123
6.4.4 The Hardening Phase 145

6.4.4.1 Introduction 145

Problems with non-hardy seedlings 145

Hardiness terminology 145

Objectives of the hardening phase 146

Factors affecting seedling hardiness 148

Scheduling the hardening phase 149

6.4.4.2 The atmospheric environment 150

Propagation structures 150

Temperature 151

Humidity 153

Light 153

Carbon dioxide 154

6.4.4.3 The edaphic environment 154

Irrigation 154

Fertilization 155

6.4.4.4 Cultural operations 156

Inoculating with mycorrhizal fungi 156

Species selection 158

Types of mycorrhizal inocula 158

How and when to inoculate 158

Cost effectiveness 158

6.4.4.5 Pests and abiotic problems 158

6.4.5 Summary 160

6.4.6 Literature Cited 161

Contents

6.4.1 Introduction 125

6.4.2 The Establishment Phase 127

6.4.2.1 The atmospheric environment 127

Temperature 127

Humidity 128

Light 128

Carbon dioxide 128

6.4.2.2 The edaphic environment 130

Irrigation 130

Mineral nutrition 130

6.4.2.3 Cultural operations 130

Monitoring germination 130

Thinning 131

Resowing 132

Transplanting 132

6.4.2.4 Pests and abiotic problems 132

Temperature extremes 132

Damping-off 133

Fungus gnats 134

Cryptogams and weeds 134

Birds and rodents 135

6.4.3 The Rapid Growth Phase 136

6.4.3.1 The atmospheric environment 136

Temperature 136

Humidity 137

Light 140

Carbon dioxide 140

6.4.3.2 The edaphic environment 140

Irrigation 140

Fertilization 140

6.4.3.3 Cultural operations 141

Culturally controlling shoot height 141

Top-pruning 143

Primary versus secondary needles 143

6.4.3.4 Pests and abiotic problems 144

Fusarium root rot 144

Insects 144

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6.4.1 Introduction

The objective of this chapter is to present a general description of 

how a typical crop is grown in a forest and conservation nursery. Of 

course, the propagation proto col and growing schedules will vary 

with the crop species as well as the type of nursery environment; 

however, we feel that going into the details of growing a typical crop 

will be informative to the novice grower. (Propagation protocols and 

growing schedules for a variety of other forest and conservation 

plants are provided on the USDA Forest Service's Home Page at 

the following uniform resource locator (URL):

» willow.ncfes.umn.edu/snti/snti.htm «

Our example will be seed propagation of a crop of conifer seedlings 

from the northwestern United States grown in a fully controlled 

greenhouse. This crop will take 17 months to produce and ship, and 

a facilities schedule is provided in table 6.4.1 A. The various 

species will be grouped together on the greenhouse benches 

according to their cultural requirements. The relatively

faster growing species are grown on one table (or bench) and the 

slower growing ones on another (table 6.4.1 B).

The propagation environment and cultural practices need to be 

planned for each of the three seedling development phases: 

establishment, rapid growth, and hardening. For each phase, we 

will discuss the potentially limiting factors of the propagation 

environment: the ambient environment, which includes 

temperature, humidity, light, and carbon dioxide, and the edaphic 

environment, which consists of water and mineral nutrients (figure 

6.4.1). It should be emphasized that the growing medium and type 

of container are critically important, not only because they provide a 

limited reservoir of water and mineral nutrients, but because they 

are the only factors that cannot be changed after seeds are sown.

Nurseries also contain a variety of living organisms that affect 

seedlings, so both pests and beneficial organisms will also be 

discussed for each development phase. Lastly, we will talk about 

typical cultural practices that can be used during each phase.

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126
127

6.4.2 The Establishment Phase

The establishment phase starts when seeds are sown and 

continues until seedlings are well established in their containers. 

With good-quality seeds, germination should be well underway and 

emerging seedlings visible by the second week (figure 6.4.2A). 

However, germination often continues into the third or fourth week, 

especially with seedlots of lower vigor. Young conifer seedlings 

("emergents") carry their seedcoats up out of the growing medium 

until they form the "bird cage" stage (figure 6.4.2B). Following this, 

seedlings do not grow appreciably in height and most of their 

energy is diverted to root growth. The radicle grows rapidly 

downward, typically reaching the bottom of the container by the time 

the seedcoat is shed (figure 6.4.2C). At the end of establishment 

phase, a rosette of primary needles forms in the center of the 

cotyledons.

The cultural objective of the establishment phase is to generate 

optimum environmental conditions to obtain a well-established 

healthy stand of seedlings with a minimum number of empty 

containers. Growers need to be careful because young plants are 

particularly susceptible to pests and cultural injury. Problems that 

show up during the establishment phase are difficult, if not impossi-

ble, to correct. For example, if poor seed quality results in a 

significant number of empty containers, the delay caused by 

resowing will result in a crop of variably sized seedlings. Resown 

seedlings will always lag behind in growth rate because of 

competition with their neighbors.

6.4.2.1 The atmospheric environment

Although it is important to keep all potentially limiting factors at 

optimal levels, seed germination in container nurseries is 

primarily affected by three environmenta l factors: temperature, 

moisture, and light.

Temperature. Warm temperatures are required to stimu late rapid 

and complete germination, so growers usually keep the greenhouse 

slightly warmer than normal during the first month of the crop cycle. 

Although there is some variation between species, an ambient 

temperature target of 27 °C: (80 °F) during the day has proven to be 

a good compromise between optimal germination and energy 

conservation. Greenhouse heating systems can maintain a 

temperature of ±2 °C (3.5 °F), which converts to a range of 25 to 29 

°C (77 to 84 °F). Most growers use different day and night 

setpoints, and their experience has shown that this practice can 

save heating costs without a significant loss in growth. For example, 

the University of Idaho's Forest Research Nursery lowers the 

temperature a
full 5°C (10 °F) at night (table 6.4.2). Because seeds begin 1o 

germinate quickly under warm-moist conditions, variations in 

seedling size can develop if it takes more than a couple of days to 

fill the propagation structure. One way to overcome this is to keep 

the containers dry and cool until all containers are in place in the 

propagation area, and then turn on the heaters and irrigate the crop.

After germination is complete, the daytime temperature can be 

lowered to 24 to 27 °C (75 to 80 °F) for the remainder of the 

establishment phase (table 6.4.2). Note that all' of these targets are 

air temperatures, but the temperature of the growing medium 

surrounding the seeds is just as important. Heating systems that 

keep the growing medium warm-such as underbench heat 

distribution tubes or overhead radiant heat-promote faster and more 

uniform germination and early seedling growth. (Heating systems 

are discussed in detail in section 3.1 .2, and operational target 

temperatures and ranges for a variety of species during the 

establishment phase can be found in table 3.1.2 of volume three of 

this series.)

Humidity. Germinating seeds can dry out rapidly in the high light 

environment of a container nursery and therefore growers must be 

particularly vigilant. Evaporation is the primary cause of moisture 

loss, but a relative humidity (RH) range of 70 to 80% (table 6.4.2) 

will keep evaporation rates low and retard fungal diseases, 

especially damping-off. Humidity is controlled with frequent, light 

mistings that keep the growing medium around seeds "moist, but 

not wet." (See the following irrigation section for more specifics.) 

Although RH is a useful measurement, the vapor pressure deficit 

(VPD) is a more relevant factor to monitor. (Operational RH and 

VPD targets and ranges for a variety of species during the establishment phase can be found in tables 3.2.5 and 3.2.6 of volume three 

of this series.)

Light. Three attributes of light affects plants: intensity, duration, 

and quality. Although research has shown that some species 

germinate best at moderate light intensities, most commercial 

conifers can tolerate full sunlight during the establishment phase. 

This fact is not appreciated by many novice growers who assume 

that shade-tolerant species such as western hemlock would 

naturally require shady conditions during germination. This belief 

probably developed from the long-standing practice of using shade 

frames over seedbeds in bareroot nurseries, which was more for 

controlling surface soil temperatures than light levels. In container 

nurseries, the type and color of

128

seed coverings and irrigation can control surface temperatures, so 

full light is recommended. In fact, it is recommended that some 

small native plants species be sown without seed coverings to 

ensure that they receive enough light during germination (Emery 

1988).

Light quality has been shown to be important to seed germination. 

Light with wavelengths in the red spectrum (660 nm) promotes 

germination of southern pine seeds, whereas far-red light (730 

nm) inhibits it. This has little practical importance, however, as 

there is sufficient red light in sunlight and all types of photoperiodic 

lighting (Barnett and Brissette 1986).

Although light intensity and quality are not operationally important, 

duration--the daylength or "photoperiod"--does have a significant 

affect on seed germination and early seedling growth of many 

species. For example, both rate and total germination of loblolly 

pine seeds were much better under photoperiodic lighting whereas 

the same treatments had no effect on slash pine (Jones 1961). 

Although not all species or seed sources of a given species may 

require it, the use of photoperiodic lighting will help overcome 

growth differences among individual seedlings and result in a more 

uniform crop. Thus, many growers turn on their crop lights after 

sowing is complete and leave them on until hardening is initiated. 

The University of Idaho's Forest Research Nursery supplies 500 lux 

(50 ft-candles) of incandescent lighting applied intermittently 

throughout the night (table 6.4.2).(A complete discussion of the 

types of photoperiodic lighting and other examples of operational 

lighting systems can be found in section 3.3 of volume three of this 

series.)

Carbon dioxide. Although carbon dioxide (CO2) enhancement is 

not effective until there is enough photo synthetic tissue to absorb it, 

the cost of operating CO2 generators is minimal and so most 

growers turn them on at the beginning of the growing cycle. A CO2 

level of between 750 parts per million (ppm) and 1,000 ppm is a 

typical target, and generators switch on several hours before 

sunlight and continue running during the day whenever vents are 

closed. The University of Idaho nursery does not use CO2 

generators, but encourages good air circulation within the 

greenhouse (table 6.4.2). (A complete discussion on managing and 

monitoring CO2 during the establishment phase can be found in 

section 3.4.3 of volume three of this series.)
129
once or twice per week with regular irrigation supplied as needed. 

In addition to the full range of 13 mineral nutrients, fertigation 

solutions typically contain a weak acid such as phosphoric acid to 

help maintain the pH of the growing medium in the range of 5.0 to 

6.0.

Each of these fertigations should last long enough to leach unused 

fertilizer salts from the container; it should then be followed by a 

clear water rinse to remove salts from the succulent foliage and 

prevent salt burn. Most growers, including the University of Idaho 

nursery, use a low-nitrogen (for example, 50 to 100 ppm) fertilizer 

solution during the establishment phase because seedlings are so 

small (table 6.4.2). Recently, growers are beginning to try an 

exponential fertilization program in which the concentration of 

nutrients is gradually increased during the growing season as 

seedlings grow in size (Timmer and others 1991). For example, 

exponential fertilizer applications produced acceptable Douglas-fir 

seedlings with 60% less N early in the growing season (Dumroese 

and others 1995). (Complete instructions on how to formulate and 

monitor fertigation can be found in section 4.1.6 of volume four of 

this series.)

6.4.2.3 Cultural operations

The establishment phase is an anxious time in the nursery 

because this is when young seedlings are most susceptible to 

cultural injury and pests. Therefore, growers should be 

particularly vigilant and inspect their crops daily (table 6.4.2).

Monitoring germination. After about 2 weeks, newly sown 

containers should be inspected to determine if germination is 

occurring normally. The seed covering can be carefully brushed 

aside and seeds inspected with a 5 to 10 power hand lens. 

Complete absence of seeds in a high number of containers 

indicates a sowing problem, or predation by birds. If seeds appear 

swollen and cracked, then they are probably healthy and should be 

returned to the container and remulched. If they do not appear 

ready to germinate, then a small sample should be removed from 

the container and cut in half with a single-edge razor blade. Inspect 

the bisected seeds with a hand lens. Healthy seed will be white to 

cream in color and firm in texture-similar to coconut meat in appearance. Diseased seed will be brown or black or with a soft texture. 

(An illustrated diagnostic key to seed problems is provided in 

section 5.1 .3 in volume five of this series.)

6.4.2.2 The edaphic environment

Irrigation. The sown containers should be thoroughly irrigated until 

they reach full saturation as soon as they are placed in the 

propagation area. Some types of peat moss are difficult to wet, 

however, so a surfactant may need to be applied with the initial 

irrigation if it was not already included as an amendment to the 

growing medium. If no surfactant is used, it may take several days 

of irrigation to completely hydrate the growing medium (Wood 

1994).

