1
2

In this volume of the Container Tree Nursery Manual, we will 

discuss the four atmospheric factors of the container nursery 

environment-temperature, humidity, light, and carbon dioxide-which 

directly control the growth rate of a crop of tree seedlings. The 

other two important factors controlling seedling growth-water and 

mineral nutrients-are supplied by the growing medium. These latter 

factors are discussed in volume four of the manual: Seedling 

nutrition and irrigation.

As explained in volume one, plants are grown in container 

nurseries so that the various factors that affect seedling growth 

and development can be kept as close to optimum levels as 

possible. The principle of limiting factors (Blackman 1905) 

states that, when a process is governed by several factors, its rate 

is limited by the factor that is closest to the minimum requirement. 

The practical significance is that, although all four atmospheric 

factors (light, temperature, humidity, and carbon dioxide) are 

necessary for growth, one usually limits the system at any 

particular time (Kramer and Kozlowski 1979).

Although they are discussed separately, these four factors must 

always be considered together because they interact with each 

other in controlling the growth of tree seedlings (fig. 3.1.1). A good 

example of these interrelationships is the one between humidity 

and temperature. The humidity in the air is typically measured by 

the relative humidity, and this value changes drastically with 

changes in temperature.

Perhaps the best example of the extreme complex ity of the 

interrelationships of the four atmospheric factors is the physiology 

of a typical leaf (fig. 3.1.2). The two gases (carbon dioxide and water vapor) and light are involved in photosynthesis, respiration, and 

transpiration:

Photosynthesis uses carbon dioxide and produces oxygen. 

Respiration uses oxygen and produces carbon dioxide.

• Transpiration is the loss of water vapor through

stomata.

All three physiological processes are regulated by the temperature

of the leaf and the functioning of the stomata (singular = stomate), 

which respond to the concentrations of carbon dioxide and the intensity of light as well as to levels of humidity and temperature. 

Stomata open in response to favorable light, carbon dioxide, and 

moisture levels, and stay open until transpirational losses of water 

vapor cause the guard cells to lose turgor pressure. The 

transpirational demand is primarily a function of temperature and 

humidity. Photosynthesis requires the stomata to remain open as 

long as possible to allow carbon dioxide to enter, but while the stomata are open, water vapor is lost (Roberts 1990).
This basic example illustrates both the extreme complexity of the 

physiological processes that control plant growth and the 

interrelationships that exist between them and the limiting factors 

of the environment.

For convenience's sake, each of the four atmospheric factors will 

be discussed separately, but the organization in each of the 

following chapters is the same to permit easy comparison: • Introduction-Discusses the basic concepts, terminology, and 

units of measurement. 

• Role in Tree Seedling Growth and Development-Explains

how these concepts can be applied in container nursery 

management. 

• Optimum Levels-Analyzes the recommendations in the 

literature and relates what other container nurseries are doing. 

• Modifying the Factor in Container Tree 

Nurseries-Discusses how container nursery growing 

structures and environmental control equipment can be used 

to keep the factor at optimum levels. 

• Monitoring and Control Systems-Explains the types of 

monitoring equipment and environmental control systems that 

are available and have been used in container nurseries.

When applying any recommendations, the reader is cautioned to 

remember that all six environmental factors are always 

inextricably interrelated. A prudent nursery manager will consider 

the implications before changing any of the environmental factors 

that affect seedling growth, and if possible, should try any 

proposed change on a small scale first.

Literature Cited

Blackman, F.F. 1905. Optima and limiting factors. Annals of 

Botany 19:281-295. 

Kramer, P.J.; Kozlowski, T.T. 1979. Physiology of woody 

plants. Berlin: Academic Press. 811 p. 

Roberts, B.R. 1990. What are stomates and how do they work? 

Journal of Arboriculture 16(12):331.

3
5
Contents

3.1.1 Introduction 7

3.1.1.1 Heat versus temperature 7

3.1.1.2 Heat exchange in the container nursery

environment 7

3.1.2 Role of Temperature in Tree Seedling 

Growth and Development 9

3.1.2.1 Cardinal temperatures 9

3.1.2.2 Shoot versus root temperatures 10

3.1.2.3 Diurnal variation 11

3.1.2.4 Genetic variation 12

3.1.3 Optimum Temperature Levels 13

3.1.3.1 Establishment phase 13

3.1.3.2 Rapid growth phase 15

3.1.3.3 Hardening phase 16

3.1.4 Modifying Temperature in

Container Tree Nurseries 20

3.1.4.1 Growing structures 20

3.1.4.2 Temperature modification equipment 22

3.1.4.3 Cooling 22

Convection ventilation 23

Fan ventilation 24

Evaporative cooling 25

Winter cooling 28

Cultural techniques 28

3.1.4.4 Heating 30

Calculating heating requirements 30

Types of fuels 31

Heaters and heat distribution systems 33

Heat conservation 36

3.1.5 Temperature Monitoring and Control

Systems 39

3.1.5.1 Sensing instruments 39

3.1.5.2 Control equipment 39

3.1.6 Conclusions and Recommendations 42

3.1.7 Literature Cited 43

6
7

water 1 ° F. Temperature scales are based on the freezing and 

boiling points of water, 0 and 100 °C or 32 and 212 ° F; 

conversion tables for Centigrade and Fahrenheit are provided in 

the appendix. Air temperature, and often the temperature of the 

growing media, are normally monitored in container tree nurseries 

(see section 3.1.4 for more information).

3.1.1.2 Heat exchange in the container nursery

environment

Heat is transferred from one place to another by three processes: 

radiation, conduction, and convection. In the container nursery 

environment, heat transfer can occur by solar and thermal 

radiation, through solids and liquids by conduction, and through 

liquids or gases by convection (fig. 3.1.1). In conduction and 

convection, heat is directly transferred by contact or mass flow of 

matter, whereas in radiation the energy form changes from 

thermal energy at the source, to electromagnetic energy for 

transmission, and then back to thermal energy upon reception 

(ASHRAE 1989).

In a container nursery all heat, except geothermal or nuclear 

waste heat, originates from the sun: either directly as solar 

radiation or indirectly from organic fuels such as wood, natural 

gas, or coal. The surfaces in a greenhouse heat up when 

electromagnetic radiation from the sun penetrates the transparent 

covering, is absorbed, and is converted to thermal radiation (fig. 

3.1.1). This "greenhouse effect" applies especially to growing 

structures cov3.1.1 Introduction

Temperature is one of the most familiar aspects of our 

environment, and it has a pervasive effect on tree seedling growth. 

It directly affects the numerous chemical reactions involved in plant 

metabolism and also controls other growth-related processes such 

as transpiration. Temperature affects plant growth in several 

different ways, ranging from minor changes in growth rate to 

severe injury or even death. Because they are succulent when 

rapidly growing, container tree seedlings are particularly vulnerable 

to temperature stresses.

Temperature is the most frequently controlled and easily 

measured environmental factor when plants are produced under 

container nursery conditions. A greenhouse is the most basic 

cultural modification to control the growing environment, and 

completely enclosed structures contain heaters, vents, fans, 

evaporative pads, and sophisticated environmental controls to 

maintain ideal growing temperatures. Before discussing the 

horticultural applications, however, it is instructive to review the 

basic concepts of heat and temperature.

3.1.1.1 Heat versus temperature

Heat is the common name for thermal energy and represents the 

total kinetic molecular energy of a substance, whereas 

temperature indicates the average energy. When heat is applied 

to a substance, and there is no change in physical state (such as 

ice to water), the molecular motion increases and the temperature 

rises. If the same amount of heat is applied to two different 

substances, however, each would exhibit different temperatures 

because they have dissimilar molecular structures or densities 

and thus different capacities to absorb heat (Schroeder and Buck 

1970). For example, a cubic meter of air at the same temperature 

as an equal volume of water does not contain the same amount of 

heat. From a heating standpoint, temperature indicates the 

direction but not the amount of heat transfer (Hanan and others 

1978).

Thermal energy is measured in calories in the metric system, or 

British thermal units (Btu) in the English system. One calorie is the 

amount of heat necessary to raise the temperature of 1 g of water 

1 °C, and 1 Btu is defined as the amount of heat required to raise 

the temperature of 1 pound of
8

Sensible heat is heat that causes an increase in temperature, of 

either the air or the growing medium. Latent heat, on the other 

hand, is heat that is required for a phase change such as that 

between liquid water and water vapor or ice, and is emitted or 

absorbed with no change in temperature (Stathers and Spittlehouse 

1990). Both evaporation and transpiration consume latent heat in 

the greenhouse environment because transpiration is essentially 

bioregulated evaporation.

ered with glazings that are transparent to solar radiation but opaque 

to thermal radiation. Thermally opaque glazings include glass, 

fiberglass, acrylics, and polycarbonates; other greenhouse 

coverings, such as polyethylene and polyester films, are 

transparent to thermal radiation and the only reason the 

greenhouse heats up is air exchange with the ambient environment 

is restricted (Mastalerz 1977). (See chapter 3 in this volume for a 

more detailed discussion of solar radiation.)

