursery and thus difficult to 

handle. Barnett and Brissette (1986) reported that some 

biodegradable containers, notably the KysTree-Start' produced 

acceptable southern pine seedlings and performed well in the field. 

Although some biodegradable conta iners are still available (table 

2.1.4), none are currently in widespread use in North American 

container tree nurseries (table 2.1 .1). These containers are 

probably useful only for growing seedlings for short periods before 

their root systems become too expansive.

20

The second type of biodegradable container consists of a shell of 

hard plastic, plastic mesh, or specially treated paper that is tilled 

with growing media and sown with seed to produce a tree 

seedling under normal nursery culture. The tree seedling is then 

outplanted in the container, which theoretically would then 

expand, decompose, or somehow allow roots to grow out into the 

surrounding soil. Hard plastic containers that were designed to 

expand from root pressure after outplanting, for example, the 

Waiters Bullet and Ontario tube (fig. 2.1 .1), enjoyed some early 

successes (Waiters 1974, Reese 1974), but there were problems 

with inconsistent root egress and some cases of root strangulation 

(Barnett and McGilvray 1981, Van Eerden 1982). Tree seedlings 

have also been successfully grown in plastic mesh containers, but 

some root restriction still occurred after outplanting (Budy and 

Miller 1984, Barnett and McGilvray 1981). Because of concern 

over such problems with root form after outplanting, interest in 

hard plastic or plastic mesh containers that can be outplanted with 

the seedling had declined by the time of the Container Nursery 

Survey (table 2.1 .1).

One of the most successful containers that is still outplanted with 

the seedling is the paperpot (table 2.1.1), which was introduced in 

eastern Canada about 20 years ago to replace the Ontario tube. 

Barnett and Brissette (1986) concluded that the paperpot is the 

best container of this type. Paperpots are bottomless, hexagonally 

shaped paper tubes that are interconnected in a honeycomb-like 

design (fig. 2.1.15). They are constructed of a special paper, 

which is a mixture of easy decomposing paper and fibers that 

resist decomposition, and are available in three grades that resist 

decomposition for different lengths of time: B for 4 to 6 weeks, V

for 7 to 9 weeks, and F for 3 to 12 months. Each individual 

paperpot is sealed with an insoluble glue to form a hexagonal, 

open-ended tube. Paperpots are interconnected to adjacent pots 

with a water-soluble glue that slowly breaks down during the 

nursery period so that the individual containers can be separated 

just before planting (fig. 2.1.15). The accordion-like sections of 

paperpots are shipped flat and must be expanded into a hard 

plastic tray before filling and sowing.

Several sizes of paperpots are available, but the 408 size was the 

most popular in North American container tree nurseries at the 

time of the Container Nursery Survey (table 2.1.1). One of the 

major concerns about paperpots is poor control of root form. Root 

spiraling and root growth between containers frequently occur 

(Barnett and
McGilvray 1981), and this characteristic is considered undesirable 

by many nursery managers and reforestation foresters. Barteaux 

and Kreiberg (1982), however, report that problems with root 

intergrowth between paperpots were "minimal," and that plantation 

checks have shown that paperpot container seedlings had better 

root form than bareroot seedlings. A newly developed product, the 

PS paperpot, is made of thin plastic containing a copper strip to 

eliminate rooting between containers during the nursery phase 

(Macdonald 1986). The Ecopot® is another new modification of the 

paperpot system that contains parallel plastic strips between the 

individual cells. This addition inhibits root growth between adjacent 

cells and produces a plug seedling that can be extracted from the 

container (Sims 1988).

The popularity of paperpots is strongly regional; in Canada, this 

container is very popular in central and eastern provinces but is 

rarely used in the West (Smyth and Ramsay 1982). Barteaux and 

Kreiberg (1982) compared paperpots to many other container 

types over a 10-year period in New Brunswick and found that 

paperpots were the least expensive and easiest to handle. 

Canadian container tree nurseries have recently shifted away from 

paperpots, however, because of concern about poor root egress 

after outplanting, especially on cold and wet sites (Sims 1988).