Following this initial soaking, subsequent irrigations should be 

frequent but short in duration until germination and emergence are 

complete. Overwatering floods the macropores that provide 

oxygen to the germinating seeds, so these light mistings should 

keep the growing medium "moist but not wet" (table 6.4.2). Proper 

watering allows the seed covering to dry out between irrigations, 

which helps control damping-off and prevents moss and algae 

from developing. As mentioned in the humidity section above, 

irrigation is used to maintain humidity as well as supply water to 

germinating seeds and young emergents.

After most seedlings are past the bird cage stage, a regular 

irrigation schedule can begin. Some growers rely on a calendar 

method based on previous experience such as two irrigations per 

week. The University of Idaho nursery uses container weight to help 

determine the amount of water to apply per irrigation (table 6.4.2). 

The fully satu rated wet weight is determined at the time of initial 

satu ration, and the target weight (when irrigation is required) is 

expressed as a percentage of wet weight. For example, white 

spruce seedlings should be irrigated when the weight of the 

containers has dropped to 80% of wet weight (Wood 1994). (The 

procedure for developing container weights is described in detail in 

section 4.2.6 of volume four of this series.)

Mineral nutrition. Some growers prefer to incorporate a small 

amount of slow-release fertilizer into their growing medium to supply 

some nutrition to young germinants. Overhead irrigation containing 

injected mineral nutrients (known as fertigation) often is not applied 

during the germination stage because seedlings receive adequate 

mineral nutrients from seed reserves and because of increased risk 

of damping-off or salt injury. Once germinants are fully established, 

however, fertigation is initiated. Some growers use a constant-feed 

program in which fertilizer is injected with each irrigation; others 

fertigate

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131

removed to avoid the possibility of disease. Leaving severed 

stems and root systems in the container has not proved to be 

detrimental, however.

Thinning must be accomplished as soon as possible to minimize 

effects on seedling development-competition for water, nutrients, 

and eventually light will quickly decrease growth. The effects of 

leaving multiple seedlings in container cells have been evaluated 

with longleaf, loblolly, and slash pine (Barnett and Brissette 1986). 

The most marked effect was on size: seedlings grown 2 or 3 to the 

cavity had 50% or less biomass than those grown with only 1 per 

cavity at the end of 14 weeks in the greenhouse (figure 6.4.313). 

Interestingly enough, this effect of intracavity competition even persisted after outplanting for one of the species: longleaf pine 

seedlings that were not thinned to 1 seedling per cavity had much 

poorer survival after 3'h years in the field (figure 6.4.3C).

Thinning. Typically, 2 or more seeds are sown per cavity to 

minimize the chances of empty cells. Extra germinants should be 

thinned as soon as possible, and two techniques are used: pulling 

and clipping.

• Pulling--This technique consists of carefully extracting ,extra 

germinants from each cavity by hand. Pulling must be done 

before seedlings develop lateral roots or it becomes difficult to 

remove the seedling without injuring the remaining crop 

seedling. Workers must be trained to remove smaller or less 

vigorous seedlings and be careful not to injure the remaining 

ones during the process. Thinned seedlings are collected in 

trays and removed from the growing area.

• Clipping--This involves using small pointed scissors to snip 

extra seedlings out of each cell (figure 6.4.3A). Seedlings can 

be clipped at a much more advanced age than they can be 

successfully pulled. Clipped seedlings must be collected in 

trays and
Resowing. If germination is erratic, then the decision must be made 

quickly whether to resow new containers to alleviate the shortfall, 

sow germinants, or transplant emergents into the empty cavities. As 

a general rule of thumb, if the percentage of empty cavities is 

between 5 and 15%, resowing with germinating seeds or transplanting emergents from seed trays is a feasible alternative (Barnett and 

Brissette 1986). If more than 15% of the cells are empty, the 

shortfall should be made up by sowing additional containers. If 

growers anticipate a germi nation problem, then they may sow extra 

containers initially to compensate for the expected empty cells--the 

oversow factor. This is one advantage to single-cell containers, 

such as RL Single Cells®, because they can be consolidated--

empty cells can be removed and replaced with cells containing 

emerging seedlings. With block containers, however, oversowing 

wastes valuable production space. Resown containers will always 

lag behind the first sowing and often have to kept growing longer 

into the growing season to reach the same size (figure 6.4.4A).

Transplanting. Another strategy for handling variable germination 

is to transplant pregerminated seeds or young emergents into the 

empty cells within the containers. Resowing with germinants or 

transplanting should be done as soon as the empty cavities become 

evident, which is usually possible about 10 to 14 days after sowing. 

Some growers sow extra seeds into germination trays or 

intentionally oversow the number of seeds per cell, and then 

transplant extra germinants back into empty cavities. Growers must 

accept that transplanted seedlings will always be smaller than those 

from the original sowing (figure 6.4.413). In a study with southern pines, seedlings from 

transplanted germinants were always smaller than direct-sown 

seedlings, especially for ]longleaf pine (table 6.4.3). Using 

germinants with longer radicles helped make up the size to some 

degree but, unfortunately, these germinants are more difficult to 

transplant correctly. In fact, nurseries that routinely transplant 

emergents from germination trays ("pricking out") find that clipping 

the tap root to about 50% of the original length increased survival 

and growth after transplanting (Singh and others 1984). However, 

with jack pine and black spruce seedlings, transplanting is not 

recommended as a routine practice because of the high incidence 

of root deformity and the high labor costs (Scarratt 1991). (The 

correct procedure for handling and positioning germinants and 

transplanting emergents can be found in section 6.2.8 of this 

volume.)

6.4.2.4 Pests and abiotic problems

Because germinating seeds and newly emerged seedlings are so 

susceptible to environmental stress and nursery pests, growers 

must be especially vigilant during the establishment phase. A 

description of the common diseases, insect pests, and abiotic 

stresses of germinants and young seedlings can be found in section 

5.1 .3 of volume five of this series, but growers should be 

particularly alert for the following abiotic problems and pests.

Temperature extremes. Because of their limited root system, 

young germinants are very susceptible to drought and direct 

heat-injury to the succulent hypocotyl tissues. Light-colored seed 

coverings and frequent light irriga132
133

protected from temperatures below freezing. Scots pine seedlings at 

20 to 30 days after sowing can be physically injured by a short 

2-hour exposure to temperatures below -4.5 °C (24 °F). After such 

an exposure, even visually healthy cotyledons and primary needles 

were found to have cellular injury that was reflected in reduced 

growth rates (Holopainen 1988; Holopainen and Holopainen 1988). 

Therefore, growers must be prepared to physically cover their stock 

or irrigate to prevent cold injury. (Frost protection of nursery stock 

with irrigation is discussed in volume seven of this series.)

Damping-off. The most common disease during the establishment 

phase is known by the traditional name of "damping-off," and there 

are two different types: pretions or "mistings" are commonly used to keep temperatures low 

around the stem. Emerging germinants are most susceptible to heat 

injury but become more tolerant as they grow older and develop 

bark tissue. The temperature at the soil surface can be monitored by 

placing the tip of a thermometer just under the seed covering and 

monitoring it frequently during sunny weather. Some nurseries 

establish guidelines to help nursery workers determine at what 

surface temperature they should begin cooling with irrigation, and 

the critical temperature becomes higher as seedlings grow older 

(table 6.4.4).

For nurseries that raise seedlings in open growing compounds, 

late-spring frosts can also cause damage. Newly emerged 

seedlings are not very cold hardy and must be
emergence and post-emergence. Pre-emergence damping-off is 

caused by several fungal pathogens and occurs before the 

germinant emerges from the medium. Unfortunately, 

pre-emergence damping-off is often mistaken for poor-quality seed, 

so growers should carefully excavate a sample of sown seeds after 

a couple of weeks and inspect them with a hand lens. The 

presence of cottony-appearing mycelia confirms fungal damping-off 

(figure 6.4.5), but a few slow-to-germinate seeds should also be cut 

in half with a razor blade to see if their contents appear healthy. 

The inside of healthy seeds should appear white to cream-colored, 

whereas that of decayed seeds is dark and watery in texture.

Symptoms of post-emergence damping-off are more obvious: the 

young emergent falls over due to a constriction at the surface of the 

growing medium. If the cause is a fungus, the hypocotyl and root 

appear discolored or decayed. Damping-off also can be caused by 

heat or chemical injury; in this case, the hypocotyl is damaged right 

at the surface of the growing medium but the root will not be 

decayed. (A damage key and color photographs to help diagnose 

the differences can be found in section 5.1 .3 of volume five of this 

series.)

fungus gnats. One of the most serious insect pests during the 

establishment phase are the larvae of dark-winged fungus gnats. 

Adult gnats (Bradysia spp.) are commonly found in greenhouses 

and, although they do not directly feed on seedlings, they have 

been shown to vector spores of other fungal pathogens, including 

Fusarium spp. and Botrytis spp. (James and others 1994). The 

larvae feed on many types of organic matter, including seeds and 

young seedlings and can become a serious problem especially 

where over-irrigation and poor water management occur. The 

larvae remain hidden in the „rowing medium and so the appearance 

of adults flying ,round the crop is the first sign of a fungus gnat 

infesta tion. Some growers monitor the relative populations with 

yellow-sticky cards and use this information to decide When control 

is warranted. (Damage keys, color photographs, and more specific 

identification and control information can be found in section 5.1.4 

of volume five of this series.)

Cryptogams and weeds. Once cryptogams-algae, moss, and 

lichens-and weeds are introduced into the containers, there is no 

easy or cheap way to remove them. They seriously compete with 

young seedlings for water, mineral nutrients, and eventually light. 

This problem should not occur during the establishment phase 

unless the containers were not clean or the growing medium was 

not sterile. In one case, weeds became a serious problem from 

seeds that mice carried into bags of stored growing medium. Spores 

of algae, mosses, and lichens and small weed seeds can also be 

introduced in unfiltered irrigation water. This can be a particularly 

serious problem when water from ponds or other surface sources is 

used. Although these plant pests will eventually invade the nursery 

through airborne spores, they should not become a problem before 

seedlings grow large enough to reach crown closure and completely 

shade the surface of the growing medium. Because they are also 

plants, it is difficult to find pesticides that kill cryptogams and weeds 

without harming the crop

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135

that doors are promptly closed. In open growing areas, covering 

sown containers with large-mesh screening until germination is 

complete is effective. It is almost impossible to completely prevent 

small rodents from entering propagation structures. However, 

establishing small bait stations with extra seed around the 

perimeter of the growing area will show when there is a problem. 

Chemical seed repellents have been used but are not 

recommended because they may reduce germination. After a 

rodent problem has been diagnosed, poison baits and snap traps 

can be effective if they are used safely and properly. (Damage 

keys, color photographs, and more specific identification and 

control information can be found in section 5.1.3 of volume five of 

this series.)

seedlings (that is, they are not phytotoxic). Ferrous sulfate and 

sterilants such as Agribrom© have been used, but prevention is the 

best solution. (Damage keys, color photographs, and more specific 

identification and control information can be found in section 5.1 .5.4 

of volume five of this series.)

Birds and rodents. Sown seeds, especially the larger pine seeds, 

are very attractive to both mice and birds. Both will disturb the seed 

covering while searching for the seeds, but birds usually carry 

seeds off before eating them, whereas rodents leave empty seed 

coats behind in containers to taunt you. Bird entry can usually be

prevented by screening greenhouse vents and making sure
6.4.3 Rapid Growth Phase

The rapid growth phase begins when the terminal shoot in the 

middle of the cotyledons begins to grow quickly and continues until 

most of the crop approaches target height. With our example crop of 

western conifers at the University of Idaho's Forest Research 

Nursery, this phase starts at about 8 weeks after sowing and 

continues for about another 3 months (table 6.4.1). Although 

terminal shoot growth is the predominant characteristic of the rapid 

growth phase, lower lateral branches also expand without forming 

buds (figure 6.4.6A). It is during the rapid growth phase that the real 

benefit of container nursery culture is dramatically illustrated as 

shoot growth can be two to three times as much as that of a normal 

seedling in the wild (figure 6.4.613). Depending on species and 

propagation environment, height growth may not be continuous but 

instead consist of a series of spurts or flushes. Because most of the 

energy resources are going to the shoot, the lateral cambium also 

becomes more active and stem diameter growth slowly increases. 