Actually, temperature regulation in a traditional greenhouse is not a 

simple task. Greenhouses get too hot during periods of sunny 

weather, and growers must utilize vents or evaporative coolers to 

provide cooling. In the temperate zones, nurseries must 

supplement this direct solar heat during periods of cloudy and cold 

weather.

Heat from solar radiation and supplemental sources is dissipated as 

both sensible and latent heat in the microclimate around a tree 

seedling (fig. 3.1.2).
9

3.1.2 Role of Temperature in Tree

Seedling Growth and Development

Plant growth is the end product of many different physical and 

chemical processes. Temperature has a direct effect on plant 

metabolism because the rate of a biochemical reaction increases 

by a factor of 2 to 3 for each 10 °C (18 °F) rise in temperature, up 

to the point where the enzymes are damaged. Each reaction has a 

different rate at any given temperature, however, and so an 

increase in temperature does not necessarily affect a complex 

process such as photosynthesis, which involves a series of 

reactions, in the same relatively simple way as a single reaction.

Temperature also affects other plant processes that control growth. 

Plants respond to temperature in a complex, buffered manner 

because of their high water content. Water has the highest specific 

heat of any common substance, and so plant foliage changes 

temperature much more slowly than the air around it. Water 

changing from a liquid to a gas requires a large amount of heat 

energy, and so transpiration helps cool the leaf and maintain 

temperatures that are most favorable for photosynthesis. Leaf 

shape has evolved to maximize interception of sunlight for 

photosynthesis, while dissipating the resultant heat through 

reradiation and transpiration.

3.1.2.1 Cardinal temperatures

The growth of tree seedlings, like that of all living things, is 

restricted to a discrete temperature range. The best temperatures 

for growing a particular seedling will vary according to species, 

ecotype, and stage of development. These ideal temperature 

ranges can be defined by cardinal temperatures: minimum, 

maximum, and optimum. These values are not rigid constants, 

however, but can shift as the plant matures or adapts to 

environmental conditions (Larcher 1975).

The general response to temperature is generic for most plants. 

For most temperate and boreal zone conifers, little seedling 

growth occurs below 10 °C (50 ° F), although basic processes 

such as photosynthesis and respiration continue very slowly at 

lower temperatures. Growth remains slow up to 15 °C (59 °F), 

and steadily increases through the optimum range of 18 to 30 °C 

(64 to 86 °F). Temperatures above 30 °C (86 °F) adversely

affect growth (Kramer and Kozlowski 1979). Although the response 

varies both between shoots and roots and between species (fig. 

3.1.3), seedling growth follows a typical bell- shaped curve over the 

range of temperatures that would normally be encountered in a 

container nursery.
The effects of temperature on seedling growth are most 

pronounced near the minimum and maximum values (fig. 3.1.3). 

As the lower limit of growing temperature is approached, the 

growth rate gradually decreases. There are exceptions: for 

example, the height growth and dry matter production of redwood 

seedlings abruptly decreases 70 to 80% when the night 

temperature drops only a few degrees, from 15 °C (59 °F) to 11 °C 

(52 °F) (Hellmers 1966). As the maximum temperatures are 

approached, there is often a rapid increase in stem length and a 

decrease in total dry matter (Downs and Hellmers 1975). Finally, 

the growth rate decreases abruptly as temperatures approach the 

thermal death point. (The pathological effects of extreme heat and 

cold on tree seedlings are discussed in volume five of this series.)

Growth can be measured in a variety of ways, but the most 

commonly measured aspects are shoot height, stem diameter 

(caliper), and whole plant dry weight (biomass). The optimum 

temperatures for maximizing growth of one of these parameters 

are not necessarily the same as those for another (Tinus and 

McDonald 1979).

3.1.2.2 Shoot versus root temperatures

The effects of temperature on growth can vary between the 

different parts of the seedling (fig. 3.1.3). Although not 

appreciated by many growers, root temperature is as important, 

if not more so, as shoot temperature in controlling the physiology 

and phenology of container tree seedlings. This influence is 

significant when the seedlings are actively growing but is 

particularly pronounced during overwinter storage when even 

brief exposures to warm temperatures can drastically change 

seedling dormancy and cold hardiness.

In the shoot, ambient air temperature affects both metabolic 

processes, such as photosynthesis and respiration, and 

biophysical processes such as transpiration. Because these foliar 

processes are all dependent on water and mineral nutrients 

transported from the root system, the temperature of the growing 

medium is just as important as air temperature. The water 

absorption rate of seedling root systems increases with 

temperature and so seedling transpiration is directly affecte d by 

the temperature

10

of the growing medium (fig. 3.1.4). Because mineral nutrient ions 

are carried along with the transpirational stream from the roots to 

the foliage, the uptake of nitrogen and other essential mineral 

nutrients is also limited at low growing medium temperatures 

(Orlander and others 1990). High root temperatures, those greater 

than 25 °C (73 °F), can also adversely affect growth, probably 

because the roots cannot take up enough oxygen (Garzoli 1988). 

Research has shown that, where growing medium temperature is 

controlled separately from air temperature, both shoot and root 

growth are influenced by the temperature of the growing medium 

(Lavender and Overton 1972). One of the significant advantages 

that container nurseries have over bareroot nurseries is that root 

temperature can be more easily monitored and controlled.

Temperature affects both the type and rate of seedling growth in 

container tree nurseries. Tinus (1984) grew several species and 

ecotypes under typical greenhouse conditions and found that shoot 

height, caliper, and biomass all vary with different combinations of 

day and night temperatures (fig. 3.1.5). He concluded that, for 

some species, these responses were different enough to warrant
separate growing environments. With most temperate zone 

conifers, high temperatures tend to produce spindly seedlings 

(Rook 1991); for example, Hellmers and others (1970) observed 

that high temperatures stimulated height growth of Engelmann 

spruce seedlings at the expense of dry matter production (fig. 

3.1.6). This effect apparently does not apply to certain southern 

conifers, however. With loblolly pine, which produces from one to 

seven flushes of shoot growth during the year (depending upon a 

seedling's age and location), Mulroy (1972) found an inverse 

relationship with temperature. Flush number was increased and 

flush length shortened with increased temperature. Thus,

these seedlings grew to about the same height under a wide 

range of temperatures.

3.1.2.3 Diurnal variation

Both day and night temperatures have been shown to affect 

seedling growth (figs. 3.1.5 and 3.1.6). Researchers have found 

that some tree species exhibit increased growth when night 

temperatures are kept lower than day temperatures; this day-night 

temperature differential is called the thermoperiod. Hellmers and 

Rook (1973) reported that radiata pine seedlings increased 

significantly in stem caliper and total and root dry weight when

11
12

night temperatures were lower than day temperatures. Daytime 

optimums for white spruce and lodgepole pine are 22 to 25 °C (68 

to 73 °F); nighttime optimums are 16 °C (60 °F). Other species, 

such as Colorado blue spruce, require a slightly higher [20 °C (68 

°F)] night temperature. A greater day-night temperature differential 

may be beneficial during the hardening phase in some species, 

but is clearly not required in others (Burr and others 1989, 1990).

Other species, however, grow better under a constant 

temperature regime. Lavender and Overton (1972) found that a 

number of different ecotypes of Douglas-fir grew best at day and 

night temperatures between 18 to 24 °C (62 to 72 °F) and that 

there was no evidence of a thermoperiod effect. Tinus (1984) 

found similar results for ecotypes of Engelmann spruce and 

interior Douglas-fir, and ponderosa pine (Tinus and McDonald 

1979). Olson and others (1959) report that eastern hemlock 

seedlings respond to total daily heat units and that distribution of 

the units between day and night had little effect. Similarly, the 

quality of black spruce container seedlings was more closely 

related to the accumulated sum of growing degree-days for a 

particular season rather than the actual temperatures (Gonzalez 

and D'Aoust 1988).