21
22
23
24
25
Paperpots have not fared well in the United States based on 

several nursery and field tests. Barnett and McGilvray (1981) 

tested paperpots with southern pines and found that the paper 

used in the Japanese paperpot degraded slowly after outplanting 

and did not allow root egress, resulting in lower field survival and 

field growth. Barnett and Brissette (1986) reported that the paper 

used in the Finnish paperpot allowed faster root egress than the 

Japanese paperpot. Budy and Miller (1984) found that Jeffrey pine 

seedlings had very poor outplanting survival and growth in the 

Japanese paperpot. Dirmarsen and Alm (1979) outplanted red and 

jack pine in Japanese paperpots in Minnesota and found that the 

paperpot~ had not degraded and that roots had not penetrated t I 

paper after four growing seasons. One reason for this discrepancy 

between geographical regions may be due to climate or soil type. 

The outplanting sites in eastern Canada are probably wetter than 

those in the United States, and this higher soil moisture may 

hasten paper decomposition rate and root egress. Paper becomes 

impervious to root penetration if allowed to dry and so seedlings 

would become root-bound on dry outplanting sites.

One recent development in this container class is the jiffy 7® 

forestry pellet, a modification of the jiffy 7® horticultural peat pellet 

(Hatheway 1988). The forestry pellet utilizes a new type of plastic 

netting that tears apart as the roots grow out of the container. This 

new container is currently being used in the Maritime Provinces of 

Canada.

2.1.3.2 Containers removed before outplanting

Containers for producing seedlings that are extracted prior to 

outplanting (plug seedlings) were by far the most popular type (91 

% of total) in forest nurseries in the United States and Canada at 

the time of the Container Nursery Survey (table 2.1.1). The term 

plug seedlings results from the fact that their roots bind the 

growing medium together into a relatively firm mass or plug (fig. 

2.1.16). Containers for producing plug seedlings should have two 

common characteristics (Tinus and McDonald 1979):

1. The walls of the container should be relatively smooth so 

that roots do not penetrate and make the plug difficult to 

remove.

26

2. The cavity of the container should be tapered from top to 

bottom so that the seedling can be easily extracted from the 

top.

Individual cells in trays. An individual container that is part of a 

larger set of containers is termed a cell or tube. These cells or 

tubes are supported in a tray or rack, which determines the 

spacing of the cells and therefore seedling growing density 

(Allison 1974). Although the term rack is more descriptive, the 

term tray will be used here because it is the more commonly used 

in the forest nursery trade.
Although there are several types of individual cell containers 

available (table 2.1.4), the Ray Leach (R/L) Single Cell® system 

is the most common in North American container tree nurseries, 

constituting 16% of total container use at the time of the Container 

Nursery Survey (table 2.1.1). Several different sizes of R/L Single 

Cells are available, and each consists of a soft plastic tube that 

fits in a hard plastic tray (fig. 2.1.14). The capacity of the various 

cells ranges from 49 to 164 cm3 (3.0 to 10.0 cubic inches), and the 

growing density ranges from 500 to 1,076 cells/m2 (49 to 100 per 

square foot) (table 2.1.1).

One of the biggest advantages of the individual cell system is that 

the containers can be handled either separately or collectively. 

This unitization feature allows consolidation of individual cells 

during culling, so that greenhouse growing space can be used 

efficiently (fig. 2.1 .14). Seedling growing density can also be 

expanded by placing cells in every other location, which promotes 

larger seedlings and also decreases foliage diseases by 

encouraging air circulation around the seedling. After firm root 

plugs have formed, seedlings can be removed from the containers 

by gently squeezing the container or tapping the top of a cell on a 

hard surface. One disadvantage of the individual cell system is that 

the containers have to be replaced in the tray if the cells are 

removed for shipping or cleaning between crops (Tinus and 

McDonald 1979). The hard plastic trays of the R/L Single Cell 

system are relatively fragile and easily damaged if they are 

repeatedly handled while loaded with heavy seedlings. With the 

current trend towards extraction of shippable seedlings from the 

containers at the nursery, however, this is becoming less of a 

problem.

R/L Single Cells are most popular in the Northwestern United 

States (Landis 1982), where they were developed. They and the 

other types of single cell systems are also used at scattered 

nurseries around the country and are especially popular at 

nurseries specializing in tree improvement stock. Another 

individual cell container, the dibble tube, is similar to the R/L Single 

Cell except that it was specifically designed to grow tropical 

species; the dibble tube is currently being used in Hawaii, Guam, 

and several foreign countries.

Extruded peat cylinders are another type of individual container. 