Roots continue to expand and grow and should occupy the entire 

cavity by the end of the rapid growth phase.

The cultural objective during the rapid growth phase is to keep all 

environmental factors in the propagation environment at optimal 

levels. Because seedlings are still very succulent, growers must 

minimize any type of unnecessary environmental or cultural stress. 

As seedlings grow older and stronger, the potential for outright 

mortality decreases but the crop is still susceptible to abiotic injury 

or pests. So, growers should still schedule regular scouting trips 

throughout the propagation area and look for areas of low vigor, 

stunting, and other injury.

6.4.3.1 The atmospheric environment

Temperature. Both the target temperature and the allowable 

temperature range during the rapid growth phase will depend on 

the type of propagation environment and the species. Fully 

controlled greenhouses can maintain both day and night 

temperatures to within a couple of degrees, whereas open 

compounds are at the mercy of the local climate. Serious growth 

delays have occurred in open compounds during cool and cloudy

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137

nursery, target temperatures are kept in the 21 to 24 °C (70 to 75 

°F) range during the day, with about a 5 °C (10 °F) setback at night 

(table 6.4.5A). Early in the season, heating is still required at night 

and in the early morning until sunlight warms the greenhouse. By 

early summer, however, cooling will become more of a problem in 

fully enclosed greenhouses because of intense sunlight and longer 

days. Convective cooling with vents is sufficient in moderate 

climates, but evaporative cooling, shadecloth, or misting is 

necessary in semi-arid environments. Horizontal airflow fans 

promote lateral air movement and help eliminate hot or cold spots 

with a greenhouse (figure 6.4.713). (Heating and cooling systems 

are discussed in detail in section 3.1.2 and operational target 

temperatures and ranges for a variety of species during the rapid 

growth phase can be found in table 3.1 .2 of volume three of this 

series.)

Humidity. Because the roots have expanded throughout the 

growing medium, seedlings are less susceptible to sudden 

moisture or heat stress and so relative humidities can be reduced 

to a target range of 60 to 80%. This moderate humidity level is 

sufficient to keep evapotranspirational losses low enough to 

encourage cell expansion and division but is not so high as to 

promote fungal diseases. A vapor pressure deficit of approximately 

1.00 kPa is a reasonable target during the rapid growth phase.

late spring or summer weather, and it is difficult, if not impossible, to 

force the crop to make up for lost growth later in the season. On the 

other hand, controlling height growth can be a real problem with 

different species in the same structure, especially with fast-growing 

species like western larch or quaking aspen. Even with relatively 

slower growing species like Colorado blue spruce, shoot growth is 

directly proportional to temperature (figure 6.4.7A) and so seedlings 

can rapidly surpass target heights under warm conditions.

Optimum day and night temperatures are one of the primary 

reasons for accelerated growth in modern greenhouses. However, 

warmer is not necessarily better. For example, a "warm" growing 

regime-20 to 27 °C (68 to 80 °F)-produced more seedlings of 3 

commercial conifers and these seedlings were of higher quality 

than a "hot" regime with temperatures reaching the 30 to 35 °C (86 

to 94 °F) range. These differences were attributed to inhibitory 

effects of the higher temperatures on seedling metabolism and 

less resistance to cold injury in the fall (Owston and Kozlowski 

1981).

Operational results have proven that many species can be grown in 

the same propagation structure and most will grow acceptably 

under a relatively wide temperature range. In our example crop at 

the University of Idaho
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139
levels adequate (table 6.4.5A). (A complete discussion on 

managing and monitoring COZ during the rapid growth phase can 

be found in section 3.4.3 of volume three of this series.)

6.4.3.2 The edaphic environment

Irrigation. During the rapid growth phase, seedlings should be 

irrigated regularly to prevent any moisture stress that might limit 

growth. However, overwatering can be as injurious as 

underwatering. The moisture available to a seedling is controlled by 

the type of growing medium and container volume, and so irrigation 

rates and timing must be adjusted for different container types and 

species. Douglas-fir seedlings exhibited optimum growth and 

morphological development when the growing medium was 

maintained at a moderate moisture range of 29 to 53% water 

content (Khan and others 1996). The University of Idaho nursery 

monitors container ("block") weights to maintain the growing 

medium moisture so that the containers weigh around 80 to 85% of 

the wet weight standard (table 6.4.5A) during this phase. Enough 

water should be applied at each irrigation so that excess salts are 

leached from the container. (A detailed discussion of irrigation 

monitoring is discussed in section 4.2.6 in volume four of this 

series).

Fertilization. Seedlings respond to the type, rate, and timing of 

fertilization. During this phase, proper fertilization is one of the 

easiest and least expensive ways to accelerate growth. On the 

other hand, growers should apply fertilizers carefully to avoid the 

potential for environmental pollution.

Most nurseries compute their fertilizer use based on the nitrogen 

(N) level and an average rate of 150 ppm is recommended for most 

species during the rapid growth phase. Of course, the other 12 

essential mineral nutrients must also be supplied at the proper 

concentration. The University of Idaho nursery uses an average N 

rate of 120 ppm for their base fertigation solution (table 6.4.5A). 

Slower growing species, such as western white pine, will require 

higher fertilization rates (200 ppm N) than faster growing ones, 

such as western larch, which can get by with only 60 ppm N (table 

6.4.513). Even different ecotypes of a species can respond quite 

differently to fertilization. For example, when six ecotypes of 

Douglas-fir seedlings were grown in the same greenhouse, those 

from coastal regions in Washington required lower N rates than 

those from Montana (figure 6.4.8A).

The University of Idaho Nursery does not monitor relative humidity 

regularly but maintains ambient conditions (table 6.4.5A). Because 

relative humidity is so dependent on temperature, short mists may 

be necessary both to cool the seedlings and to raise the humidity to 

the target range during hot days when exhaust fans are on frequently. A critical time to monitor humidity is toward the end of the 

rapid growth phase when seedling crowns begin to close, because 

humidity will remain near 100% within the canopy, ideal conditions 

for foliar pathogens such as Botrytis spp. (Operational relative 

humidity and vapor pressure deficit targets and ranges for a variety 

of species during the rapid growth phase can be found in table 3.2.5 

and 3.2.6 of volume three of this series.)

Light. Light intensity rarely becomes limiting during the rapid growth 

phase because light levels still exceed the light saturation point for 

most seedlings, even on cloudy days. Under extremely high sunlight 

levels, young succulent tissue of shade-loving species could be 

damaged by solarization, although overheating is a more common 

problem. The saturation light intensity varies considerably by 

species, however, and range of values for common tree species in 

provided in table 3.3.4 of volume three of this series.

Light duration is critical to maintain high growth rates during this 

phase, and photoperiod lighting is used to extend the daylength. 

The University of Idaho nursery uses continuous intermittent 

photoperiod light during the night to keep seedlings actively growing 

(table 6.4.5A). The reliability of the lighting system should be 

checked regularly because if the lights fail for only 1 or 2 nights, 

sensitive species or ecotypes may stop height growth and set a 

resting bud. Daylength also affects the type of foliage that is 

produced in some species-see the following section on primary vs. 

secondary needles. (A complete discussion of the types of 

photoperiodic lighting and other examples of operational lighting 

systems can be found in section 3.3.4 of volume three of this 

series.)

Carbon dioxide. If a greenhouse is so equipped, the CO2 

generators should continue to operate during the early mornings of 

rapid growth phase. Although the actu al time that they are running 

will be less because solar heating will cause vents to open earlier, 

the high photo synthesis rates require a steady supply of CO2. In 

addition to generators, horizontal air flow fans help encourage good 

CO2 exchange in the growing area in addition to cooling (figure 

6.4.713). The University of Idaho nursery relies on frequent 

ventilation to keep ambient CO2

140
The timing of fertilization is critical. Nutrient uptake begins in a 

matter of minute s, but its effects on seedling growth are not 

apparent for days. This effect is especially critical for species, such 

as spruce, that exhibit free growth after a period of predetermined 

growth. For example, Norway spruce seedlings produced both 

more predetermined and free shoot growth when fertilization began 

earlier in the growing season (figure 6.4.813).

Monitoring shoot growth rates provides a good indication of how 

well seedlings are responding, and foliar tissue samples can also 

help to determine if fertilization levels are appropriate. If one 

species is lagging behind the target growth curves, increasing the N 

level of the fertilizer can accelerate shoot growth and help to make 

the target by the end of the growing season-for an example, see 

Figure 6.1 .10 in chapter 1 of this volume. (A complete discussion 

of nutritional targets and fertilization procedures is provided in 

volume four of this series.)

6.4.3.3 Cultural operations

Although the primary cultural objective of the rapid growth phase is 

to promote shoot growth, it also is important to produce a 

well-balanced seedling with a sturdy stem diameter (caliper) and 

roots that are well distributed throughout the plug. However, these 

are secondary concerns because root and stem diameter growth 

will be encouraged during the hardening phase.

Culturally controlling shoot height. One of the most common 

mistakes during the rapid growth phase is to allow shoot growth to 

continue for too long, which results in seedlings that are too tall for 

their corresponding caliper (figure 6.4.9A). These top-heavy 

seedlings are undesirable because they dry out quickly in the 

nursery and, in addition, are physically difficult to handle and 

expensive to plant. Finally, because of their excessive shoot to root 

ratio, they are subject to high moisture stress after transplanting or 

outplanting.

The key cultural controls to produce a sturdy shoot are (1) seedling 

growing density, (2) light, and (3) N fertilization rate. As far as 

growing density goes, the basic rule of thumb is the greater the 

distance between seedlings, the greater their caliper because they 

are less likely to grow too tall (figure 6.4.913). The low light intensity 

in overly-dense seedlings will cause them to grow disproportionately 

in height ("stretch"). Of course, once the seeds are sown there is no 

way to control density unless you are using single-cell containers 

that could be spaced further

41

The University of Idaho's Forest Research Nursery has grown five 

species of northwestern conifers in one greenhouse by manipulating 

N rate and the length of the fertilization period. Western white pine is 

the slowest growing species; Engelmann spruce, Douglas-fir, and 

ponderosa pine have intermediate growth rates; and western larch 

is the fastest growing. The slower the species grows, the more N the 

seedlings receive and for a longer time period (table 6.4.513).
142
143

pruning is more difficult with others such as Douglas-fir because the 

timing is so critical (figure 6.4.9C). The cultural window when shoot 

tissue is still soft enough to tolerate pruning is only a few weeks in 

most cases. Shoots that are pruned into woody tissue are 

permanently damaged and often exhibit growth abnormalities 

(figure 6.4.9D). Other conifers, such as longleaf pine, can be 

top-pruned several times during the rapid growth phase (Barnett 

and McGilvray 1997). Most broadleaved species are much more 

tolerant to top pruning and can be pruned without damage. Still, 

growers should strive to control the rate of shoot height growth and 

use top-pruning only when absolutely necessary.

Primary versus secondary needles. Besides the increased growth 

rate, one of the most visible differences between the establishment 

and rapid growth phases is the nature of seedling foliage. Several 

species of conifers and some hardwoods produce primary leaves or 

needles from the apical meristem in the middle of the cotyledons 

that are very different from mature foliage. For example, the primary 

needles of pine seedlings are flat and needlelike (figure 6.4.10A/B), 

whereas primary needles of junipers are short spikes. Some 

customers prefer their seedlings to have secondary needles (Rose 

and others 1990); for instance, lodgepole pine seedlings with secondary needles have better outplanting performance (van Steenis 

1993). Some species of pine seedlings normally produce primary 

needles during the first flush of shoot growth and, under natural 

daylength, will continue to do so throughout the first growing 

season.

apart in their racks. This is generally uneconomical (and impossible 

with block containers), so the best solution is to choose a container 

with the proper volume and spacing to produce the desired target 

seedling specifications. The traditional use of shadecloth to lower 

temperatures is not recommended because it lowers available light; 

if high temperatures are a problem, light irrigation or mistings should 

be used. Nitrogen fertilization is the one cultural factor that can be 

easily controlled in any nursery. Nitrogen acts like the "accelerator" 

in an automobile--the more you apply, the faster the shoot grows. 

