73
Contents

3.3.1 Introduction 75

3.3.1.1 Biophysics of light 75

3.3.1.2 Definitions and units 76

3.3.2 Role of Light in Tree Seedling

Growth and Development 80

3.3.2.1 Photosynthesis 80

Effect of light intensity 81

Use of artificial light 83

3.3.2.2 Photomorphogenesis 84

The phytochrome system 85

Extending photoperiod 85

Shortening photoperiod 88

Phototropism 91

3.3.3 Optimum Light Levels 92

3.3.3.1 Crop scheduling 92

3.3.3.2 Establishment phase 93

3.3.3.3 Rapid growth phase 94

3.3.3.4 Hardening phase 95

3.3.4 Modifying Light in Container Tree

Nurseries 96

3.3.4.1 Effect of structural coverings 96

3.3.4.2 Shading the growing area 97

3.3.4.3 Types of lamps 98

Incandescent lamps 98

Fluorescent lamps 103

High-intensity discharge lamps 104

Reflectors 106

3.3.4.4 Photosynthetic lighting 106

Light intensity 106

Light quality 107

Lamp fixtures and positioning 107

3.3.4.5 Photoperiodic lighting 107

Types of photoperiodic lighting 107

Light intensity and quality 108

Lamp fixtures and positioning 108

3.3.4.6 Short-day treatments 111

3.3.5 Light Monitoring and Control

Systems 115

3.3.5.1 Measuring light levels 115

Photometers 115

Radiometers 116

3.3.5.2 Control systems 116

3.3.6 Conclusions and Recommendations

117

3.3.7 Literature Cited 118

74
vision and radio waves pass harmlessly through most objects. The 

electromagnetic spectrum ranges from very long wavelength AM 

radio waves (1 km), to visible light (10-9 m), to extremely short 

wavelength cosmic waves (10-15 m)(fig. 3.3.2).

The term sunlight is the common name for electromagnetic 

radiation that originates from our sun, some 150 million km (93 

million miles) away. All materials above absolute zero, which is 

-273 °C (-469 °F), emit some electromagnetic radiation, and the 

wavelength of this radiation is a function of the temperature of the 

source. The sun, with its surface temperature of 6,000 °C (10,800 

°F), produces a wide range of electromagnetic radiation; 99% of 

which is between 200 and 2,000 manometers (nm) in wavelength 

(fig. 3.3.3).

The earth's atmosphere selectively screens out most ultraviolet 

radiation and also some visible wavelengths (fig. 3.3.3). Because 

clouds absorb and reflect so much visible sunlight, the total 

amount of solar radiation that is available for growing plants is 

affected by the latitude and number of cloud-free days (fig. 3.3.4). 

Because of this geographic variation, many horticultural 

greenhouses are located in the southwestern part of the United 

States, where skies remain sunny throughout the

75

3.3.1 Introduction

The nature of light has fascinated humans for thousands of years, 

but it was not until a little more than a century ago that its true 

nature began to be understood. Light is the most complex and 

variable of the environmental factors that affect plant growth and is 

arguably the most important (Smith and Whitelam 1990). In 

addition to its biological importance, light has both practical and 

economical implications for container tree nurseries. Light is a 

subjective phenomenon, and so container nursery managers must 

remember that their own response to light is not the same, in terms 

of wavelength (color) or intensity, as the response of their tree 

seedlings (fig. 3.3.1).

3.3.1.1 Biophysics of light

Light is defined as that part of the electromagnetic spectrum that 

is visible to the human eye. Our planet is bathed in a wide variety 

of electromagnetic waves from both natural and artificial sources. 

This radiation can be described by either wavelength or energy 

terms (fig. 3.3.2). Because shorter wavelengths carry more energy 

than longer wavelengths, cosmic rays are deadly whereas tele
year. (The effects of climate on container nursery location are 

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

3.3.1.2 Definitions and units

The various types of electromagnetic radiation originating from the 

sun can be distinguished by their wavelengths. Although any units 

of length can be used, the wavelength of solar radiation is 

commonly measured in nanometers (1 nm = 10-9 m). English 

conversions for these units are available (Bickford and Dunn 1972) 

but are almost never used. The portion of the electromagnetic 

spectrum that has biological significance can be roughly separated

into ultraviolet radiation (UV = < 400 nm), visible light (400 to 

700 nm), and infrared radiation (IR = > 700 nm) (fig. 3.3.3).

The wavelengths that have significance to horticulture have been 

divided into 8 bands, based on their biological effects (table 

3.3.1). Two specific physiological processes that are important for 

plant growth can be manipulated culturally by light of specific 

wavelengths. Photosynthesis is greatest in red light (band 3), with 

some activity in the blue-green wavelengths (band 5), whereas 

photomorphogenesis is most affected by far-red, red, and 

blue-violet light (bands 2, 3, and 5). Most of the other 

wavelengths are reflected, giving plants their

76
characteristic green color. However, for practical purposes, plant 

scientists combine the wavelengths from 400 to 700 nm into one 

term: photosynthetically active radiation (PAR).