3.1.2.4 Genetic variation

As shown in the previous sections, different species have different 

responses to temperature; in general, southern species and 

ecotypes have higher temperature requirements than those from 

further north. This intraspecific variation can exist even between 

species that occur in similar habita ts. Lopushinsky and Max (1990) 

found that four different northwest conifers had significantly 

different root and shoot growth over a range of different soil 

temperatures (fig. 3.1.3). Different ecotypes of the same species 

can respond differently to temperature; the same authors found 

that ponderosa pine seedlings from a low-elevation seed source 

had shoot and root growth patterns that were different from those 

of seedlings from a high-elevation source (see also Tinus and 

McDonald 1979).
3.1.3 Optimum Temperature Levels

Seedlings grow over a wide range of temperatures, but 

somewhere within this range there are temperatures that 

promote optimal growth. The nursery manager needs to 

identify these optimum temperatures for different species 

and ecotypes at the various stages of seedling development, 

and determine how much variation can be tolerated while 

still producing the desired crop. Tree seedling growth in 

container nurseries can be divided into three stages:

• Establishment phase-the period from seed germination through primary leaf development and

root extension throughout the container.

• Rapid growth phase-the period when seedlings

grow in height and weight at an exponential

rate.

• Hardening phase-the period after bud set

when caliper and root growth continue (stage 1),

and the seedling is cold hardened for outplanting (stage 2).

3.1.3.1 Establishment phase

The first stage in container seedling production is to promote 

rapid and complete seed germination, and the most critical 

environmental variables are moisture and temperature 

(Tinus 1982). Some temperate zone species require cold, 

moist stratification before their seeds will germinate and this 

process can take from 1 to 6 months depending on the 

species. Seeds of other species and more southerly sources 

require only a short presoak so that they can imbibe water 

(presowing seed treatments are described in detail in 

volume six of this series).

Minimum, optimum, and maximum temperatures for seed 

germination vary widely among seeds of different species 

and generally are lower for temperate zone species than for 

tropical species. Optimum germination (the highest 

percentage germination in the shortest time) is 

temperature-sensitive and varies somewhat between 

species and between genotypes within a species. However, 

germination temperatures between 22 to 24 °C (72 to 75 ° 

F) are optimal for a wide range of species (McLemore 1966, 

Barnett 1979, Heidmann 1981).

Most research results on seed germination are obtained under 

standard laboratory conditions with constant temperatures. Such 

results cannot be applied directly to operational greenhouse 

conditions, however, where temperatures fluctuate both diurnally 

and from day to day. Some germination tests have been 

conducted with fluctuating temperatures that are more 

representative of greenhouse conditions. Of four southern pines, 

longleaf pine seed was the only one that was adversely affected 

by temperatures alternating between 24 to 35 °C (75 to 95 °F) 

compared to a constant 24 °C (75 °F) that would be ty pical of an 

operational greenhouse (table 3.1.1). All species showed serious 

reductions in germination when grown under constant high 

temperatures [35 °C (95 °F)]. Short periods of high temperatures 

may not reduce seed germination of some species; for example, 

Dunlap and Barnett (1982) found that exposing loblolly and 

shortleaf seeds to periods of 35 °C (95 °F) up to 12 hours per day 

speeded germination without adversely affecting total germination. 

However, the germination rate of longleaf pine seed was reduced 

by this exposure to high temperatures.

After the germinant begins to emerge from the container (fig. 

3.1.7), growers must carefully regulate temperatures to promote 

rapid seedling growth and development and minimize the potential 

for damping-off disease or abiotic injury. Tinus (1982) 

recommends that temperatures during the establishment phase be 

maintained at or slightly below the optimum because slightly 

cooler temperatures slow hypocotyl elongation and result in 

sturdier seedlings. A recent survey of operational container tree 

nurseries revealed that growers use a relatively narrow range of 

temperatures during the establishment phase (table 3.1.2). 

Daytime temperatures ranged from 21 to 27 °C (70 to 80 °F), 

whereas overnight temperatures were only slightly cooler [18 to 27 

°C (65 to 80 °F)].

Obviously, because of the relatively high temperatures that are 

necessary, heated greenhouses should be used during the 

establishment phase and some nurseries have constructed 

special germination rooms (fig. 3.1.8). Even in the production 

greenhouse, the placement of the sown containers can affect 

seedling emergence; Hallett and others

13
14

(1985) reported that the emergence of black spruce seedlings 

varied considerably between heated and unheated growing 

structures, and at different locations within a heated structure (fig. 

3.1.9).

(More information on the effects of growing structures on 

temperature is provided in section 3.1.4.1.)
3.1.3.2 Rapid growth phase

Once the seedlings have emerged and become established in the 

containers, nursery managers must control the temperature in the 

growing area to promote rapid shoot expansion (fig. 3.1.10). The 

shoots of forest tree seedlings either grow continuously or in a 

series of sequential flushes, and temperatures in the growing area 

must be kept in an optimum range, or the seedlings may stop 

shoot growth and set a terminal bud (Kramer and Kozlowski 1979). 

Once this happens, it is often difficult to break bud dormancy 

without satisfying the chilling requirement, which can take months. 

In an operational nursery, premature bud set can be disastrous 

and can prevent the crop from meeting height standards within the 

crop schedule (Tines 1982).

Recommended temperatures derived from growth chamber 

experiments (table 3.1.3) are within the optimum range for a 

variety of species. These temperatures consider not only biological 

requirements, but also the economic constraints of heating and 

cooling. These recommendations also indicate some differences in 

response between seed sources but not to the extent that different 

greenhouse temperature regimes are needed.

Tines (1984) and Barnett and Brissette (1986) have reported 

inconsistencies in optimum temperatures that are attributed to 

different ages and size of seedlings. For example, optimum day 

temperatures for Engelmann spruce reported by Hellmers and 

others (1970) were in agreement with data reported by Tines 

(1984) but optimum night temperatures were quite different. The 

optimum temperatures for container loblolly pine seedlings 

according to the work of Bates (1976) is a 29/23 °C (84/'73 °F) 

day/night regime. This is quite different from the data of 

Greenwald (1972), who studied loblolly seedlings more than 6 

months old. The

15
16

differences between these research results suggest that, as 

seedlings develop, there is a shift in the temperatures that are 

optimum for growth.

Nevertheless, selection of target temperatures need not be 

complicated. A survey of successful container nurseries showed 

that nursery managers are producing a variety of tree seedling 

crops under a relatively narrow range of temperatures (table 3.1.2). 

Daytime targets ranged from 21 to 27 °C (70 to 80 ° F) and target 

night temperatures varied from 16 to 24 °C (60 to 75 °F); these 

values agree with published data from Canadian container tree 

nurseries (Edwards and Huber 1982, Hallett 1982). These 

moderate temperature regimes reflect the operational realities of 

most container tree nurseries, including structural and equipment 

capabilities and the economics of energy use.

For new growers, the values listed in tables 3.1.2 and 3.1.3 can 

be used until actual experience or new research provides better 

data. If more than one species must be grown in the same greenhouse, then nursery managers must select a compromise that will 

provide acceptable growth for the entire crop. Growers must 

consider that, although some species and ecotypes are more demanding than others, most seedlings will produce acceptable 

growth under a rather wide range of temperatures and that the 

"best" temperature will also depend on the type of growth that is 

desired (fig. 3.1.5).

3.1.3.3 Hardening phase

Once the crop seedlings have reached the desired height or dry 

weight target, they must be culturally
induced to stop height growth and set a terminal bud (fig. 3.1.11A). 

During the hardening phase, stem caliper and root growth are 

encouraged and the seedlings are gradually hardened to tolerate 

handling, storage, and outplanting conditions. Temperature has a 

pervasive effect on seeding physiology, and thus bud development 

(fig. 3.1.11), dormancy, and cold hardiness are all affected by the 

temperature regimes maintained during the hardening phase. Other 

environmental factors also affect these processes, and so growers 

use one or more combinations of cultural treatments, including 

reducing temperature, shortening day length, decreasing nitrogen, 

and inducing moisture stress to initiate hardening.

For crops grown for more than one season, temperate zone 

species require a cold treatment

before shoot growth can resume (Kramer and Kozlowski 1979). The 

chilling requirement is more pronounced in northern or continental 

climate species and ecotypes, but there is usually some 

requirement for cool temperatures in southern and maritime 

species. For example, Garber and Mexal (1980) reported that about 

7 weeks of cool temperatures were needed before bareroot loblolly 

pine seedlings would resume normal growth. The exact age or 

stage of development when the chilling requirement becomes 

necessary is not known for most species. (The use of low 

temperatures in developing cold hardiness and in meeting chilling 

requirements are described in greater detail in volume six of this 

series.)