The N proper concentration for the species being grown and the 

type of N fertilizer are also important. For example, fertilizers high in 

ammonium or urea will produce taller seedlings of many species 

than will those with a higher proportion of nitrate.

Top-pruning. Unfortunately, excessive shoot height is a difficult 

problem to control. Some conifer species, such as western 

redcedar, can be top-pruned easily as long as it is done before the 

tissue becomes too woody. Top-
Photoperiod treatments, such as short days ("blackout') that are 

applied for as short as 2 weeks during the summer will cause 

secondary needles to form on some species of pines such as 

Scots pine (Rosvall-Ahnebrink 1982). In an operational nursery 

trial, two morphologically different types of lodgepole pine 

seedlings were created by manipulating photoperiod. Seedlings 

with mostly primary needles were produced in greenhouses with 

natural daylength whereas seedlings with mostly secondary 

needles were the result in greenhouses with supplemental lighting 

(Omi and others 1993; Omi and Eggleston 1993). The 

primary-needled seedlings were significantly shorter and had more 

roots than those with secondary needles (figure 6.4.1013). This 

effect is species specific, however, because other pines such as 

western white pine were unaffected by the light treatments.

6.4.3.4 Pests and abiotic problems

Fusarium root rot. One of the most common diseases of 

northwestern conifer seedlings during the rapid growth phase is a 

root rot caused by Fusarium spp. These ubiquitous fungi typically 

are introduced into the nursery in growing media, on contaminated 

containers, or on seeds (figure 6.4.5). Fusarium spp. sometimes 

attack germinating seedlings causing damping-off, but more often 

produce minor root infections without obvious symptoms. Warm 

temperatures favor the Fusarium fungus, however, and so 

advanced root rot often develops during the

warm moist conditions of the rapid growth phase. Then, during the 

early stages of hardening, foliar symptoms often develop after a 

few days of high temperature and moisture stress (Sutherland 

1990). Fusarium root rot is difficult to diagnose because most 

nursery managers are unaware that they have a problem until the 

typical curled, necrotic needles develop (James and others 1991). 

When root systems of symptomatic seedlings are examined, the 

cortex easily strips away to reveal brown, decayed tissue. 

Fusarium root rot can spread from seedling to seedling through 

windborne or waterborne spores (figure 6.4.11), resulting in 

disease pockets. (Damage keys, color photographs, and more 

specific identification and control information can be found in 

section 5.1.4 of volume five of this series.)

Insects. Although insects are not a serious problem in nurseries 

that practice good pest prevention practices, some can cause small 

localized problems. The larvae of webworms (that is, grubs), crane 

flies, root weevils, and fungus gnats feed on the root systems; only 

the adults of the fungus gnats are readily visible. The type of 

chewing injury is diagnostic and so the reader is referred to the 

damage key in section 5.1.4 of volume five of this series. Lygus 

bugs and thrips can also cause distorted shoot growth in localized 

pockets of seedlings. (Damage keys, color photographs, and more 

specific identification and control information can be found in 

section 5.1.4 of volume five of this series.)

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145

roots had been killed. The problem became obvious several weeks 

after outplanting when seedlings began dying, In such cases, the 

economic impact also includes the cost of shipping and planting, 

which far exceeds the face value of the seedlings.

Although seedling mortality is the most dramatic result of non-hardy 

stock, other types of sublethal injury may not be immediately 

apparent. Roots will grow whenever temperatures are favorable 

(figure 6.4.12A) and, because they are exposed, can be easily 

damaged by heat or cold temperatures (figure 6.4.1213). Plants 

with sublethal root injury do not exhibit symptoms immediately but 

may gradually decline over time or may fall prey to some 

opportunistic pathogen. Conifer seedlings often reflect hardiness 

problems as "transplant shock," with characteristic chlorosis and 

bottlebrush foliar symptoms and consequent poor performance 

during the first growing season.

Hardiness terminology. The terminology used to describe the 

hardening process can be quite intimidating For example, one 

recent system for categorizing dormancy uses complicated terms 

such as "photoperiodic endodormancy" and "hydrational 

ecodormancy" (Lang 1987). We prefer simpler definitions. In forest 

and conservation nurseries, the terms "dormancy" and "hardiness" 

are often used interchangeably, but there are important differences. 

Hardiness can be defined as a condition of durability, or resistance 

to stress (Landis 1988). Although hardiness can refer to one 

specific stress, it stands to reason that seedlings that are cold 

hardy also are resistant to all the stresses that will be encountered 

during handling, storage, and outplanting. Cold hardiness is 

operationally useful because it is relatively easy to monitor. 

Hardening or acclimation can be defined as the process of 

inducing hardiness and occurs in concert with the dormancy 

process.

6.4.4 The Hardening Phase

6.4.4.1 Introduction

Hardening is one of the most important periods in the culture of 

forest and conservation seedlings because they must be prepared 

to endure a considerable amount of stress after they leave the 

nursery. For example, conifer seedlings in the western United 

States must be lifted when dormant, stored long periods under 

refrigeration, shipped long distances, and then outplanted in 

relatively harsh environments with no postplanting care. In Ontario, 

the success or failure of spruce crops can almost always be traced 

to poor hardening practices (Odium 1992). In the southern United 

States, properly hardened container seedlings have been shown to 

survive and grow better than those which are not (table 6.4.6).

Problems with non-hardy seedlings. Hardiness problems may not 

be apparent during nursery culture and often do not appear until 

seedlings are stored or after they are outplanted. It is relatively easy 

to grow a crop of seedlings to target height and caliper 

specifications, but it is much more difficult to harden them to endure 

the stresses of handling, storage, and outplanting. This is evidenced 

by several catastrophic losses caused by non-hardy stock in recent 

years. Container tree seedling nurseries in British Columbia lost 

over 20 million seedlings during an early cold snap in October 1985, 

valued in Canadian dollars at $4.5 million. A couple of years later, 

Ontario Ministry of Natural Resources reported a loss of 7 million 

black spruce seedlings, worth approximately $1 million, that was 

attributable to poor seedling quality (Lewis 1988). In February 1996, 

a crop of 400,000 container longleaf pine seedlings being 

overwintered in an open compound in southern Mississippi were 

exposed to unexpected low temperatures. After the freeze, the 

seedlings still looked fine, so the nursery shipped them, not realizing 

that the
Dormancy can be defined as the inability of a plant tissue to grow, 

even under environmentally favorable conditions (Lavender 1985). 

Note that dormancy refers to specific meristematic tissue 

(buds, lateral meristems, or root tips), whereas hardiness 

refers to shoots or roots and, what is more significant 

operationally, often the entire seedling. Be aware that much of 

the published literature only deals with bud dormancy and, although 

that is important, nursery managers must deal with the entire 

seedling. There is some overlap between dormancy and hardiness; 

in fact, dormancy is a prerequisite to high levels of hardiness. 

Although non-dormant tissue can harden to some degree, seedlings 

cannot achieve full hardiness if they are still growing (Weiser 1970). 

The highest levels of hardiness are reached after the plant has 

already passed through dormancy (figure 6.4.13).

Objectives of the hardening phase. A well-designed 

hardening phase can have several different objectives:

1. Manipulate seedling morphology--The first objective of the 

hardening phase is to stop shoot growth and promote bud set 

while encouraging caliper and root growth. In fact, the crop will 

grow very little in height during the hardening phase, which 

allows photosynthates to be shifted to the stem and root 

meristems (see figure 6.1.8 in this volume). At the end of the 

rapid growth phase, the seedlings are dis146
147

2. Acclimate seedlings to the natural environment--

Container seedlings have been growing at an accelerated 

rate during the rapid growth phase and therefore are very 

succulent and susceptible to a variety of stresses. For crops 

that will be outplanted during summer or fall, this is the 

primary objective of the

hardening phase, as the seedlings must rapidly acclimatize to 

the outplanting site when they leave the

nursery.

3. Develop stress resistance for handling, storage, and

shipping--When the hardening phase is completed, the crop 

will either be moved to sheltered storage or harvested and 

packed for shipment. For our example crop of western

conifers, the seedlings will be pulled from the growing

container, wrapped or bagged in

proportionately tall fro their caliper and root system and the 

hardening phase should be used to develop a study stem and 

subsequently a better shoot-to-root ratio. Because some

customers prefer mature needles on their conifer crops,

developing secondary needles may be another cultural

objective (see section 6.4.3.3). Most western conifers develop 

dormant buds that contain preformed shoot primordia for the 

next season's growth (figure 6.4.14A). Although a terminal 

bud does not develop on all species, many customers prefer 

their seedlings to have them. Likewise, the presence of

numerous lateral buds is considered desirable because they 

will help develop a full crown the following season (figure 

6.4.14 B).
plastic, and then stored under refrigeration for 2 to 4 months 

(table 6.4.1 A). Therefore, seedlings must be resistant to heat, 

cold, and moisture stress and be able to tolerate mechanical 

shocks.

4. Fortify the seedling for survival and growth after 

outplanting--Once seedlings are harvested, they will not be 

able to manufacture food through photosynthesis until they are 

established on the outplanting site. This means that they must 

have accumulated enough stored food reserves to maintain 

them for a period of days in the case of summer "hot" outplanting, or months in the case of frozen storage for spring 

outplanting at high-elevation sites.

Factors affecting seedling hardiness. There are a number of different factors that can affect seedling hardiness

in container nurseries and three are particularly

noteworthy:

1. Genetic factors--Different species go through the hardening 

process at different rates under the same environmental 

conditions. Even within a species, there are genetic differences 

in hardiness between ecotypes. For example, seedlings of four 

northwestern conifers that grow together in nature were shown 

to harden at different rates under nursery conditions. Interior 

spruce not only hardened more rapidly but also reached a 

greater level of cold hardiness than the other species (figure 

6.4.15).

2. Type of tissue--Stems, buds and foliage of a seedling harden 

at different rates (figure 6.4.16). Part of the reason for this 

variable response is due to the physical nature of different 

types of cells. Thin-walled meristematic cells are more 

vulnerable to cold injury

than are older cells with thicker walls. Thus, in shoots, the 

terminal meristem is most often damaged by cold 

temperatures because it is the last to quit growing and 

become hardy. One of the most sensitive areas on container 

tree seedlings is the root collar. The exact reason for this is 

uncertain. Either this tissue is the last to harden or the tissue 

may be more similar to root tissue and therefore less cold 

tolerant. Another consideration is that this root collar area is 

physically protected by the container and the lower

148
Scheduling the hardening phase. For practical purposes, the 

hardening phase can be divided into two consecutive time periods: 

dormancy induction and stress conditioning. In a typical growing 

schedule, container seedlings must become dormant before they 

can develop full hardiness (Weiser 1970). Because objectives are 

different for these two stages, the cultural regimes will be different. 

The dormancy induction stage "shocks" the terminal shoot into 

dormancy while still encouraging stem diameter and root growth. 

For most of the western conifers in our example crop, lateral and 

terminal buds form, enlarge in size, and become covered with tight 

bud scales.

Once shoot dormancy has been achieved, the crop can be 

stress-conditioned. Seedlings are naturally hardened by exposing 

them to ambient conditions, but it is possible to achieve a greater 

level of stress resistance in a shorter time by imposing special 

cultural treatments in the nursery. These treatments must not be 

too severe, however, because overly stressed seedlings will 

actually be less hardy. Plants that have low levels of photosynthetic reserves cannot acclimate properly (Weiser 1970). The 

relative amount of time in dormancy induction and stress 

conditioning varies with crop species and type of hardening 

regime. A combination of 3 weeks of warm temperatures/short 

photoperiod to induce dormancy followed by 3 weeks of cold 

temperatures/short photoperiod to develop stress resistance was 

found to be best for interior spruce. Lodgepole pine, on the other 

hand,

149

seedling foliage and therefore may never become as hardy as 

other parts of the stem.

Seedling root systems never become as hardy as the shoots 

and some species, such as longleaf pine, do not harden at all. 

Older, mature roots of commercial conifer seedlings from 

climates with cold winters are able to achieve moderate cold 

hardiness of -10 to -15 °C (5 to 14 °F) during late fall and 

winter. Newer roots are much less hardy than older ones, and 

young white roots harden little if at all (figure 6.4.12B). 