They are formed by extruding a peat-water mixture into a 

continuous, thin plastic casing; the filled casing is then sliced into 

uniform cylinders or "sausages." While conifer seedlings were 

successfully grown

in these containers, they have never been used extensively in 

North American container tree nurseries (Mitchell and others 1972, 

Hocking and Mitchell 1975).

Other types of individual cell containers, such as the polytube or 

polybag (fig. 2.1 .17), are constructed of plastic film. These 

containers are inexpensive and easy to ship and store and are 

popular in nurseries in developing countries or remote locations. 

Several different sizes of polybags are commercially available; the 

20 x 15 cm (8 x 6 in) size was found to produce better quality 

casuarina seedlings (Vinaya Rai and Natarajan 1987). Polybags 

come in clear and black plastic, but black polybags have proven to 

be superior because they retard the growth of algae (Liegel and 

Venator 1987). Proper root development is a problem in these 

containers because their smooth sides promote root spiraling and 

root balling is common in the bottom of the container (Venator and 

others 1985). Wilson (1986) evaluated the

27
Book and sleeve containers are available from several distributors 

in the United States and Canada (table 2.1 .4). Rootrainers come 

in a variety of sizes, and the most popular sizes range in cell 

capacity from 40 to 350 cm3 (2.5 to 21 .5 cubic inches) and in cell 

density from 516 to 1,280 cells/m2 (48 to 1 19 per square toot) 

(table 2.1 .1). A new, large capacity S/L container (1,300 cm3 or 80 

cubic inches) has been recently developed that has a growing 

density of 172 cells/m2 (16 per square foot).

Both book and sleeve containers are tapered and have 

well-defined vertical ribs that discourage root spiraling and guide 

roots to the drainage hole at the bottom of the container. One very 

practical feature is that they can be easily opened up to check the 

condition of the growing medium and root growth at any time 

during the growing cycle, even before a firm plug has developed 

(fig. 2.1 .7 and 2.1.13A). The durability of these containers 

depends on the type of plastic: the original polystyrene containers 

tended to breakdown in sunlight and could only be used for a 

couple of years. For extended use, the newer model Rootrainer 

constructed of ABS (acrylonitrilebutadienestyrene copolymer) 

plastic is considerably more durable. Another advantage is that 

the plugs are easily extracted

28

use of polybags and polytubes in tropical nursery culture, and 

concluded that their popularity was more a function of the 

hardiness of the seedlings being grown than in the beneficial 

attributes of the container system. Yharma (1987) discussed the 

use of polybags in tropical nurseries and the causes of root 

deformation, and listed some cultural practices that reduce root 

problems.

Book and sleeve containers. This container type consists of a 

row of cells made of relatively thin plastic, and is designed to be 

opened and closed without damaging seedling roots. About 10% 

of the containers used in forest nurseries in the United States and 

Canada at the time of the Container Nursery Survey were book or 

sleeve containers (table 2.1 .1). The original and most popular of 

these containers is the Spencer-Lemaire (S/L) Rootrainer® (fig. 

2.1 .18), which is called a book container because it consists of 

two matched forms that are hinged together on the bottom and 

produce a row of rectangular cells when snapped together. A 

sleeve container consists of two matched sections that are 

independent but snap together to form a row of cells when 

assembled (fig. 2.1 .13). Once assembled, book and sleeve 

containers are held together in specially designed trays or taped 

together to form blocks of cells.
29

for grading and packing. Book and sleeve containers are 

somewhat less durable than some of the other container types, 

and individual cells cannot be removed if they fail to produce a 

germinant or contain a diseased seedling. Because they are easily 

opened, however, empty cavities can be filled with other seedlings 

once they have formed a firm plug (Tinus and McDonald 1979).

Because the S/L Rootrainer was originally developed in Alberta, 

Canada (Spencer 1974), it is popular in that region, as well as 

other Canadian provinces (Smyth and Ramsay 1982, Kelly 1982). 

In the United States, both the S/L Rootrainer and sleeve containers 

are used at various locations around the country with no strong 

regional pattern.

Block containers. Block containers consist of a block, generally 

rectangular, that contains a number of cavities or cells arranged in 

a regular pattern (fig.2.1.19 and 2.1.20). The individual cells are 

cylindrical cavities that gradually taper from the top opening to the 

drainage hole at the bottom. Block containers were the most 

popular container type in use at the time of the Container Nursery 

Survey, and several brands and sizes are commercially available 

(table 2.1.4). In North America, the most common block containers 

are the Styrofoam® block (fig. 2.1.19) and the Multi-Pot® (fig. 