Container nursery managers routinely use cooler temperatures, 

especially at night, to help induce

17
shoot dormancy and set terminal buds (table 3.1.2). The response 

to cooler temperatures is varied, however, and interacts with 

photoperiod in some species. Lodgepole pine seedlings will 

continue height growth over a wide range of temperatures as long 

as an extended daylength is maintained whereas other species, 

such as bur oak, may set a terminal bud in response to cool nights 

regardless of the photoperiod (Tinus 1982). For Scotch pine, the 

length of the shoot growth period was substantially shorter with 

warm nights; warm days had a much smaller effect (table 3.1.4).

Many nurseries move their crops from the greenhouse or remove the 

covering at the beginning of the hardening phase; thus, some of the 

temperatures reported in table 3.1.2 reflect outside ambient 

conditions. Even in enclosed growing structures, ambient 

temperatures are not precisely controlled unless damaging 

temperature extremes are encountered. Crops that must be left in 

enclosed facilities are hardened by lowering the temperature in two 

stages. The first stage reduces day and night temperatures to a level 

that is suboptimum for rapid height growth but warm enough for cali18
per and root growth to continue and for the me tabolic changes to 

occur that result in the onset of hardening. These temperatures are 

maintained for 4 to 6 weeks. The second stage of hardening requires temperatures just above freezing, especially at night, for 4 to 

6 weeks. During this time bud chilling requirements are met, cold 

hardiness develops, root growth potential rises, and the seedlings 

become resistant to mechanical damage. The hardening process 

will proceed most quickly if temperatures during these two stages 

are kept close to their optimum. Operationally, however, it will often 

be cheaper to reduce the temperature incrementally, which 

happens naturally in a crop hardened outdoors, where 

temperatures eventually reach, or fall below, freezing.

19
cause container seedling root systems must be protected against 

both hot and cold temperature extremes.

3.1.4.1 Growing structures

Producing a uniform growing environment within an enclosed 

greenhouse is a more complex problem than most people realize. 

One of the primary reasons to grow seedlings in a transparent 

growing structure is to trap solar radiation and convert it to thermal 

energy (fig. 3.1.1), but this free source of heat energy can also 

create problems:

1. Heat builds up very rapidly on sunny days,

even in the winter.

2. Cooling occurs rapidly in poorly insulated structures at night 

and during cloudy weather.

3. The high humidities that are maintained in

enclosed growing structures significantly affect

the heat budget of a container nursery because

of the latent heat consumed in evapotranspiration or released during condensation.

The type of greenhouse or other growing structure can vary with 

the growth stage of the seedlings. Enclosed, heated greenhouses 

are routinely used during the establishment phase to speed 

germination and early seedling growth. Even the location within a 

heated greenhouse is important; many nurseries grow their 

container seedlings on benches and some provide underbench 

heating. Underbench heating increases temperature uniformity 

within the greenhouse. Hallet and others (1985) found that the 

emergence rate for black spruce seedlings was significantly faster 

in heated than in unheated greenhouses and was most rapid when 

the sown containers were placed on raised benches or directly on 

a heated floor (fig. 3.1.9).

Some container nurseries even have special germination rooms 

that are kept uniformly warm, humid, and light to stimulate rapid 

germination (fig. 3.1.8). Matthews (1982) stated that, in British 

Columbia, well-insulated greenhouses are modified to serve as 

germinators by installing efficient heating systems and special mist 

irrigation equipment. Sown containers are stacked 12 high on 

pallets and moved into the germinators for approximately 7 days. 

He concluded that germinators improve both the rate

3.1.4 Modifying Temperature in Container

Tree Nurseries

The primary purpose of a greenhouse is to control temperature. A 

greenhouse without temperature control is usually too hot in the 

day and too cold at night; this is why the seedling environment

must be heated or cooled. The recommended targets in tables 

3.1.2 and 3.1.3 refer to the temperature setpoints on the heating 

and cooling systems, and the range gives the temperatures within 

which the greenhouse should operate most of the time. When 

temperatures go outside the range, growth is slowed but usually 

no permanent harm is done unless damaging hot or cold 

temperatures are exceeded for an extended time. If temperatures 

are allowed to remain outside the ranges during the establishment 

or rapid growth phases, seedlings may be stressed into a 

temporary state of dormancy. When this stressed condition 

persists, the entire crop production schedule may be seriously 

delayed. During the hardening phase, bud set or cold hardiness 

may be delayed or even reversed, leaving the crop in poor 

condition to be cold stored or outplanted. However, a certain 

amount of temperature fluctuation is permissible and even 

desirable. Within this recommended range, a greenhouse is 

cheaper and easier to operate if its temperature is allowed to 

fluctuate a few degrees rather than being held to very close 

tolerances.

In a properly designed greenhouse, stable night temperatures are 

relatively easy to maintain, because there is no incoming heat load 

from the sun. However, ambient night temperatures are almost 

always lower than optimum and supplemental heat is required in 

the greenhouse. Many species have surprisingly high nighttime 

optimum temperatures and maintaining them can be expensive, 

especially during cold weather. One strategy is to lower the night 

temperature from the optimum to a temperature at which both the 

growth rate and the cost of heating are acceptable. Another option 

is to avoid rearing seedlings during the coldest months of the year. 

(Crop scheduling is discussed further in section 3.3.3.1 and in 

volume six of this series.)

Even container seedlings grown in outdoor compounds have a 

significant benefit over bareroot seedlings in that the small volumes 

of growing media in containers warm much faster than field soil. 

This feature can also be a hazard, however, be

20
and percentage of germination and may also reduce total heating 

costs. Other nurseries move their seeded containers directly into 

the production greenhouse.

Most container tree nurseries still grow their seedlings in some 

sort of greenhouse during the rapid growth phase, and all growing 

structures have specific design features to help control 

temperature. Fully controlled greenhouses are equipped with 

heating and cooling equipment to control temperature throughout 

the growing season (fig. 3.1.12A). Semicontrolled 

environments, often called shelterhouses, have roll-up side and 

end walls that can be raised to promote cross ventilation (fig. 

3.1.12B). Shelterhouses are particularly useful during late spring 

and early summer when solar radiation is most intense and during 

the hardening phase when low night temperatures are desired. In 

recent years, some forest nursery managers are growing their 

stock in uncontrolled environments, sometimes called outdoor 

growing compounds (fig. 3.1.12C), in which temperature cannot 

be controlled. (More information on types of growing structures 

can be found in volume one of this series.)

21
3.1.4.2 Temperature modification equipment

Although greenhouses are not as sophisticated as growth 

chambers, modern environmental control equipment can regulate 

the indoor climate very well. However, most growers cannot afford 

and do not need truly precise control and thus must be content 

with some fluctuation in environmental conditions, including 

temperatures outside the permissible range for a small portion of 

the time (usually less than 5%). Daytime temperatures will 

fluctuate more than those at night because solar radiation load 

varies tremendously. Nighttime temperatures in a well-designed 

greenhouse will be quite stable and very close to the setpoint. 

Poor control of temperature can cause serious problems during 

critical periods in the crop cycle such as germination and 

emergence. Excessive temperature variation should not be 

tolerated and should be interpreted as a sign of poor greenhouse 

design or equipment malfunction.

There are many consultants who can help new nursery 

managers with the details of greenhouse design, layout, and 

equipment selection. Hiring a greenhouse consulting service 

may be too expensive for a small nursery operation, however. 

The other option is for the nursery developer to learn as much as 

possible about the types of structures and equipment. Without 

the services of a knowledgeable consultant, a basic 

understanding of the calculations for heating and cooling a 

growing structure is essential. Armed with this knowledge, the 

developer can critically review construction proposals provided 

by manufacturers from both a technical and an economical 

standpoint.

A critical first step is to visit other local nurseries, especially those 

with a similar crop, and see what structures and equipment they 

are using. Most growers will gladly point out both the good and 

bad points about their facilities, and especially what they would do 

if they had to build their operation over again. They should also be 

able to evaluate the reliability and competence of local 

greenhouse distributors and consultants. Nursery developers 

should deal with distributors who carry a full line of greenhouses 

and equipment to assure several choices of component 

combinations and prices. Economics of design and frequency of 

sale

22

of a given structure or equipment model often can affect prices 

drastically. Whenever dealing with greenhouse suppliers or 

consultants, nursery managers should be sure to point out the 

unique requirements of a tree seedling crop compared to other 

horticultural crops, such as the long growing cycle and the need to 

harden the seedlings adequately. The temperature control system 

for a growing structure is usually designed around a particular crop 

and a specific growing season but with enough flexibility to change 

crops or growing schedules to some extent.