Because roots will grow whenever soil temperatures allow, 

they can be damaged during sudden frosts. Root injury is not 

always apparent in container stock under nursery conditions 

due to frequent irrigation but can be devastating after outplanting. For example, cold injury to Norway spruce seedlings 

resulted in a 50% reduction in root growth capacity and a 40% 

reduction in shoot growth (Lindstrom and Nystrom 1987).

3. Growth stage--Cold hardiness develops in a typical seasonal 

pattern in natural environments. Seedlings harden gradually 

through a series of stages in fall but deharden rapidly in spring 

(figure 6.4.16). Several models have been developed to 

describe growth and hardening cycles in plants. One of the 

most popular is the degree growth stage model, which shows 

the yearly cycle of plant growth as a series of different stages 

(Fuchigami and Nee 1987) that can be described in terms of 

shoot dormancy, hardiness, and stress resistance (Burr 1990). 

The model represents the annual seedling growth cycle as a 

sine wave, starting with bud break at 0% and ending with bud 

break the following season (360%) (figure 6.4.17). Active shoot 

growth occurs in the first (upper) half of the cycle with bud set 

occurring at 180%. The second (lower) half goes through shoot 

dormancy, which actually is initiated during the 90 to 180% 

stage, and ends with the post dormant phase from 315 to 

360%. Again, note that this model deals only with buds and 

does not address dormancy or cold hardiness of lateral 

meristems or roots. Roots, in particular, have been shown to 

grow whenever temperatures are favorable and therefore do 

not have a true dormancy, as such. Therefore, nursery 

managers should use the degree growth stage model as a 

good conceptual framework guide but realize that they must 

view seedling dormancy and hardiness in a true holistic sense.
their crops in greenhouses move seedlings to a shadehouse or 

open-growing compound at the end of the rapid growth phase so 

that the stock will be exposed to the ambient environment. Growers 

with poly-covered structures sometimes remove the greenhouse 

roof to achieve the same effect. Semi-controlled environments, 

such as shelterhouses, are ideal structures for hardening because 

it is easy to roll up the sides and expose the crops to ambient 

conditions. The latest models of semicontrolled structures feature 

retractable roofs that are controlled by computers. The roof can be 

opened at the beginning of the hardening phase and closed in the

case of frost. The University of Idaho's Forest Research Nursery 

currently hardens its seedlings in the propagation greenhouse 

(table 6.4.7). Some fast-growing species, such as western larch, 

are moved to an outdoor growing compound to begin the hardening 

phase. (See section 1.3.2 in volume one of this series for a 

complete discussion on the attributes of various propagation 

environments.)

Regardless of the type of propagation environment, four factors are 

especially critical to inducing and maintaining

required a longer (5 to 7 weeks) stress conditioning period for a 

total hardening period of 8 to 10 weeks (Simpson and Macey 

1992).

Scheduling enough time for the hardening phase is one of the most 

critical concepts in container seedling culture, but it is not 

appreciated by inexperienced nursery managers and especially 

novices growing their first crops. Proper hardening takes time, and it 

is a common mistake to not allow enough. This often happens when 

more than one crop is grown per season or when growers try to 

force a little extra height growth with crops that grow more slowly 

than expected.

Many growers do not appreciate the fact that stem and root growth 

require a steady supply of photosynthate, so the hardening phase 

must be scheduled when there is still enough solar energy to fuel 

this growth. While it is possible to achieve stress conditioning during 

the late fall and early winter, there is just not enough sunlight at this 

time of year to support caliper and root growth. After seedlings 

reach their target height at the end of the rapid growth phase, they 

need about 2 months to continue growing stem tissue and roots and 

then harden enough to tolerate the stresses of harvesting, storage, 

shipping and outplanting. Mexal and others (1979) found that 

loblolly pine container seedlings required at least 6 weeks of 

hardening, whereas Douglas-fir seedlings exposed to a sequence of 

hardening treatments required 18 weeks for full hardening (figure 

6.4.18). For our example of western conifers at the University of 

Idaho nursery, the hardening phase typically takes from 2 to 4 

months (table 6.4.1 A).

6.4.4.2 The atmospheric environment

In this section, we will discuss the same potentially growth-limiting 

environmental factors as in the establishment and rapid growth 

phases but, in addition, nurseries often change the propagation 

structure to initiate the hardening phase.

Propagation structures. Whereas the establishment and rapid 

growth phases are often easiest in fully controlled propagation 

structures such as greenhouses, this is not always the case for 

hardening. It is difficult to induce full hardening in a greenhouse. For 

example, Scots pine seedlings that were hardened in a greenhouse, 

even with short photoperiod treatments, suffered more postoutplanting injury than those receiving ambient treatments (figure 

6.4.19). Traditionally, growers who produce

150
hardiness in forest and conservation seedlings: temperature, 

moisture, mineral nutrients, and photoperiod (figure 6.4.20). 

Hardening treatments are applied simultaneously or in sequence. 

Douglas-fir seedlings reached maximum hardiness when exposed to 

chilling temperatures, short photoperiods, and finally an exposure to 

below-freezing temperatures (figure 6.4.18). By contrast, only one of 

these factors is important to the dehardening process--temperature 

(van den Driessche 1969)--which is of critical importance during the 

storage period (see volume seven in this series.)

Temperature. Because temperature has such a pervasive effect 

on physiological processes, the control of day and night 

temperature in the propagation area is crucial to the hardening 

process. Exposure to cold temperatures that simulate normal fall 

conditions affects many aspects of seedling dormancy and cold 

hardiness, and a combination of cold temperatures and shortened 

photoperiod is often effective. For example, a sudden switch to 

short photoperiod under warm temperatures followed by a change 

to cool temperatures induces dormancy and develops the 

maximum stress resistance of northern conifers (Simpson and 

Macey 1992).

Dormancy induction. Because the objective of this stage is to stop 

height growth but encourage stem and caliper growth, the 

temperatures in the growing area are gradually reduced. This has 

the effect of maintaining sufficient rates of photosynthesis and 

respiration to promote caliper and root growth. Changing to cool 

temperatures affects the timing of growth cessation and bud set and 

dormancy initiation (Fuchigami and Nee 1987) as well as the 

number of primordia that form in the developing bud (Templeton and 

others 1993). Moderately warm temperatures are required to allow 

the number and size of the primordia to increase; for example, night 

temperatures of 15 to 20 °C (59 to 68 °F) were found to promote 

bud initiation and development in white spruce seedlings (Odium 

1992).

The final effect of temperature during the hardening phase is 

dormancy release. The bud dormancy of most woody plants is 

released by long-term exposure to temperatures slightly above 

freezing-5 to 7 °C (40 to 45 °F). This time/temperature treatment is 

commonly known as the chilling requirement (Burr and others 

1989). Other species require exposure to freezing. For example, two 

species of birch seedlings broke bud more quickly and complete ly 

when exposed to below-freezing temperatures after reaching full 

dormancy (Rinne and others 1997).

During the hardening phase, the University of Idaho nursery 

gradually lowers the day and night temperatures in the greenhouse 

at 5 to 10 °F increments until they reach their target temperatures 

(table 6.4.7). This begins the acclimatization process but keeps the 

temperatures warm enough to develop stem diameter and root 

growth (Wenny and Dumroese 1998).

Stress conditioning. Most temperate zone seedlings must be able 

to tolerate temperatures well-below freezing during outside 

overwinter storage or after outplanti ng. When about 90% of the crop 

has reached the target height and bud set is complete, 

temperatures can be lowered to begin conditioning the seedlings. 

Night temperatures have been shown to be more important than 

day temperatures for developing cold hardiness in Douglas-fir (van 

den Driessche 1969), Norway spruce, and Scots pine (Aronsson 

1975). One interesting but not widely appreciated fact is that some 

species can be hardened to below-freezing temperatures without 

ever subjecting them to these temperatures. For example, Tinus 

(1974) was able to fully harden several western conifers to tolerate 

-30 °C (22 °F) with a maximum day tempera151
152
ture of 10 °C (50 °F) and a night temperature just above freezing.

Around the first week in November, the University of Idaho nursery 

continues the hardening process by exposing seedlings to ambient 

temperatures. This prepares them for the freezing temperatures 

they will experience in late fall and during storage. To keep root 

plugs from freezing solid and to protect the irrigation system, temperatures in the greenhouses are not allowed to go lower than 28 

°F (-2 °C) (Wenny and Dumroese 1998). (Operational target 

temperatures and ranges for a variety of species during the 

hardening phase can be found in table 3.1 .2 of volume three of this 

series.)

Humidity. During the hardening phase, relative humidity is 

maintained at ambient levels, which helps prepare seedlings for the 

conditions they will experience on outplanting sites. Irrigations are 

done early in the morning and the greenhouse is ventilated 

immediately afterwards to exhaust humid air and help dry seedling 

foliage (table 6.4.7). This is a critical procedure to prevent 

condensation, which favors fungal development. Backpack leaf 

blowers work well to dry foliage and stimulate caliper development. 

(Operational relative humidity and vapor pressure deficit targets and 

ranges for a variety of species during the hardening phase can be 

found in tables 3.2.5 and 3.2.6 of volume three of this series.)

Light. Both light intensity and duration are important to the 

hardening process and daylength (photoperiod) is a critical factor for 

species and ecotypes from northern latitudes and high elevations. Species may even have photoperiodic 

ecotypes-Scots pine seedlings from northern latitudes stopped 

shoot growth up to 50 days earlier than those from more southerly 

locations (Oleksyn and others 1992). Those species from coastal 

climates and lower latitudes are less affected by shortened 

photoperiod treatments.

Dormancy induction. A short photoperiod treatment is one of the 

most important environmental factors triggering the termination of 

shoot growth and formation of buds in western conifers. Photoperiod 

lights extend the natural daylength during the rapid growth phase, 

and sometimes just shutting-off these lights will induce a rapid bud 

set. Growers should be aware that it is the relative rather than the 

absolute photoperiod that is effective. For example, seedlings that 

were grown under a 24hour intermittent photoperiod initiate 

hardening under a 18-hour treatment even though the latter is their 

normal summer daylength. For seedlings from northern latitudes 

that will be outplanted during summer, blackout curtains have been 

used to shorten the daylength to 8 or 10 hours to induce dormancy. 

With Scots pine, it has been demonstrated that short days are the 

most important cultural treatment for inducing dormancy and cold 

hardiness (figure 6.4.19). The effect of shortened photoperiod can 

happen quickly. Birch seedlings responded to shortened photoperiod treatments after only 5 days by slowing shoot growth, 

which ceased completely after 10 days (Rinne and others 1997). As 

mentioned earlier, sunlight intensity must remain high enough 

during this stage of the hardening phase to promote growth of buds, 

stems, and roots.

Stress conditioning. Short photoperiods also help induce cold 

hardiness in many species, especially when combined with cold 

temperatures. A short (8-hour) photoperiod was found to induce 

cold hardiness levels in loblolly pine comparable to seedlings that 

had been acclimated naturally outdoors (Mexal and others 1979).

The University of Idaho nursery shuts off the photoperiod lights at 

the beginning of the hardening phase. This, combined with mild 

nutrient and moisture stress, slows height growth, promotes 

terminal buds, and begins the stress acclimatization process (table 

6.4.7). (Wenny and Dumroese 1998). (The cultural management of 

light during the hardening phase is discussed in section 3.3.3.4 of 

volume three of this series.)

153
Carbon dioxide. Carbon dioxide does not have a significant effect 

on the hardening process and so nurseries that USE! generators 

leave them running through the dormancy induction stage. High 

carbon dioxide levels retard normal leaf abscission of some 

broadleaved species, however, and so generators should be shut 

off at the beginning of stress conditioning. The University of Idaho's 

Forest Research Nursery does not use carbon dioxide generators 

but ventilates to encourage good air exchange (table 6.4.7) (Wenny 

and Dumroese 1998). (A complete discussion on managing and 

monitoring CO2 during the hardening phase can be found in section 

3.4.3 of volume three of this series.)

6.4.4.2 The edaphic environment

Irrigation. A mild moisture stress has been shown to reduce shoot 

growth and promote dormancy and hardiness in some species of 

container seedlings such as Colorado blue spruce and Douglas-fir. 

With other species, however, even moderate moisture stress can be 

detrimental to the hardening process. For example, moisture stress 

had no effect on induction of shoot dormancy in western hemlock 

and actually inhibited the beneficial effects of the other dormancy 

treatments (O'Reilly and others 1989). Mild moisture stress should 

be considered a hardening technique only for species that have 

demonstrated a positive response and for those species where 

other treatments are ineffective.