2.1.20), which made up 51 and 11 % of container use at the time 

of the Container Nursery Survey, respectively (table 2.1.1).
Styroblocks® are one specific brand of Styrofoam block, and the 

term should not be used as a generic term for all Styrofoam 

containers.

An important characteristic of Styrofoam blocks is their inherent 

insulating value, which protects seedling root systems against 

extreme temperatures. One cultural problem with Styrofoam 

containers is that roots of some species grow into the pores in the 

cavity walls, making the seedlings difficult to extract and the blocks 

difficult to clean and sterilize between crops. A new Styrofoam 

block, called the Ventblock®, offers a series of ventilation holes 

between the cavities to promote air circulation between seedlings; 

this feature has proven effective in controlling the grey mold fungus 

(Botrytis cinerea), which thrives in the humid environment in 

densely growing seedlings (Peterson and Sutherland 1989).

Multi-pots are composed of high-density polyethylene and are 

available in a range of sizes, although only the 57 and 65 cm3 (3.5 

and 4.0 cubic inches) were commonly used in the United States 

and Canada at the time of the Container Nursery Survey (table 

2.1.1). Multi-pots are one of the most durable containers that are 

currently available, and their smooth inner cell walls facilitate 

seedling extraction and make cleaning and sterilizing between 

crops easy.

The Ecopot® is another block container that is a modification of 

the paperpot system. This container resembles the paperpot in the 

way it is bought and used, but contains parallel plastic strips in 

between the individual cells. These plastic strips are removed one 

row at a time during packing, producing a plug seedling. For 

nurseries that already have the paperpot sowing and handling 

system, the Ecopot is an inexpensive way to shift to a plug 

seedling system (Sims 1988).

Block containers have a regional usage pattern in Canada: they 

are most popular in the provinces where they were originally 

developed. Styrofoam blocks are most popular in British 

Columbia, and Multi-pots are gaining increasing popularity in the 

Maritime Provinces (Smyth and Ramsay 1982) and the southern 

United States. In the United States, Styrofoam blocks are one of 

the most common containers in the Northwest (Landis 1982), and 

about 80% of the container seedlings produced in the Lake States 

come from Styrofoam blocks (Alm 1982). Barnett (1982) reports 

that southern pine seedlings grew well and also had good 

outplanting performance when produced in Styrofoam blocks.

Block containers are single, lightweight units that are easy to 

handle and have no individual cells or sleeves of cells that can 

become dislodged. Many of the Styrofoam blocks and Multi-pots 

with different cavity sizes have standard outside dimensions so that 

they can be used with the same filling, sowing, handling, and 

extraction equipment (Tinus and McDonald 1979). This feature is 

also useful during storage, because the Styrofoam blocks stack 

easily and the Multi-pots will nest together. Both types of block 

containers are resistant to breakdown in sunlight and are reusable; 

Multi-pots claim an operational life of 6 to 10 years. A new liquid 

coating, Speedling® Super-Cote, can be applied to Styrofoam 

blocks to extend their life expectancy. It has been used successfully 

on several southern pine seedlings, and reportedly makes the plugs 

easier to extract. Root control ribs inside the cavity are also 

standard on both types of block containers, although Barnett (1982) 

reported that roots of loblolly pine seedlings were deformed at the 

bottom of the plugs because the anti- spiral ribs did not extend all 

the way to the bottom of some Styrofoam block cavities. One of the 

drawbacks of all types of block containers is that empty cavities 

cannot be replaced, making high seed quality and proper sowing 

procedures necessary.

Styrofoam blocks are composed of expanded polystyrene foam 

and come in a wide variety of container capacities, ranging from 41 

to 492 cm3 ( 2.5 to 30.0 cubic inches), and growing densities 

ranging from 270 to 1,108 cells/ m2 (25 to 103 per square feet). It 

is worth noting that environmentally polluting chloro-fluorocarbons 

are not used in the manufacturing of Styrofoam blocks.