Other aspects of the nursery (benches, containers, etc.) must be 

considered in the design of the original growing structure and 

selection of environmental control equipment. For example, raised 

benches are often used in container tree nurseries to allow for 

underbench heating and ventilation. However, if the containers will 

be grown on special pallets that will be handled with forklifts, then 

the floor area must remain clear of obstructions and the heating 

and cooling equipment situated overhead.

Several good technical references are useful guides in designing a 

heating or cooling system. These publications contain good 

discussions of the basic principles and tables of environmental and 

engineering specifications, as well as step-by-step worksheets and 

examples. The Greenhouse Climate Control Handbook (Acme 

1988), ASHRAE (1989), and Ball RedBook (Ball 1985) are 

particularly useful. Good information can also be found in various 

greenhouse books (Hanan and others 1978, Langhans 1980, 

Nelson 1985, Garzoli 1988, Aldrich and Bartok 1989).

3.1.4.3 Cooling

In many parts of North America, cooling a greenhouse is more of a 

problem than heating it, especially in the spring and summer, 

when the maximum amount of sunlight is available and most tree 

seedling crops are grown. Efficient cooling presents a serious 

technological challenge (Garzoli 1988). The sunlight that is so 

necessary for photosynthesis generates excess heat that can 

adversely affect plant growth. On a clear day, solar radiation will 

generally provide more thermal energy than is lost through the 

structure, even if the outside tempera-
ture is below freezing (Hanan and others 1978). In addition to 

cooling, adequate ventilation is often necessary to control 

excessive humidity. (The effects of humidity are discussed in 

chapter 2 of this volume.)

Greenhouse cooling and heating systems typically consist of a 

series of successive stages. The first stage of most cooling 

systems mixes the hot air in the structure with cooler air from the 

outside. The cooling capacity of a ventilation system is expressed 

as the rate of air exchange and is measured in the number of air 

changes per hour, which is the number of times that the volume of 

air in the greenhouse can be exchanged for outside air in 1 hour. 

Two site factors affect the efficiency of cooling with air exchange: 

1) intensity of solar radiation, because air warms as it moves 

across the greenhouse from the intake vents to the exhaust vents, 

and 2) elevation, because the lighter air at high elevations is less 

effective in removing solar heat. As an example, an air exchange 

rate of 2.4 m3/min/ m2 (8 cubic feet per minute per square foot) of 

greenhouse space is considered to be adequate for a greenhouse 

located at elevations below 300 m (1,000 feet), with an interior light 

intensity of 53.8 klux (1,000 μmol/s/m2) and a temperature rise of 4 

°C (7 °F) from the cooling pads to the exhaust fans (Nelson 1985).

Three basic types of greenhouse cooling systems are used: 

convection ventilation, fan ventilation, and evaporative 

cooling. They may be employed singly but are often used in 

combination. A fourth method-refrigerated cooling-is rarely used 

because it is uneconomical except in special situations.

Convection ventilation. This basic type of greenhouse cooling 

uses the least amount of energy; it operates on the fact that hot air 

is lighter than cooler air. Vents at the top and sides of the 

greenhouse are opened, which allows hot air to escape by 

convection and be replaced by cool air from the side vents (fig. 

3.1.13A). The vents can be either manual or automatic, and 

thermostatically controlled vents can provide a reasonable degree 

of temperature control when the outside air remains cool.

With convection ventilation, the degree of temperature control 

depends upon four factors: type of growing structure, location and 

position of vents, wind speed and direction, and the temperature 

differential between inside and outside. The curved roofs of some 

growing structures, such as polyethylene-covered quonset 

houses, do not have conventional ridge ventilation or vents on the 

side walls (Garzoli 1988). Taller greenhouses generally ventilate 

better than those with low profiles because they generate a better 

convection column. Shelterhouse structures are specifically 

designed to promote good natural ventilation, and their roll-up 

sides can be operated either manually or automatically (fig. 

3.1.12B). Some greenhouses are now equipped with Poly-vent° 

walls, which create a rigid side wall when inflated and collapse to 

allow good cross ventilation when deflated (fig. 3.1.14).

Vents must be properly located and operated for effective natural 

ventilation. Under calm conditions, all vents should be wide open 

and the greater the temperature differential between inside and 

out, the greater the airflow. When the wind speed exceeds about 

10 km/h (6 miles per hour), the side vents and leeward roof vent 

should be opened to their widest position and the roof vent on the 

windward side should be closed (fig. 3.1.13B). This causes a 

slight lowering of the air pressure right above the leeward vent 

and increases airflow (Langhans 1980). Cooling with convection 

ventilation is widely and effectively used in container tree 

nurseries in the Pacific Northwest and the

23
A fan ventilation system consists of a series of vents for air 

intake on one end wall of the greenhouse and a combination of 

exhaust fans and vents on the opposing end (fig. 3.1.15). 

Greenhouse exhaust fans are generally of the propeller

Canadian Maritimes, where temperatures rarely exceed 38 °C 

(100 °C) at mid-day in the summer.

Even with well-designed systems, there are some drawbacks to 

convection ventilation systems. Convective systems cannot 

reduce temperatures below the outside ambient environment, and 

will increase evapotranspiration in dry, sunny climates. Roof vents 

must be closed during unusually high winds or they may be 

damaged.

Fan ventilation. When convection ventilation is ineffective, 

exhaust fans can be used to increase the air exchange rate 

through a greenhouse. Fan ventilation is more efficient than 

convection ventilation because the laminar flow from a properly 

designed system cools only the area around the crop, not the air 

in the upper strata of the greenhouse (Langhans 1980).

24
type and must be able to function against the slight negative 

pressure that exists in a closed greenhouse. Special louvered 

intake vents are often used and are designed to remain closed 

normally, and to open only under the negative pressure created by 

the exhaust fans or when their opening motor is turned on.

The position of the fans and vents will depend on the type of 

greenhouse, its orientation, and the prevailing wind direction. Fans 

should be located to draw air across the longest dimension up to a 

maximum of 70 m (230 feet). This limitation is a compromise 

between efficient use of the tempered air and the temperature rise 

from the vents to the fan. It is more efficient to locate fans and 

vents on the end walls because fewer exhaust fans are needed, 

and it is much easier to create uniform conditions due to the 

laminar flow that is created. To be most effective, fans should be 

on the side opposite the normal direction of the wind and, where 

greenhouses are located close together, fans should not be 

positioned directly opposite one another. Other aspects of fan 

placement are covered in Nelson (1985) and Langhans (1980).

The capacity of fan ventilation systems can be measured in air 

exchange rates; obviously, the greater the exchange rate, the 

better the ventilation (table 3.1.5).

To achieve better air exchange and mixing, some greenhouses 

have installed tube ventilation systems, which use large-diameter, 

flexible plastic tubes with regularly spaced holes to draw cooler 

outside air into the warm top of a greenhouse (fig. 3.1.16). Fan-jet 

ventilation systems (fig. 3.1.17A) comb ine exhaust fans with a 

tube ventilation system to regulate air temperature and humidity 

efficiently within a growing structure (Langhans 1980). When the 

temperature or humidity is optimum, the system recirculates the air 

within the greenhouse (fig. 3.1.17B), but when they become too 

high, the exhaust fan removes inside air and the inlet louver opens 

to let in cooler outside air (fig. 3.1.17C).

There are also disadvantages to fan ventilation. The amount of air 

that can be economically drawn through a greenhouse with fan 

ventilation has a limit, and three air changes per minute is 

considered to be the maximum (Langhans 1980). If there is a 

power failure and greenhouse cooling is totally dependent on fans, 

the temperature can rise to damaging levels quickly.

Evaporative cooling. This popular greenhouse cooling system is 

based on the fact that a significant amount, 585 kcal/I (8,900 

Btu/gal), of latent heat is absorbed when water evaporates. 

Therefore, when dry air from outside the greenhouse is

25
26

drawn by a fan through a water-soaked pad, it is cooled by the 

evaporating water, and the amount of heat removed can be 

substantial (table 3.1.5). The common home "swamp cooler" is a 

miniature version of an evaporative cooling system.

The potential for using an evaporative system to cool a container 

nursery structure depends on the water vapor pressure deficit of 

the outside air and the thermodynamic efficiency of the system. 

The drier the outside air, the lower the temperature to which that 

volume of air can be cooled (fig. 3.1.18). The procedure for 

evaluating the cooling potential utilizes a psychrometer and a 

psychrometric chart to determine the wet-bulb depression, which 

indicates the drop in temperature that can be expected through 

evaporation (Langhans 1980).