Dormancy induction. Inducing moderate levels of plant moisture 

stress can be achieved by withholding irrigation and lowering 

humidity. The trick is to allow the seedlings to dry clown until they 

have just begun to wilt. Then, they should be irrigated and 

maintained under a mild moisture stress for several weeks. This 

procedure is more of an art than a science however, and requires 

constant vigilance to make sure that stresses do not reach injurious 

levels. When this happens, the apical meristem tissues where cells 

are actively dividing and elongating are injured. In the case of a mild 

stress, the meristems will recover but when stresses reach severe 

levels the damage can be long-term ("blown tops") (figure 6.4.21 A). 

For example, a moderate plant moisture stress (PMS) treatment of 

-1.5 MPa induced bud set and shoot dormancy in blue spruce 

seedlings but, if the stress reached higher PMS levels of -1 .8 to -2.0 

MPa, foliar injury occurred (Young and Hanover 1978). Periodic 

moisture stress treatments of -1.7 MPa was effective in inducing 

termi nal bud formation in white spruce although the number of 

needle primordia was significantly lower (figure

154
problems during the hardening period because the continued 

release of N may delay onset of dormancy and hardiness. 

However this is not the case with newer controlled-release 

fertilizers, which give growth benefits after outplanting.

Dormancy induction. In general, the continued application of 

high-N fertilizers promo tes shoot growth and succulence in 

container seedlings. For example, red maple seedlings grown at 

high (300 ppm) N levels retained their leaves about 3 weeks longer 

than those grown at more normal rates (Gilliam and others 1980). 

Reducing N concentration in the fertigation solution is a standard 

cultural technique at the start of the hardening phase and some 

nurseries withhold nitrogen completely for a few weeks to induce a 

stress. Nitrogen stressing is commonly used for species, such as 

western larch, that growth rapidly in height and do not respond to 

other hardening techniques (figure 6.4.22A). Eliminating fertilization 

for 2 weeks reduced the number of bud primodia in white spruce 

seedlings (figure 6.4.21 B), but Bigras and others (1996) found no 

evidence that N fertilization had any effect on the number of bud 

primordia of black spruce. Inducing N stress in Colorado blue 

spruce seedlings caused bud set but the buds were small and 

light-colored (Young and Hanover 1978).

Stress conditioning. It is widely known that high fertilization levels, 

especially of N, cause plants to grow later into the fall when they 

are more susceptible to cold injury. Pellett and Carter (1981) did an 

exhaustive literature review and concluded that seedlings grown at 

normal fertilization rates will be more cold hardy, and therefore 

stress resistant, than those grown under either very low or 

excessively high fertilization levels. This has been confirmed with 

research on commercial conifer seedlings. Although it varies with 

plant species, fertigation solutions with a N concentration of from 50 

to 100 ppm during the hardening phase should produce seedlings 

with a foliar N concentration in the range of 2.0 to 2.5%.

Nutrient-deficient seedlings have problems with dormancy and are

just as susceptible to cold and other stresses as those that have 

been over fertilized. Black spruce seedlings grown under 

suboptimal fertilization were less hardy than those with higher foliar 

nitrogen levels (Bigras and others 1996). The same conclusion was 

reached in a study with Scots pine as well-fertilized seedlings had 

less cold injury than those which were nutrient deficient (Rikala and 

Repo 1997). Over fertilizing is a more com55

6.4.21 B). Tinus (1974) concluded that moisture stress should be 

considered a temporary shock treatment and not maintained any 

longer than necessary (a complete discussion of moisture stress 

terminology and treatments can be found in section 4.2.2 of 

volume four of this series).

Stress conditioning. Moisture stress in combination with other 

cultural practices is sometimes used for hardening some species of 

seedlings. A sequence of moderate moisture stress (-1 .0 MPa at 

mid-day) followed by a shortened photoperiod and a low-nitrogen 

fertilizer regime was recommended for coastal ecotypes of 

Douglas-fir and ponderosa pine (Lavender and Cleary 1974). In 

another study, however, water stress alone was shown to be 

ineffective in the hardening of coastal Douglas-fir seedlings unless 

followed by a period of cold temperatures (Tanaka and Timmis 

1974).

Monitoring block weights is a popular way of regulating irrigation in 

many western container nurseries and so this technique is 

particularly useful during the hardening process. As seedlings 

transpire, the weight of the block decreases until some 

predetermined level when the seedlings are irrigated. Container 

weight scales can be developed from experience and observation or 

from soil moisture retention curves and provide a visual and 

repeatable method of determining when moisture stress has 

reached a mild levels. For example, the University of Idaho nursery 

allows the weight of the containers to drop to 70 to 80% of the wet 

weight before irrigating (table 6.4.7) (Wenny and Dumroese 1998). 

Of course, this will vary considerably between species and with 

weather conditions (see section 4.2.7 in volume four of this series 

for details).

Fertilization. Lowering mineral nutrient levels in the growing 

medium will logically slow shoot growth, and so reducing or even 

temporarily eliminating fertilizer is used to shock seedlings into 

dormancy and hardiness. just as high nitrogen (N) is one of the 

primary cultural factors used to stimulate shoot growth during the 

rapid growth phase, lowering the N level is a logical and effective 

way to control height and induce hardiness (Young and Hanover 

1978). Specialized fertilizers have been developed for the hardening 

period in container nurseries, but recent operational trials have 

shown that they are really not necessary. Nitrate, rather than 

ammonium, and increased calcium levels have also proven 

beneficial in hardening fertilizer solutions. Some slow-release fertilizers incorporated into the growing medium can cause
Inoculating with mycorrhizal fungi. Probably more research has 

been done on mycorrhizae than on any other single aspect of 

nursery culture. Yet, most nursery managers are either unsure 

about whether their seedlings have mycorrhizae or have no idea of 

which organisms are involved. A mycorrhiza is the anatomical 

structure resulting from the symbiotic association between a plant 

root and a fungus. There are two main types that are distinguished 

by their morphology: ectomycorrhizae (ECM) and 

endomycorrhizae--which are more correctly known as 

vesicular-arbuscular mycorrhizae (VAM). What type is present at a 

nursery will depend on what species of seedlings are being grown. 

ECM are the mycorrhizae that are most often noticed in forest and 

conservation nurseries because of their mushroom fruiting bodies 

(figure 6.4.23A) or the colored sheath of fungal hyphae with 

surrounding mycelia can be seen with a hand lens on the short 

feeder roots (figure 6.4.23B).

When considering inoculation with mycorrhizal fungi, growers 

should think about what they hope to gain. The benefits of 

mycorrhizae can be separated into nursery effects and outplanting 

effects. The experience at many nurseries has been that 

high-quality crops can be grown without mycorrhizal inoculation 

because the nursery environment supplies all the growth 

requirements of a seedling. The other major nursery benefit of 

mycorrhizae is protection against root pathogens but, with a sterile

mon problem in container nurseries, however, as numerous studies 

have shown that seedlings with high nitrogen contents suffer more 

cold injury than those grown under more moderate fertilization 

(figure 6.4.22B). (See section 4.1 .10 in volume four of this series 

for more information on the effects of heavy fertilizer use.)

The University of Idaho nursery uses mild moisture and nutrient 

stress at the beginning of the hardening phase to initiate bud set 

and begin the cold acclimatization process, and continues the 

moisture stress until harvest (table 6.4.7) (Wenny and Dumroese 

1998). Foliar fertilizers can be used to recharge seedling reserves 

after nutrient stress and as a way to fertilize when irrigations are 

undesired in order to maintain mild moisture stress (Montville and 

others 1996).

6.4.4.3 Cultural operations

There are few special cultural operations during the hardening 

phase beyond the normal procedures discussed earlier. However, 

many customers are requesting that their seedlings be inoculated 

with beneficial microorganisms such as mycorrhizal fungi. Although 

there are three possible times to inoculate (at the time of sowing, 

during the crop cycle, or during outplanting), the use of low-N 

fertilizer and the presence of many new root tips make the 

hardening phase an ideal time.

156
157
tainers are filled. ECM fungal spores can also be applied in a water 

suspension either by hand, or through the existing irrigation 

system starting as soon as seedlings have enough roots for 

successful colonization. Many nursery cultural practices, especially 

high fertilization rates, inhibit development of mycorrhizae and so 

inoculating during the hardening phase with its lower N fertilization 

has some merit. The drawback is that roots may already be 

infected with other mycorrhizal fungi, especially the ubiquitous T. 

terrestris, which thrives in the nursery environment.

Cost effectiveness. Mycorrhizal inoculation must make sense from 

an economic as well as biological point of view. If inoculation is 

believed to improve seedling quality, a cost-benefit analysis should 

be performed. The total cost of inoculation (including inoculum price 

and application costs) should be compared to the benefits either in 

the nursery or on the outplanting site. The savings associated with 

less fertilizer, disease reduction, higher seedling survival, and 

increased growth need to be documented. The marketability of 

seedlings that have been inoculated with a specific mycorrhizal 

fungus may be a significant benefit. A nursery could advertise that 

its seedlings were mycorrhizal and stress the associated benefits, 

just like they do for seedling size, seed origin, and vigor.

The decision whether to inoculate seedlings is complicated. So, 

unless there is a good understanding of the various fungi/host 

combinations and the inoculation process, a mycorrhizal 

consultant may be helpful. (A comprehensive discussion of 

mycorrhizae in nurseries can be found in volume five of this 

series.)

6.4.4.4 Pests and abiotic problems

The major pest during the hardening phase is the fungus Botrytis 

cinerea, which causes a foliar blight called gray mold. Because this 

fungus thrives in moist conditions with low light intensity, it becomes 

more of problem at the time of canopy closure. Botrytis has a wide 

host range, including most weeds, so its spores are ubiquitous 

around nurseries (figure 6.4.24A). Another reason why this disease 

is so prevalent during the hardening phase is that cooler 

temperatures favor condensation of moisture, which promotes both 

spore germination and growth of fungal mycelia (figure 6.4.24B). 

Disease pockets can quickly develop and therefore frequent 

scouting is necessary (figure 6.4.24C; table 6.4.7).

growing medium and containers, these pests should not be much 

of a problem. One of the most widely advertised benefits of 

mycorrhizae is increased survival and growth on afforestation 

sites. And, of course, one of the most important benefits is from a 

marketing standpoint because seedlings with well-developed 

mycorrhizae are widely considered to be high-quality nursery 

stock.

When considering a nursery inoculation program, several things 

need to be considered:

1. The proper species of fungi for each crop

2. The most appropriate type of inoculum for your

nursery system

3. The proper timing and technique

4. Cost effectiveness

Species selection. There are two possible routes to take when 

selecting a mycorrhizal fungus for inoculation: (1) selection of a 

species adapted to a broad range of hosts or site conditions, and (2) 

selection of a species adapted to a specific host or particular type of 

outplanting site. Obviously, the first step is to select a species that 

can colonize the plant species that is desired. Most VAM fungi and 

ECM species such as Cenococcum geophilum, Pisolithus tinctorius, 

and Thelephora terrestris have broad host ranges. Broadly adapted 

mycorrhizal fungi are advantageous because many nursery crops 

can be inoculated at the same time, and inoculated seedlings would 

be adapted to a wide variety of outplanting site conditions. On the 

other hand, host-specific or site-specific mycorrhizal fungi will 

produce maximum seedling performance in a given application.

Types of mycorrhizal inocula. There are two basic cate gories of 

inocula currently being used in forest and conservation nurseries: 

spores and vegetative inocula (Figure 6.4.23C). Fungal spores of 

ECM fungi are obtained from fruiting bodies collected from wild 

stands, or vegetative inoculum is produced from fungal mycelia 

grown in pure culture on an artificial medium. Both spore inoculum 

and vegetative inoculum are available commercially. Several firms 

have developed sophisticated techniques for culturing ECM fungi on 

artificial media. Other companies collect fruiting bodies of ECM 

fungi, harvest the spores, and sell them to nurseries.