30
31

Miniature containers. A recent innovation in the container tree 

nursery industry, borrowed from the vegetable transplant industry, is 

the use of very small containers (mini-containers) to produce young 

plug seedlings for container/bareroot transplants (Hee and others 

1988, Klapprat 1988). Seedlings are produced in rectangular plastic 

or Styrofoam containers (fig. 2.1.21 A), that contain a grid of small 

cells [4 to 18 cm3 (0.25 to 1.1 cubic inches)] and produce high 

seedling growing densities [948 to 3,813 cells / m2 (88 to 354 per 

square foot)]. After a relatively short growing period in a 

greenhouse, the container seedlings are transplanted to bareroot 

nursery beds for an additional period of growth. Although a number 

of different miniature container systems are on the market, two of 

the most commonly used in container tree nurseries in North 

America are the Mini Plug® system (Hee and others 1988), and the 

Techniculture® plug system (Klapprat 1988).

The objective of the mini-container system is to produce a fully 

extractable seedling with a dimensionally stable root plug that will 

tolerate transplanting within a relatively short growing period (3 to 4 

months) (fig. 2.1.21 B). Both the Mini-plug and Techniculture 

systems are fully automated, from sowing the seed through 

transplanting. Because the seedlings grow in them for only a short 

time, mini-containers are radically different from typical container 

types: the cells are very short and closely spaced, and do not have 

ribs to control root spiraling. In some systems, the mini-containers 

are carried to the bareroot nursery where they are used as 

magazines for feeding the seedlings to the transplanter; other 

nurseries extract the seedlings before transplanting.

The future of miniature container systems appears promising. They 

can be used to shorten the rotation for slow-growing species or 

species that do not grow well in bareroot nurseries. Because all 

transplants are relatively more expensive than seedlings, economic 

analyses and outplanting trials of mini- container transplants are 

currently being performed. A series of trials with the Mini-plug 

system showed that the container transplants performed as well or 

better than traditional bareroot seedling or transplant stock types 

on a variety of coastal sites in the Pacific Northwest (Tanaka and 

others 1988).
seedling is removed from the container, creating a more natural, 

branched root system after outplanting (Burden and others 1983, 

McDonald and others 1984a).

Burden and Martin (1982) treated the inside of Styrofoam block 

containers with latex paint containing CuCO3 and grew 10 different 

species of conifer seedlings in them (fig. 2.1.23). They reported 

that this chemical root pruning prevented root spiraling, but their 

results varied with seedling species, container volume, type of 

growing medium, and the concentration of CuCO; in the wall 

coating. McDonald and others (1984a) found that both CuCO3 and 

IBA arrested root growth of ponderosa pine seedlings when 

applied to container walls, but the copper treatment was more 

effective. Romero and others (1986) reported that treatment with 

CuCO3 significantly increased the number of lateral roots and both 

root and shoot weight in Caribbean pine container seedlings. 

Green ash and red oak seedlings grown in CuCO3--treated 

containers were larger than controls and had more fibrous and 

evenly distributed root systems (Arnold and Struve 1989). Dong 

and Burden (1986) reported that treating paper contain2.1.4 Chemical Root Pruning

There has been widespread concern over the root form of 

container seedlings and the potential problems with root-bound 

plug seedlings after outplanting. Burden and others (1986) relate 

the problem of physical instability (toppling) of lodgepole pine 

seedlings in British Columbia plantations to poor lateral root egress 

after outplanting, and discuss chemical and mechanical root 

pruning treatments to overcome this problem. Carlson and others 

(1980) studied the root system morphology of Sitka spruce that 

had been direct- seeded, or grown as bareroot or container 

seedlings, and concluded that the root deformation that was 

caused by outplanting was probably not severe enough to cause 

instability or growth retardation. Obviously, species rooting 

characteristics, type of planting tool, and soil characteristics on the 

outplanting sites have a significant effect on the root form of the 

seedling after outplanting. Nevertheless, container nursery 

managers and reforestation foresters are interested in any cultural 

treatment that will generate a better root system in container tree 

seedlings.

One option is to coat the interior walls of the containers with 

chemicals that inhibit root growth, such as cupric carbonate 

(CuCO3) or indolebutryic acid (IBA), carried in a binding material 

such as latex paint (Pellett and others 1980, McDonald and others 

1984a). Chemicals for root pruning must inhibit root growth and 

remain in the application zone throughout the growing season 

without diffusing into the growing medium or becoming phytotoxic 

to the seedling. These chemicals must also not be toxic to nursery 

personnel or the environment (Hulten 1982). Tinus (1987) tested a 

variety of heavy metals including copper, silver, cobalt, nickel, 

lead, zinc, and antimony and found that only copper would stop 

root growth without injuring the seedling. CuCO3 has been the 

most popular root pruning chemical used in subsequent trials, and 

recommended application rates range from 60 to 200 g/liter (2.0 to 

6.7 ounces per gallon) depending on seedling species and 

container type (Wenny and others 1988).