A typical evaporative cooling system consists of absorbent pads 

with a water pump and sump to keep the pads wet on one wall, 

and exhaust fans on the opposing wall to draw air uniformly 

through the pads and across the growing area (fig. 3.1.19A). The 

efficiency of an evaporative system is dependent on many design 

and engineering factors, such as type and thickness of the pad 

material and position of pads and fans and is expressed as the 

temperature of the air emerging
from the wet wall divided by the wet-bulb depression (Nelson 1985). 

A well-designed system should be able to reduce the in-house 

temperature to approximately 85% of the wet-bulb depression, 

although the temperature can be expected to rise from 3 to 4 °C (5 

to 7 °F) across the greenhouse due to solar reheating (Ball 1985).

Evaporative cooling systems are common in container tree 

nurseries in warmer and drier climates, and the most popular is the 

fan and pad system. Exhaust fans were discussed in the previous 

section on fan ventilation. Pads are typically mounted vertically 

(Fig. 3.1.19A/B) and are made of specially treated cardboard (fig. 

3.1.19C), which can give 10 years of service. Aspen excelsior, 

which lasts only one season, was previously used. Horizontal 

pads, which contain porous gravel or lava rock that is kept wet by 

mist nozzles, have also been used successfully but are much less 

common (Nelson 1985).

27
28

Winter cooling. Due to the high solar radiation loads during 

clear fall or winter days, it is often necessary to introduce cool 

outside air into the greenhouse. Cold air can damage succulent 

seedlings, however, and so must be mixed with the warm, inside 

air before it is distributed throughout the greenhouse. It is 

sometimes prudent to introduce dry outside air after irrigation to 

avoid excessive humidity and the resultant condensation (fig. 

3.1.20), which can cause disease problems such as grey mold.

The fan-jet system (fig. 3.1.17A and C) that was described in the 

previous section on fan ventilation has proven to be an effective 

method of mixing and distributing outside air throughout the 

greenhouse during cold weather. Several companies sell specially 

designed environmental control systems that can accommodate 

the demands of winter cooling.

Cultural techniques. In addition to the abovementioned 

structural and equipment options, container nursery managers can 

cool their crops with several cultural techniques including shading, 

irrigation, and seed mulches.

Shade. Because internal greenhouse temperature is directly

related to solar radiation, reducing the amount of light also has a 

cooling effect (table 3.1.6). The application of a shading material to 

the outside of a greenhouse to help control summer temperatures 

is a traditional horticultural practice, but unfortunately, permanent 

shade can also reduce sunlight levels to the point that seedling 

photosynthesis and subsequent growth are adversely affected, 

especially during cloudy periods (Langhans 1980). At least with 

some species, shade can reduce root development more than 

shoot growth (Barnett 1989).

In horticultural applications, shade can be either seasonally fixed or 

movable. Shading compounds, including special shade paints, are 

considered fixed because they are applied for the entire growing 

season and may be difficult to remove. Because they function by 

reflecting a portion of incoming sunlight, only white colors are 

recommended and the amount of shade is controlled by the 

thickness of the layer (Hanan and others 1978). Shade paint 

usually weathers or washes away, or is designed to peel off with 

the first hard frost.

Fixed lath can also be used to produce permanent shade, and 

some container nurseries use special lath houses to both shade 

and protect seedlings during the hardening phase and for 

overwinter storage. Standard shadeframes are made from 

snowfence, which consists of strips of wooden lath connected by 

wires with alternate open spaces that produce 42% shade. 

Although some container nurseries produce seedlings in 

shadehouses, this amount of shade is generally considered to be 

too high for most species.

A variety of different brands of synthetic shadecloths can be used 

to generate permanent shade (fig. 3.1.21A). They are available in 

different materials or weaves that will generate any amount of 

desired shade, ranging from 25 to 90%. Although shadecloth can 

be mounted inside the growing structure, this is not 

recommended because it will absorb solar radiation and interfere 

with proper ventilation, thereby contributing to the heat load 

(Davis and Cole 1976). Mounting shadecloth outside is a better 

option because, if properly installed, it will not be damaged by 

wind or hail
and will dissipate the heat outside the greenhouse. Deploying and 

removing shadecloth are often labor intensive (fig. 3.1.21 B).

In the past few years, mechanically controlled retractable shade 

curtains have become commercially available and can be installed 

either inside or outside the greenhouse. Screens of woven or spunbonded polyester can be mounted inside the greenhouse to cool 

the greenhouse in summer and to retard heat loss at night and 

during cold weather. A novel type of shade curtain with alternating 

bands of aluminized and clear material has the added advantage of 

actually reflecting diffuse light back into the greenhouse while 

reflecting away unwanted thermal radiation. Garzoli (1988) 

considered these reflective shade screens invaluable for cooling 

greenhouses across Australia. (Retractable shade curtains also 

function to retard heat loss at night and control daylength, so 

additional information is provided in section 3.1.4 and chapter 3 of 

this volume.)

Irrigation. Container nursery managers can utilize

the high latent heat of evaporation to help cool their crops by 

scheduling short bursts ("mists") of irrigation during the hottest time of 

day. This is particularly effective during the establishment

phase, when the young, succulent germinants can be easily damaged 

by high temperatures at the surface of the growing medium. Mist 

cooling can also supply the young seedlings with enough water

without saturating the medium. Some nurseries have outfitted their 

moveable irrigation booms (fig. 3.1.22A) with multiple-position nozzles 

that contain a mist head in addition to the standard irrigation nozzle 

(fig. 3.1.22B). Older seedlings can also be cooled with irrigation, but 

this is usually done on a periodic basis to supplement the standard 

cooling system during unusually hot weather.

29
30

Running water over the outside of a greenhouse will reduce 

internal air temperatures, and it can be applied with a sprinkler in 

emergency situations. However, research trials that selectively 

filtered out all of the thermal wavelengths of solar radiation by 

continuously running a film of water or dye solution over the 

surface of the greenhouse did not provide adequate cooling 

(Garzoli 1988).

Seed mulches. Using a light-colored seed mulch after sowing (fig. 

3.1.7) will also help prevent damaging surface temperatures. Dark 

mulches absorb more solar radiation and quickly reach 

temperatures that can scald the succulent stems of young tree 

seedlings. Light-colored mulches in combination with mist cooling 

are particularly effective in preventing heat damage to the stems of 

young seedlings. (Heat injury is discussed in greater detail in

volume five of this series.)

3.1.4.4 Heating

Even if greenhouses receive a surplus of free solar heat during 

daylight hours, they must still be equipped with heating systems to 

keep the crops warm at night and to supplement solar radiation on 

cloudy days. Heating systems have become quite efficient over the 

years, but a greenhouse is a difficult structure to keep warm 

enough to grow a crop. Growing structures are designed to capture 

as much sunlight as possible, but are inherently poorly insulated. 

In northern climates, the cost of fuel is usually a limiting factor and 

so container nurseries must be carefully designed so that they can 

be heated both efficiently and economically.

Calculating heating requirements. The basic concept in heating a 

growing structure is to add heat at the same rate as it is lost, and so 

the heating system must be matched to the properties of the 

growing structure. The majority of heat is lost by conduction 

through the supporting materials that hold the greenhouse cover 

and the covering material itself. Remember that heat losses are not 

the same for all covers. For example, fiberglass panels lose only 1 

% of the total incoming radiation, compared to 4.4% for glass and 

70.8% for a single layer of polyethylene film. Adding a second layer 

of polyethylene to create an insulating air
space reduces the heat loss by almost 40% (Nelson 1985). Cold air 

infiltrating through cracks and around doors also accounts for a 

considerable loss of heat. Greenhouse heating specialists take 

measurements of greenhouse surface area and apply coefficients 

of heat loss to compute a rough estimate of total heat loss for the 

structure. If a more accurate estimate of heating requirements is 

needed, other factors such as wind velocity must be considered 

(Langhans 1980, Acme 1988). The heating requirements for a 

container nursery will vary during the calendar year; they are 

highest in the winter when solar input is least and cloud cover is 

greatest, but become negligible during the summer months (table 

3.1.7).

Types of fuels. The choice of a fuel to heat a greenhouse is critical 

because fuel can significantly affect operating costs if crops are to 

be grown during periods of cold weather. Although cost is often the 

most obvious consideration, other fuel and heating equipment 

factors must be evaluated: availability (particularly dependability of 

supply), convenience of use and storage, and cleanliness. 

Considerations related to the heating equipment include operating 

requirements, service requirements, and ease of control (ASHRAE 

1989). Most common types of fuels have been used to heat 

greenhouses, but some locally cheap and abundant fuels, such as 

coal, may be unsuitable because of their air pollution potential. The 

following general discussion of common greenhouse fuels is 

presented in order of preference, based on the Container Nursery 

Survey (table 3.1.8). More specific information can be found in 

Langhans (1980), Nelson (1985), and ASHRAE (1989).