How and when to inoculate. In container nurseries, ECM fungal 

spores can be applied to seeds before sowing. Vegetative 

inoculum of ECM or VAM fungi can be incorporated into the 

growing medium before the con158
Fungicides can be effective in preventing development of the 

disease but do little to eradicate existing infections. Also, there are 

strains of Botrytis that have become resistant to many common 

fungicides, making it necessary to use several chemicals in rotation 

for effective control. Integrated pest management (IPM) is a better 

option, as cultural controls have proven quite effective. Using containers with more space between cells, managing the timing of 

irrigation, providing good ventilation, and roguing diseased individuals can greatly reduce the chances of 

developing a serious problem (Russell 1990). Some growers inject 

a surfactant during irrigation that helps foliage dry more quickly. The 

University of Idaho nursery has greatly reduced its use of fungicides 

through an aggressive IPM program (Dumroese and others 1990) 

and uses cultural controls such as vacuuming dead needles from 

western larch to reduce Botrytis infections (Dumroese and Wenny 

1992).

159
160

Discussion of the potentially limiting factors of the ambient 

environment (temperature, humidity, light, and carbon dioxide) and 

the edaphic environment (water and mineral nutrients) provide 

growers with an idea of how to maximize growth and improve 

seedling quality. Sections on typical cultural operations and pests 

and abiotic stresses help growers prevent problems at each phase 

of the growing season.

6.4.5 Summary

Once a nursery has decided to propagate a species or group of 

plants, it is necessary to develop propagation protocols and growing 

schedules. We followed a typical crop of western conifer seedlings 

from sowing to hardening. Species should be grouped together in 

the growing area according to their cultural requirements. The 

propagation environment and cultural practices need to be planned 

for each of the three seedling development phases: establishment, 

rapid growth, and hardening.
6.4.6 Literature Cited

Aronsson A. 1975. Influence of photo- and thermoperiod on the 

initial stages of frost hardening and dehardening of 

phytotron-grown seedlings of Scots pine (Pinus sylvestris L.) 

and Norway spruce (Picea abies (L.) Karst.). Stud ia Forestal ia 

Suecica128: 1-20.

Aronsson A. 1980. Frost hardiness in Scots pine (Pinus sylvestris 

L.): 2. Hardiness during winter and spring in young trees of 

different mineral nutrient status. Studia Forestalia Suecica 155: 

1-27.

Barnett JP, McGilvray JM. 1997. Practical guidelines for producing 

longleaf pine seedlings in containers. Gen. Tech. Rep. 

SRS-14. Asheville, NC: USDA Forest Service, Southern 

Research Station. 28 p.

Barnett JP, Brissette JC. 1986. Producing southern pine seedlings 

in containers. Gen. Tech. Rep. SO-59. New Orleans: USDA 

Forest Service, Southern Forest Experiment Station. 71 p.

Bigras FJ, Gonzalez A, D'Aoust AL, Hebert C. 1996. Frost 

hardiness, bud phenology and growth of containerized Picea 

mariana seedlings grown at three nitrogen levels and three 

temperature regimes. New Forests 12(3): 243-259.

Burr KE. 1990. The target seedling concepts: bud dormancy and 

cold-hardiness. In: Rose R, Campbell SJ, Landis TD, eds. 

Target seedling symposium. Proceedings, Combined Meeting of 

the Western Forest Nursery Associations. Gen. Tech. Rep. 

RM-200. Ft. Collins, CO: USDA Forest Service, Rocky Mountain 

Forest and Range Experiment Station: 79-90.

Burr KE, Tinus RW, Wallner SJ, King RM. 1986. Comparison of 

four cold hardiness tests on three western conifers. In: Landis 

TD, ed. Western Forest Nursery Council and Intermountain 

Nursery Association Meeting, Combined Proceedings. Gen. 

Tech. Rep. RM-137. Ft. Collins, CO: USDA Forest Service, 

Rocky Mountain Forest and Range Experiment Station: 80-87.

Burr KE, Tinus RW, Wallner SJ, King RM. 1989. Relationships 

among cold hardiness, root growth potential and bud 

dormancy in three conifers. Tree Physiology 5(3): 291-306.

Dumroese RK, Wenny DL. 1992. Reducing Botrytis in 

container-grown western larch by vacuuming dead needles. 

Tree Planters' Notes 43(2): 30-32.

Dumroese RK, Wenny DL, Page-Dumroese DS. 1995. Nursery 

waste water: the problem and possible reme dies. In: Landis TD, 

Cregg B, tech. coords. National Proceedings, Forest and 

Conservation Nursery Associations. Gen. Tech. Rep. 

PNW-GTR-365. Portland, OR: USDA Forest Service, Pacific 

Northwest Station: 89-97.

Dumroese RK, Wenny DL, Quick KE. 1990. Reducing pesticide 

use without reducing yield. Tree Planters' Notes 41(4): 

28-32.

Emery DE. 1988. Seed propagation of native California plants. 

Santa Barbara, CA: Santa Barbara Botanic Garden. 115.

Fuchigami LH, Nee CC. 1987. Degree growth stage model and 

rest-breaking mechanisms in temperature woody perennials. 

HortScience 22(5): 836-845.

Gilliam CH, Still SM, Moor S, Watson ME. 1980. Effects of three 

nitrogen levels on container-grown Acer rubrum. HortScience 

15(5): 641-642.

Green Timbers Nursery. 1993. Personal communication. Surrey, 

BC: British Columbia Ministry of Forests, Green Timbers 

Nursery.

Holopainen JK. 1988. Growth and visible responses of Scots 

pine (Pinus silvestris L.) seedlings to simulated summer frost. 

European Journal of Forest Pathology 18(2): 85-92.

Holopainen JK, Holopainen T. 1988. Cellular responses of Scots 

pine (Pinus silvestris L.) seedlings to simulated summer frost. 

European Journal of Forest Pathology 18(3/4): 207-216.

James RL, Dumroese RK, Wenny DL. 1991. Fusarium diseases of 

conifer seedlings. In: Sutherland JR, Glover SG, eds. 

Proceedings, First Meeting of IUFRO Working Party S2.07-09, 

Diseases and Insects in Forest Nurseries. Info. Rep. BC-X-331. 

Victoria, BC: Forestry Canada, Pacific Forestry Centre: 181-190.

James RL, Dumroese RK, Wenny DL. 1994. Fungi carried by adult 

fungus gnats (Diptera: Sciaridae) in Idaho greenhouses. Pest 

Rep. 94-5. Missoula, MT: USDA Forest Service, Northern 

Region. 10 p.

Jones L. 1961. Effect of light on germination of forest tree 

seed. Proceedings of the International Seed Testing 

Association 26(3): 437-452.

Khan SR, Rose R, Haase DL, Sabin TE. 1996. Soil water stress: 

its effects on phenology, physiology, and morphology of 

containerized Douglas-fir seedlings. New Forests 12(1): 19-39.

Landis, T.D. 1988. Cold hardiness: conditioning plants to 

promote hardiness and dormancy. The Digger (January 

1988): 17-19.

Lang GA. 1987. Dormancy: a new universal terminology. 

HortScience 22(5): 817-820.

Lavender DP. 1985. Bud dormancy. In: Duryea ML, ed. 

Evaluating seedling quality: principles, procedures, and 

predictive abilities of major tests. Corvallis, OR: Oregon State 

University, Forest Research Laboratory: 7-15.

161
Owston PW, Kozlowski TT. 1981. Growth and cold hardiness of 

container-grown Douglas-fir, noble fir, and Sitka spruce 

seedlings in simulated greenhouse regimes. Canadian Journal 

of Forest Research 11 465-474.

O'Reilly C, Arnott JT, Owens JN. 1989. Effects of photoperiod 

and moisture availability on shoot growth, seedling 

morphology, and cuticle and epicuticular wax features of 

container-grown western hemlock seedlings. Canadian 

Journal of Forest Research 19: 122-131 .

Pawuk WH. 1982. The effects on growth of transplanti ng 

germinating seeds into containers. Tree Planters' Notes 33(1): 

38-39.

Pellett HM, Carter JV. 1981. Effect of nutritional factors on cold 

hardiness of plants. Horticultural Reviews 3: 144-171.

Powell GR. 1982. A comparison of early shoot development of 

seedlings of some trees commonly raised in the Northeast of 

North America. In: Proceedings, Northeastern Area 

Nurserymen's Conference, 1982 July 25-29; Halifax, NS. Truro, 

NS: Nova Scotia Department of Lands and Forests: 1-25.

Rikala R, Repo T. 1997. The effect of late summer fertilization on 

the frost hardening of second-year Scots pine seedlings. New 

Forests 14: 33-44.

Rinne P, Hanninen H, Kaikuranta P, Jalonen JE, Repo T. 1997. 

Freezing exposure releases bud dormancy in 8etula 

pubescens and 8. pendula. Plant, Cell and Environment 

20(9): 1199-1204.

Rose R, Carlson WC, Morgan P. 1990. The target seedling 

concept. In: Rose R, Campbell SJ, Landis TD, eds. 1990. Target 

seedling symposium: Proceedings, Combined Meeting of the

Western Forest Nursery Associations; 1990 August 13-17; 

Roseburg, OR: Gen. Tech. Rep. RM-200. Ft. Collins, CO: USDA 

Forest Service, Rocky Mountain Forest and Range Experiment 

Station: 1-8.

Rosvall-Ahnebrink G. 1982. Practical application of dormancy 

induction techniques to greenhouse-grown conifers in Sweden. 

In: Scarratt JB, Glerum C, Plexman CA, eds. Proceedings, 

Canadian Containerized Tree Seedling Symposium. COJFRC 

Symp. Proc. O-P-10. Sault Ste. Marie, ON: Canadian Forestry 

Service, Great Lakes Forest Research Centre: 163-170.

Russell K. 1990. Gray mold. In: Hamm PB, Campbell SJ, Hansen 

EM, eds. Growing healthy seedlings: identifi cation and 

management of pests in Northwest forest nurseries. Spec. Pub. 

19. Corvallis, OR: Oregon State University, Forest Research 

Laboratory: 10-13.

Lavender DP, Cleary BD. 1974. Coniferous seedling production 

techniques to improve seedlings establishment. In: Tinus RW, 

Stein WI, Balmer WE, eds. Proceedings, North American 

Containerized Forest Tree Symposium; 1974 August 26-29; 

Denver, CO. Pub. 68. Great Plain Agricultural Council: 177-180.

Lewis FA. 1988. Who killed cock robin? Silviculture Magazine 

3(1): 18-19.

Lindstrom A, Nystrom C. 1987. Seasonal variation in root 

hardiness of container-grown Scots pine, Norway spruce, and 

lodgepole pine seedlings. Canadian Journal of Forest Research 

17: 787-793.

Macey DE, Arnott JT. 1986. The effect of moderate moisture and 

nutrient stress on bud formation and growth of container-grown 

white spruce seedlings. Canadian Journal of Forest Research 

16(5): 949-954.

Mexal JG, Timmis R, Morris WG. 1979. Cold-hardiness of 

containerized loblolly pine seedlings. Southern Journal of 

Applied Forestry 3: 15-19.

Molina R, O'Dell T, Luoma D, Amaranthus M, Castellano M, Russell 

K. 1993. Biology, ecology, and social aspects of wild edible 

mushrooms in the forests of the Pacific Northwest: a preface to 

managing commercial harvest. Gen. Tech. Rep. PNW-GTR-309. 

Portland, OR: USDA Forest Service, Pacific Northwest Research 

Station. 42 p.

Montville ME, Wenny DL, Dumroese RK. 1996. Impact of foliar 

fertilization on container-grown ponderosa pine seedling 

viability. Western Journal of Applied Forestry 11(4): 114-119.

Odlum KID. 1992. Hardening and overwintering container stock 

in Ontario: practices and research. In: Donnelly, F.P.; 

Lussenburg, H.W. comp. Proceedings of the 1991 Forest 

Nursery Association of British Columbia Meeting, Prince 

George, BC: 29-35.

Oleksyn J, Tjoelker MG, Reich PB. 1992. Growth and biomass 

partitioning of populations of European Pinus sylvestris L. under 

simulated 50° and 60° degrees N daylengths: evidence for 

photoperiodic ecotypes. New Phytologist 120(4): 561-574.

Omi SK, Eggleston KL. 1993. Photoperiod extension with two 

types of light sources: effects on growth and development of 

conifer species. Tree Planters' Notes 44(3): 105-112.