Chemicals for root pruning can be either sprayed into the 

container cavities or used as a dip for the entire block. The blocks 

are then filled and sown in the usual manner, but when the 

seedling roots contact this chemical barrier they cease growing 

and suberize (fig. 2.1.22). New lateral roots are generated and are 

subsequently pruned when they reach the treated container wall, 

resulting in a more fibrous, branched root system that is evenly 

distributed throughout the container. These pruned root tips 

resume normal root growth when the

32
ers with cupric sulfide (CuS) prevented root spiraling in paper 

containers with a polyethylene coating and also stopped root 

growth between non-polyethylene-coated paper containers. Root 

distribution within the container has often been a problem, with 

the majority of new root tips forming at the drain hole at the 

bottom of the container. Copper-treated containers produced 

more new roots in the middle and upper zones of the container, 

which should promote better seedling stability after outplanting 

(Wenny and others 1988).

Chemical root pruning has also been found to have other benefits 

for container tree seedlings. Romero and others (1986) analyzed 

root development of CuCO3-treated Caribbean pine seedlings and 

found that treated seedlings had more lateral roots as well as 

significantly larger stem diameters than control seedlings. They 

also noted that the CuCO3 treatment caused a change in root 

morphology: treated seedlings had finer, more fibrous root 

systems than did untreated seedlings. Ruehle (1985) studied the 

effect of CuCO3 on the root morphology of southern pine seedlings 

and found few significant differences in shoot or root dimensions, 

weight, or ectomycorrhizal formation. McDonald and others 

(1984b) found that a combination of CuCO3 treatment

and inoculation with ectomycorrhizal fungi increased the number 

of short roots of lodgepole pine seedings, and ectomycorrhizai 

formation on both lodgepole pine and ponderosa pine seedlings. 

Donald (1986) reported that copper salts also kept bryophytes 

from growing on the containers.

The real benefits of chemical root pruning should occur after 

outplanting, and results have generally been favorable. Green ash 

and red oak seedlings transplanted from containers treated with 

CuCO; had higher root growth potential compared to the controls; 

Arnold and Struve (1989) attributed this to the greater number of 

root tips produced after chemical root pruning. McDonald and 

others (1984a) planted CuCO3-treated ponderosa pine seedlings in 

vermiculite, and found that treated seedlings had 3 times more root 

egress and were also significantly taller than untreated seedlings. 

Burden and others (1983) studied the root development of 

chemically pruned lodgepole pine seedlings after outplanting and 

found that treated seedlings have more uniform root growth ;fig. 

2.1.24) and significantly better height growth than controls. 

However, seedlings of three western conifer species, when 

examined 3 years after outplanting, showed no significant increase 

in survival or growth

33
even though the seedlings did have more new roots in the upper 

zones of the soil (Wenny 1988). The practical benefits of chemical 

root pruning may only become obvious when the outplanted 

seedlings ire exposed to physical or environmental stresses.

Although the use of copper-treated containers has not been 

implemented operationally in North America, chemical root pruning 

is currently being used in South Africa (Donald 1986); containers

are dipped in a polymer suspension of Cu2+ ions called Styrodip 

prior to filing and sowing (Nelson 1989). Beaver Plastics has 

recently introduced the Trimroot Styroplug®, which uses a 

"differential coating" system (fig. 2.1.25) to control and prune roots 

within a Styrofoam container; they stress that operational trials are 

needed to test this new system, however. Another option is to 

stimulate natural air pruning of roots around the periphery of the 

plug by designing a container with air slits at regular intervals on 

the sides. The Rootmaker® container incorporates a series of 

drainage slots around the perimeter but is currently available in 

only one relatively large size (Whitcomb 1988).

34
2.1.5 Conclusions and Recommendations

As evidenced by the variety of containers that are currently in use 

in container tree nurseries, acceptable seedlings can be produced 

in many different types of containers. There is no one container 

that exhibits all the many different characteristics, and thus, no 

single type of container is best for all nurseries and outplanting 

sites. In the final assessment, the choice of a container system 

depends on the objectives and cultural characteristics of each 

individual container tree nursery operation (Landis 1982).

35
2.1.6 Literature Cited

Allison, C.J., Jr. 1974. Design considerations for the RL single cell 

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