Gas. Natural gas is composed mostly of methane and ethane, 

depending on its geological source; for safety purposes, 

mercaptans are added to produce a noticeable odor. Where 

available, natural gas is a preferred fuel for heating greenhouses 

because the initial installation of heaters is easy and inexpensive, 

storage tanks are not required, and the gas burns hot and clean. 

Natural gas was used in 33% of the forest nurseries surveyed in 

1984 (table 3.1.8). Air pollution is not a problem because the

only byproducts of natural gas combustion are water vapor and 

carbon dioxide.

Liquefied petroleum (LP) gases (propane, butane, and mixtures of 

the two) are commercially produced as byproducts of oil refineries 

or by stripping natural gas. They share most of the advantages of 

natural gas but are generally more expensive and must be stored 

in tanks (fig. 3.1.23). Propane is the only practical LP gas for most 

forest nurseries because it can be used at below-freezing 

temperatures whereas butane cannot. In spite of its higher cost, 

propane was the second most commonly used fuel because it has 

the same characteristics as natural gas and can be transported by 

truck to remote locations (table 3.1.8).

Oil. Heating (or fuel) oil is also commonly used in container tree 

nurseries (table 3.1.8). Heating oil comes in several grades that are 

determined by viscosity (which increases with the number of the 

grade) and other suitability factors. The heavier oils cost slightly less 

and have a higher heat content but must be preheated before they 

can ignite (ASHRAE 1989). Grade #1 is kerosene, which is not 

typically used for greenhouse heating; #2 is the common fuel used 

for unit heaters because it does not have to be preheated. Because 

of their higher viscosities, grades 4, 5, and 6 are only use31
32

ful for the larger furnaces used in central heating systems (Bartok 

1990). Their sulfur contents are also a consideration and are 

related to grade, ranging from an allowable maximum of 0.64% for 

#2 heating oil to 4.00% for #6. In addition to their air pollution 

potential, high-sulfur fuels are more corrosive to heating 

equipment.

Electricity. Electric heat is not a common source of heat in 

ornamental nurseries, but was the fourth most popular in the 

Container Nursery Survey (table 3.1.8). This popularity is probably 

a reflection of the fact that electricity can be easily transmitted to 

locations where few other fuel options are available. In addition to 

being the most effi cient source of energy for heating (table 3.1.8),

electricity has the advantage of being quiet, clean, and nonpolluting. However, it is by far the most expensive energy source.
Waste heat. A few forest nurseries have converted to using 

heat generated from other industries for growing tree seedlings 

(table 3.1.8). As an example, for every 1 billion kcal that are 

converted into electricity at generating plants, another 1 billion kcal 

are lost in the cooling water. This hot water could easily be used 

for heating greenhouses (Langhans 1980). One nursery reported 

that cooling water at 32 °C (90 °F) from a power plant supplied 

750,000 kcal/h of heat for their greenhouses (Container Nursery 

Survey).

Solar heat. By their very design, all greenhouses use solar 

energy to supply at least part of their heating requirements. A few 

forest nurseries reported that solar energy was their main source 

of heat (table 3.1.8). Many different solar heating systems have 

been designed for greenhouses (fig. 3.1.25A), but they have not 

been widely adopted because of the low cost and availability of 

fossil fuels. However, container nursery managers can make minor 

structural modifications to capture the solar energy that is so 

abundant during the daytime and store it for use during the night 

(fig. 3.1.25B). Thermal insulating blankets are another simple 

design feature that are very effective in retaining captured solar 

energy. (See the following section on heat conservation for more 

details.)

Beaters and heat distribution systems. Once the type of fuel 

has been selected, the next step is to convert it into usable heat 

and the first decision is whether to use central or unit heaters. 

With a large central heating system, one or two large boilers are 

located in a single position (fig. 3.1.26A), and steam or hot water 

is piped to the various surrounding growing structures (fig. 

3.1.26B). These systems require a larger initial investment-$16 to 

$32/m2 ($1.50 to $3.00 per square foot)-and thus are only 

economical for large-scale nurseries.

Unit heaters are much smaller and are located in each growing 

structure (fig. 3.1.27A), where they distribute th e heat-hot air, 

water, or thermal radiation directly to the crop (fig. 3.1.27B). They 

are more practical for smaller nursery operations and cost 

approximately $2.70 to $10.80/mz ($0.25 to $1.00/ft2) (Nelson 

1985).

33

Wood. In some areas of North America, where wood is cheaply 

and readily available, wood is being used to heat container tree 

nurseries (fig. 3.1.24). Compared to other fuels, wood has a 

relatively low heat output (table 3.1.8), is bulky to handle, and 

creates a considerable volume of ash, which must be removed. 

However, wood is a good, relatively nonpolluting fuel when burned 

in a properly designed and maintained furnace. Wood pellets are a 

recent innovation that may prove practical for heating greenhouses, 

especially in areas where other fuels are unavailable.

Coal. Coal was a traditional fuel for heating greenhouses but 

is not commonly used in container tree nurseries (table 3.1.8). 

Where readily available, coal is generally the cheapest fuel but is

bulky to handle and store. Some grades are high in sulfur and are 

therefore unsuitable because of air pollution concerns.

Since the fuel crises of the 1970's, container nursery managers 

have become much more cost conscious when it comes to heating 

fuels and many new options are being explored:
34
Steam or hot water. A traditional system for heating greenhouses is 

to heat steam or hot water in a central boiler (fig. 3.1.26A), but new 

high-efficiency boilers are small enough to fit within individual 

houses. The hot water or steam is circulated through either plain or 

tinned tubing around the walls, overhead, under benches (Fig. 

3.1.26B), or in the floor. Heat radiating from these tubes circulates 

throughout the greenhouse by convection, and so the location of the 

heating tubes is critical. ,Although steam is a more efficient way to 

transfer heat, it has more maintenance problems. Hot water 

systems ;generally provide more uniform temperatures (Langhans 

1980). Large central heating systems are most efficient when 

operated at full capacity, and so must be properly matched to the 

size of the nursery operation. Steam or hot water heating was used 

by approximately 26% of the container tree nurseries surveyed in 

1984.

Warm air. Many container nursery operations use small, 

self-contained unit heaters to heat individual growing structures with 

warm air (fig. 3.1.27A). These forced air heaters burn fuel in a lower 

combustion chamber and the hot gases rise through heat 

exchanger tubes and are exhausted through an outside stack. A fan 

in the rear of the heater blows through the heat exchangers and the 

warm air is either discharged directly into the greenhouse or into a 

fan-jet distribution system (fig. 3.1.27B). The latter is much 

preferred because direct discharge systems create problems with 

heat circulation, and warm air blowing directly onto the crop can 

cause excessive drying (Langhans 1980). Forced air unit heaters 

are the most common type (71 %) of heating system used in forest 

tree nurseries. The distribution tubes are often placed under the 

benches (fig. 3.1.27B) so that the warm air will rise through the 

seedlings, warming the growing medium and drying the foliage. 

Under-bench heating is particularly effective in reducing the 

incidence of certain fungus pests, such as grey mold.

Radiant heaters. In the past decade, some container tree 

nurseries (2%) have installed overhead radiant heating systems 

(fig. 3.1.28A), also called infrared heaters. These low-energy 

unit heaters consist of a series of small gas burners that are 

regularly spaced along a metal pipe (fig. 3.1.28B).

35
36

Hot flue gases are drawn along the pipe and heat it to the 

temperature that generates infrared radiation and the gases are 

then exhausted at the far end. The thermal radiation produced is 

directed down onto the plants by aluminum reflectors mounted 

above the hot pipe. Infrared heaters are quite energy efficient; 

some ornamental growers have reported a 30 to 50% reduction in 

fuel use (Nelson 1985).

One unique feature of overhead radiant heaters is that the plants 

are kept warmer than the surrounding air because the infrared rays 

are not converted to heat until they are absorbed by the plants. 

This practically eliminates condensation, which is one of the major 

causes of foliar disease. Overhead radiant heaters are also 

appreciated by nursery workers because the thermal radiation 

warms individuals without unnecessarily warming the air (fig. 

3.1.28A). One drawback of the system is that, after seedling crown 

closure, the foliage absorbs all the heat and so the growing 

medium may remain too cool (Langhans 1980).