Omi SK, Eggleston KL, Marshall JD, Wenny DL. 1993. Primary vs. 

secondary needle development in lodgepole pine: update on 

current investigations from Coeur d' Alene Nursery. In: 

Proceedings, 12th Annual Meeting of the Forest Nursery 

Association of British Columbia; 1992 September 12-October 

1; Penticton, BC. Vernon: BC Ministry of Forests: 35-42.

162
Timmer VR, Armstrong G, Miller BD. 1991. Steady-state nutrient 

preconditioning and early outplanting performance of 

containerized black spruce seedlings. Canadian Journal of 

Forest Research 21 : 585-594.

Timmis R, Worrall J. 1975. Environmental control of cold 

acclimation in Douglas-fir during germination, active growth, 

and rest. Canadian Journal of Forest Research 5: 464-477.

van den Driessche R. 1969. Influence of moisture supply, 

temperature, and light on frost-hardiness changes in 

Douglas-fir seedlings. Canadian Journal of Botany 

47:1765-1772.

van Steenis E. 1993. Lodgepole pine culture: current trends in B.C. 

In: Proceedings, 12th Annual Meeting of the Forest Nursery 

Association of British Columbia; 1992 September 12-October 

1; Penticton, BC. Vernon; BC Ministry of Forests: 93-96.

von Wuehlisch GV, Muhs HJ. 1991. Environmental influences 

on juvenile shoot growth in Picea abies. Scandavian Journal 

of Forest Research 6: 479-498.

Weiser CJ. 1970. Cold resistance and acclimation in woody 

plants. HortScience 5(5): 403-410.

Wenny DL, Dumroese RK. 1998. Personal communication. 

Moscow, ID: University of Idaho, College of Forestry, Wildlife, 

and Range Sciences.

Wood B. 1994. Conifer seedling grower guide. Smoky Lake, AB. 

Environmental Protection. 73 p.

Young E, Hanover JW. 1978. Effects of temperature, nutrient, 

and moisture stresses on dormancy of blue spruce seedlings 

under continuous light. Forest Science 24(4): 458-467.

Scarratt JB. 1991. Effect of early transplanting upon growth and 

development of spruce and pine seedlings in paperpot 

containers. New Forests 4(4): 247-259.

Simpson DG. 1990. Frost hardiness, root growth capacity, and field 

performance relationships in interior spruce, lodgepole pine, 

Douglas-fir, and western hemlock seedlings. Canadian Journal 

of Forest Research 20(5): 566-572.

Simpson DG, Macey DE. 1992. Development of physiological

quality in interior spruce and lodgepole pine seedlings. In: 

Donnelly FP, Lussenburg HW, comp. Proceedings of the 1991 

Forest Nursery Association of British Columbia Meeting, Prince 

George, BC: 78-85.

Singh O, Sharma HP, Sharma SK. 1984. Effect of root clipping 

on the growth of transplanted spruce seedlings. Journal of 

Tree Science 3(1/2): 149-152.

Sutherland JR. 1990. Fusarium root rot. In: Hamm PB, Campbell 

SJ, Hansen EM, eds. Growing healthy seedlings: 

identification and management of pests in Northwest forest 

nurseries. Spec. Pub. 19. Corvallis, OR: Oregon State 

University, Forest Research Laboratory: 8-9.

Tanaka Y, Timmis R. 1974. Effects of container density on growth 

and cold hardiness of Douglas-fir seedlings. In: Tinus RW, Stein 

WI, Balmer WE, eds. Proceedings, North American 

Containerized Forest Tree Symposium; 1974 August 26-29; 

Denver, CO. Pub. 68. Denver: Great Plains Agricultural Council 

181186.

Templeton CWG, Odium KD, Colombo SJ. 1993. How to identify 

bud initiation and count needle primordia in first-year spruce 

seedlings. Forestry Chronicle 69(4): 431-437.

Thompson G. 1995. Nitrogen fertilization requirements of 

Douglas-fir container seedlings vary by seed source. Tree 

Planters' Notes 46(1): 15-18.

Tinus RW. 1974. Characteristics of seedlings with high survival 

potential. In: Tinus RW, Stein WI, Balmer WE, eds. 

Proceedings, North American Containerized Forest Tree 

Symposium. 1974 August 26-29; Denver, CO: Pub. 68. Denver: 

Great Plains Agricultural Council 276-282.

163
164

Index of Common and Scientific Names

Alaska-cedar 

Antelope bitterbrush 

American chestnut 

American elm 

American plum 

Arizona sycamore 

Atlantic white- cedar 

aralia 

Arizona pine 

acacia 

araucaria 

alder 

ash

balau 

baldcypress 

basswood 

beech 

beefwood 

birch 

bitterbrush 

blackberry 

black locust 

black walnut, walnut 

black spruce 

blue oak 

boxwood 

bulrush bunchberry 

bur oak

California walnut 

chamisso sedge 

cherry, chokecherry, plum 

citrus cliffrose 

Colorado blue spruce 

cottonwood

Dipterocarps

Douglas-fir

Chamaecyparis nootkatensis 

Purshia tridentata 

Castanea dentata 

Ulmus Americana

 Prunus Americana

 Platanus wrightii

 Chamaecyparis thyoides 

Aralia spp.

 Pinus ponderosa var. arizonica 

Acacia spp.

 Araucaria hunsteinii

 Alnus spp. 

Fraxinus spp.

Shorea spp.

Taxodium distichum

Tilia spp.

Fagus spp.

Casuarina spp.

Betula spp.

Purshia

Rubus spp.

Robinia psuedoacacia

Juglans nigra

Picea mariana

Quercus douglasii

Buxus spp.

Scirpus spp.

Cornus canadensis

Quercus macrocarpa

Juglans californica

 Carex pachystachya 

Prunus spp. 

Citrus spp. 

Cowania stansburiana 

Picea pungens 

Populus spp.

Dipterocarpus spp.

Pseudotsuga menziesii

99, 103

38, 94

93

96

37

112

81

110

61

56, 59

57

39

42, 59

56

66

10, 59, 66

49, 56, 64

56

9, 10, 56, 59, 151, 153

50

110

47, 58

47, 49

78, 93, 96, 109, 132, 145, 155

42

58

113

60

14, 15

75

39

8, 39, 46, 56, 59

56, 112

50

24, 115, 137, 154, 155

103, 105, 106, 119

56

19, 20, 36, 39, 42, 43, 44, 48, 52, 53, 62,

64, 66, 69, 81, 118, 126, 130, 133, 139,

140, 141, 143, 147, 148, 150, 151, 154, 155
eastern redcedar 

eastern redbud 

eastern cottonwood 

eastern white pine 

elm 

Engelmann spruce 

Eucalypts 

shining gum eucalyptus 

red-ironbark eucalyptus 

rosegum 

European beech

fir 

Fraser fir 

gooseberry

hardwickia 

hawthorn 

hemlock 

hickory 

honeylocust 

hopea 

horsechestnut

Indian paintbrush 

interior spruce

jack pine 

juniper

larch 

lilac 

loblolly pine 

locust 

lodgepole pine 

longleaf pine

magnolia 

mahogony 

manzanita 

maple 

monkeyflower

noble fir 

northern bayberry 

northern red oak 

Norway spruce 

oak

Juniperus virginiana

Cercis canadensis

Populus deltoides

Pinus strobus

Ulmus spp.

Picea engelmannii

Eucalyptus spp.

Eucalyptus nitens

Eucalyptus sideroxylon

Eucalyptus grandis

Fagus sylvatica

Abies spp.

Abies fraseri

Ribes spp.

Hardwickia binata 

Crataegus spp. 

Tsuga spp. 

Carya spp. 

Gleditsia triacanthos 

Hopea spp. 

Aesculus spp.

Castilleja spp.

Picea glauca x P. engelmannii

Pinus banksiana

Juniperus spp.

Larix spp.

Syringa spp.

Pinus taeda

Robinia spp.

Pinus contorta var. contorta

Pinus palustris

Magnolia spp. 

Swietenia spp. 

Arctostaphylos spp. 

Acer spp. 

Mimulus spp.

Abies procera

Myrica pennsylvanica

Quercus rubra

Picea abies

Quercus spp.

59

61, 65

57

70

9

126, 139, 141, 148

12, 56, 59, 68, 80, 117

75

102

47

5 7, 64

49, 76

5, 14

98, 102

76

59, 66, 110

49

39, 49, 56

51, 58, 61

56

49, 56

14

42, 53, 118, 133, 148, 149

78, 132

9, 10, 13, 76, 143

117

115

40, 47, 59, 61, 64, 128, 131, 145, 150, 153

49, 59

6, 7, 43, 52, 53, 133, 143, 144, 148, 149,

159

62, 69, 70, 128, 131, 133, 143, 145, 149

59

48

59, 61

14, 42, 48, 50, 51, 66, 76, 109

78

50

66

47, 57

141, 149, 151

9, 46, 49, 50, 56, 57, 58, 59, 76, 109

165
95

110

110

57

9, 12, 44, 46, 47, 53, 56, 59, 68, 72, 74, 80,

109, 112, 117, 118, 135, 143

143 3

7, 40, 46, 48, 49, 53, 57, 61, 64, 66, 126,

139, 141, 147, 155

56, 98, 99, 100, 101, 103

5, 17, 39, 47, 61, 110, 137

96, 118

115

117

57, 58

47

61, 155

61, 65, 66, 94, 103

14, 51

47

113

14

94, 99, 102

43, 118, 133, 144, 150, 151, 153, 155

113

37

133

116, 117

82

57

43

128, 131

113

61

39

53, 64, 69, 80, 128, 131, 132

57

10, 56, 69, 117, 118, 141, 145

60

53, 69

57, 61

66

78,110

47, 57, 66

49

49

Corpus nuttallii 

Paulownia tomentosa 

Asimina spp. 

Shorea robusta 

Pinus spp.

Pinus edulis

Rhus diversiloba

Pinus ponderosa var. ponderosa

Populus spp.

Populus tremuloides

Pinus radiata Cercis spp. 

Sequoia sempervirens 

Quercus spp. 

Alnus rubra 

Acer rubrum 

Juniperus scopulorum 

Rosa spp. 

Eucalyptus grandis 

Juncus spp.

Artemisia spp. 

Salix scouleriana 

Pinus sylvestris 

Carex spp. 

Carya ovata 

Pinus echinata 

Hackelia venusta 

Larix sibirica 

Acer saccharinum 

Picea sitchensis 

Pinus elliottii 

Juncus effusus 

Ceanothus velutinus 

Magnolia grandiflora 

Pinus spp. 

Quercus falcata 

Picea spp. 

Rhus trilobata 

Abies lasiocarpa 

Acer saccharum 

Pinus lambertiana 

Rhus spp. 

Liquidambar styraciflua 

Platanus spp.

Nyssa spp.

Pacific dogwood 

[royal] paulownia 

pawpaw 

Philippine mahogany 

pines

pinyon pine 

poison-oak 

ponderosa pine 

poplar 

quaking aspen

radiata pine 

redbud 

redwood 

red oak red alder 

red maple 

Rocky Mountain juniper 

rose 

rosegum 

rush

sagebrush 

Scouler willow 

Scots pine 

sedge 

shagbark hickory 

shortleaf pine 

showy stickseed 

Siberian larch 

silver maple 

Sitka spruce 

slash pine 

soft rush 

snowbrush ceanothus 

southern magnolia 

southern pines 

southern red oak 

spruce 

squawbush 

subalpine fir 

sugar maple 

sugar pine 

sumac 

sweetgurn 

sycamore 

tupelo

166
167

37, 61

46, 49, 56

81, 128, 148, 154

4, 17, 42, 69, 94, 103, 126, 137, 139, 140,

141, 155, 156, 159

68, 133, 143

17, 19, 43, 44, 47, 53, 65, 94, 95, 126, 139,

140, 141, 144

40

57, 58, 65

6, 7, 21, 22, 23, 52, 130, 154, 155

56, 57, 98, 99, 100, 101, 102, 103, 105,

106, 119

78

Acer circinatum

Juglans spp.

Tsuga heterophylla

Larix occidentalis

Thuja plicata

Pinus monticola

Fraxinus americana 

Quercus alba 

Picea glauca 

Salix spp.

Gaultheria spp.

vine maple

walnut western 

hemlock western 

larch

western redcedar 

western white pine

white ash 

white oak 

white spruce 

willow

wintergreen