Heat conservation. During the energy crisis of the early 1970's, 

the increasing cost and scarcity of traditional heating fuels 

prompted an active research and development effort to reduce 

greenhouse energy requirements. Now, well-designed growing 

structures are much better insulated and equipped with 

energy-efficient heating systems. The following suggestions should 

be considered when designing a container tree nursery:

Select sheltered nursery sites. Container tree nurseries should 

not be located in frost pockets or in windy, exposed locations. 

Windbreaks can be effective in reducing conductive heat loss as 

long as they do not shade the growing area. (Site selection is 

discussed in more detail in volume one of this series.)

Maximize the capture of solar energy. Prevent shading within the 

growing structure by minimizing overhead piping and frameworks. 

Overhead heating or cooling equipment can create shade patterns 

that may reduce photosynthesis (fig. 3.1.16). Keep the greenhouse 

covering material clean and regularly replace weathered sections.
Insulate the growing structure. Insulation is particularly 

important in container nurseries because greenhouses can lose 

heat 5 to 10 times faster than an average residential house. 

Langhans (1980) discusses options that can significantly reduce 

heat losses. These are addition of 1) an extra plastic cover over 

the greenhouse to create an insulation air space, 2) retractable 

heat curtains, and 3) permanent insulation for north wall and roof 

areas.

An extra plastic covering is effective on structures constructed of 

either polyethylene sheeting or rigid panels. Houses covered with 

double layers of polyethylene are equipped with a small blower 

that maintains an insulating space of dead air between the layers, 

and energy savings of 30 to 40% over single layer structures have 

been reported. Approximately two-thirds of the nurseries in the 

Container Nursery Survey had energy-efficient double-layer 

coverings. (See volume one in this series for more information on 

growing structures.)

Retractable heat curtains can be extended out over the crop at 

night to reduce both radiation and convection losses (fig. 3.1.29A) 

while not interfering with needed sunlight during the day (fig. 

3.1.29B). Almost any material that can be suspended on wires and 

pulled back and forth with a pulley system can be used as a 

thermal blanket. Sophisticated automatic systems are 

commercially available and can be installed in any type of growing 

structure without interfering with photoperiod lights or the irrigation 

system (fig. 3.1.29C). Retractable curtains are also used for shade 

cooling (see section 3.1.4.3) and photoperiod control (see chapter 

three in this volume).

Adding insulation, especially below bench height and on north 

walls (fig. 3.1.30A), can be effective in retarding heat loss. Tests 

have shown that a net loss of sunlight occurs through the north 

wall of a growing structure, and so covering the north wall and roof 

with reflective insulating material can retard heat loss while 

actually increasing photosynthetic active radiation; other inflatable 

insulating layers can be installed overhead (fig. 3.1.30B), or along 

the sidewalk (fig. 3.1.14).

37
Be space efficient. Utilize greenhouse growing space 

effectively by keeping the structure filled with plants because 

the crop creates a thermal mass that will retain heat better than 

an empty structure. Minimize aisle and edge space and use 

moveable benches, if possible.

Modify cultural procedures to conserve energy.

Maintain temperatures in the growing area to produce the most 

seedling growth for the least energy input. Because heating 

requirements are highest at night, lower the night temperature as 

much as possible. Langhans (1980) reported that lowering the 

night temperature a mere 3 °C (5 °F) will result in a 17% savings 

in heating costs; fuel savings as high as 50% are possible (table 

3.1.9).

Design growing schedules around the solar calendar. Careful 

crop scheduling can result in considerable heat savings, 

especially when producing more than one crop per year. Growers 

should design their schedules so that trees are being hardened 

during the coldest months and winter crops consist of species that 

will tolerate lower temperatures. In some locations, crops may not 

be economical during the winter season because of the high cost 

of heating. (Crop scheduling is described in more detail in section 

3.3.3.1 and in volume six of this series.)

38
3.1.5 Temperature Monitoring and

Control Systems

Because temperature is so critical to tree seedling growth, growers 

should monitor air and soil temperature throughout the crop cycle. 

Successful growers use temperature-sensing devices to detect 

when conditions reach damaging levels, and alarms are activated 

when temperatures become either too hot or too cold. Many 

modern container nurseries have sophisticated environmental 

control systems that are continuously monitored by computer, but 

even the simplest facility should have certain instruments for 

monitoring and controlling temperature.

3.1.5.1 Sensing instruments

Growers have traditionally measured ambient temperature with 

standard glass thermometers. The "max-min" thermometer (fig. 

3.1.31A) is an inexpensive instrument that should be used in all 

container tree nurseries. Not only can the current temperature be 

read, but the maximum and minimum temperature extremes are 

automatically recorded. A record of daily temperature fluctuations 

can be easily obtained by recording the values and resetting the 

instrument. A number of relatively inexpensive electronic 

thermometers are commercially available, and some have long 

probes that are useful for checking the temperature of the growing 

medium inside the container. Some of these instruments feature 

digital displays and data storage, yet are small enough to be 

carried in a pocket (fig. 3.1.31 B).

Because they are not inherently accurate, thermometers and other 

environmental monitoring equipment should be calibrated before 

initial use and at regular intervals. The most important source of 

error in measuring temperature is the effect of solar radiation. 

Thermometers should always be shaded when in use, and 

permanently mounted instruments must be placed out of direct 

sunlight. Some instruments are both shielded and aspirated, that 

is, a steady stream of air is drawn over the instrument with a small 

fan. (The proper location of environmental control equipment is 

discussed further in volume one of this series.)

Another indispensable, yet inexpensive, piece of nursery 

equipment is the recording hygrothermograph (fig. 3.1.31 C), 

which continuously records both air temperature and relative 

humidity. The basic hygrothermograph contains a bimetallic strip 

thermometer, which measures temperature based on the 

differential expansion of two strips of different metals bonded 

together, and a hair hygrometer, which uses strands of human hair 

to record changes in relative humidity. This handy instrument 

contains a recording drum, and models can be purchased which 

record for either a week or a month between servicings. The drums 

are powered by either clock springs or batteries and so must be 

rewound or checked regularly.

Growers should plan to have at least one hygrothermograph for 

each growing structure. Hygrothermographs can be moved to 

various locations to check for hot or cold spots, or they can also 

serve as a standard while other locations are checked with 

thermometers. As with all temperature monitoring equipment, 

hygrothermographs should be kept shaded. Hygrothermographs 

must be calibrated regularly with an accurate thermometer and a 

sling psychrometer.

Most modern greenhouses are outfitted with sophisticated 

electronic control equipment that monitors temperature as well as 

many other environmental factors.

3.1.5.2 Control equipment

A mechanical thermostat, which consists of a temperature 

sensor and a switch, can be used to activate everything from 

motorized vents to valves on mist irrigation lines. The temperature 

sensitivity of a thermostat is called the differential, which is 

expressed as the number of degrees between switching actions; 

most greenhouse thermostats have a differential of 0.5 to 2 °C (1 

to 4 °F). The range of a thermostat is the temperature span within 

which the switch will operate, generally the range of 2 to 40 °C (35 

to 105 °F). Thermostats provide a simple on-off control function, 

and multiple functions can be gained by using more than one 

thermostat (fig. 3.1.32A), which is referred to as staging (Aldrich 

and Bartok 1989). Mechanical ther39
The next step in sophistication and cost is the thermistor, which is a 

solid- state temperature sensor that changes voltage output in 

response to temperature and activates a switch. Thermistors can 

be integrated into a circuit with a microprocessor or even a 

computer to form an "intelligent" control system (Nelson 1985). 

Sophisticated environmental control systems can sense and 

integrate several environmental variables at once and control both 

heating and cooling stages (fig. 3.1.32B). (More information on 

environmental control systems is presented in volume one of this 

series.)

40
41
3.1.6 Conclusions and Recommendations

Although seedling growth occurs over a wide range of 

temperatures, container nursery managers need to identify those 

optimum temperatures for the different species and ecotypes that 

they will be growing. Optimum temperatures will vary with the 

stage of seedling development, and it is critical to learn how much 

temperature variation can be tolerated while producing an 

acceptable crop. Temperature guidelines are provided in tables 

3.1.2 and 3.1.3. If more than one species must be produced in the 

same growing structure, then growers must accept a compromise 

temperature regime that will provide acceptable growth for the 

entire crop.

Approaches to temperature control in container nurseries will vary 

by type and location of the growing structure, availability of fuel 

sources, and type of temperature modification equipment. The 

information provided in this chapter can be used in the design of 

new growing structures or to improve the operation of existing 

facilities. Although economics will dictate which options are best, 

the nursery manager's objective should always be to optimize 

temperature regimes to the extent necessary to produce quality 

seedlings.

42
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