Radiant energy can be measured in many different ways, and 

numerous terms and units have been used. The choice depends 

on use; for horticultural purposes, radiation should be measured in 

terms of power (watts) or photon energy (micromoles) within the 

PAR spectrum. Although they also describe light with power units, 

lighting engineers use units that reflect the sensitivity of the human 

eye within the visual spectrum (lux). Three systems are commonly 

used to describe and measure light (table 3.3.2):

Energy units- micromoles per second per square meter 

(μmoI/s/m2)--measure the rate of radiation energy (photon flux 

density) per unit area. Simply stated, energy units describe the 

amount of power

(in the case of sunlight, solar power) that is intercepted by an 

object (Bickford and Dunn 1972). The amount of energy per unit 

varies inversely with wavelength: 1 mole of red light (670 nm) 

carries 42,000 calories compared to 1 mole of blue light (470 nm), 

which has 60,000 calories (Reifsnyder and Lull 1965). Energy 

units are always in the PAR wavelengths unless otherwise stated. 

One mole of photons is also called an einstein (ASHRAE 1989).

Radiation units-watts per square meter (W/m2)--also measure 

radiation energy per unit area, but the specific wavelength and 

time interval also must be stated. Radiation units are often used 

by ecologists and plant scientists to describe the total amount of 

solar radiation that is received at a given location. In horticulture, 

radiation units are most useful for describing the intensity of 

artificial lighting because lamps are rated in watts.

77
Illumination units--lux (Ix)—are a standardized measure of 

artificial light per unit area at a given distance from the source. 

They cover the visual radiation from 380 to 710 nm and peak at 

555 nm, corresponding to the visual sensitivity of the human eye 

(fig. 3.3.1). The standard unit of illumination is the lumen. A 

lumen that is evenly

78

distributed over an area of 1 ml is defined as 1 Ix; a lumen 

distributed over 1 square foot is 1 foot-candle (table 3.3.2). 

Although the sensitivity spectrum of the human eye is quite 

different from the needs of plants, the cheapest and most 

commonly used light-measuring instruments are calibrated in lux 

or foot-candles.
79

The best light unit to use in horticulture has been the subject of 

much debate. Both energy and radiation measurements are 

concerned with the same aspects of light but, for horticultural 

purposes, the use of energy units has been recommended as the 

most appropriate. When radiation units must be used, as when 

working with artificial lighting, the factors for converting to energy 

units will vary with the type of lamp (table 3.3.3). Complete 

conversion tables can be found in Thimijan and Heins (1983).

There are two common units for measuring energy that are 

numerically equal (table 3.3.2): micromoles or microeinsteins per 

second per square meter (μmol/s/m2 or μE/s/m2). Although 

microeinsteins are commonly used by seedling physiologists 

(Kramer and Kozlowski 1979), micromoles are most widely 

recommended (Thimijan and Heins 1983) and will be used in this 

manual. Because illumination units (lux) are the most common way 

of measuring light in operational nurseries, light measurements will 

be given in both illumination and energy units. The conversion between energy and illumination 

units varies with the light source, however (table 3.3.3). To 

avoid confusion and yet remain accurate, the value in 

micromoles will be followed by its approximate lux conversion 

in parentheses. For example, there is considerably more light on 

a sunny day [2,000 μmol/s/m2 (~108,000 Ix)] than on one that is 

heavily overcast [60 μmol/s/m2 (~3,200 Ix)] (table 3.3.3). For those 

using the English illumination units of foot-candles, the conversion 

is simple because the two values differ by a factor of 10 (1 

foot-candle = 10.8 Ix).

The proper units must also be considered when purchasing alight 

meter or measuring light. For operational purposes, growers must 

use a light meter that will measure the relevant wavelengths. To 

measure the amount of sunlight or artificial light that is available 

for photosynthesis, PAR should be measured. For dormancy 

prevention, only red and far-red need be measured. (See section 

3.3.5 for more information on types of light meters.)
3.3.2 Role of Light in Tree Seedling

Growth and Development

There are three main properties of light that affect plant growth: 

intensity, duration, and quality (Kramer and Kozlowski 1979). For 

the accelerated rate of growth that is desired in container tree 

nurseries, the source of light must provide enough radiant energy 

for photosynthesis. Light intensity is also a factor in preventing 

dormancy, although the required intensity is much less. The 

duration of light is primarily concerned with the daylength (actually, 

the length of the dark period is more important) that is required to 

prolong vegetative growth. Light quality refers to the fact that 

different wavelengths trigger different functions in plants; within the 

PAR range, however, light quality is considered to be less 

important than intensity and duration (ASHRAE 1989).

On a physiological basis, the effects of light on tree seedling 

growth and development can be grouped into two categories: the 

high-energy requirements of photosynthesis and the low-energy 

effects that are collectively known as photomorphogenesis 

(Kramer and Kozlowski 1979). Each of these responses is 

sensitive to specific wavelengths (fig. 3.3.1):

• Photosynthesis—Radiant energy (most PAR wavelengths 

but predominately red) is captured by the carotene and 

chlorophyll pigments and converted into the chemical energy 

needed for plant growth and metabolism, using carbon 

dioxide and water as raw materials.

• Photomorphogenesis—Radiant energy (far-red, red, and 

blue wavelengths) is captured by phytochrome and other 

pigments. Phytochrome is sensitive to the ratio of red to 

far-red light and acts as an environmental sensor to measure 

daylength. The phytochrome system controls seedling 

phenology, such as seed germination and bud set and also 

triggers other morphological reactions, such as the response 

to shade. Blue light is important to normal morphological 

development, particularly in regard to branching and shoot 

sturdiness.

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3.3.2.1 Photosynthesis

Photosynthesis is one of the most important chemical process on 

earth-most life would impossible without it. Plants produce the 

basic building blocks of life such as carbohydrates, amino acids, 

and fats and also generate oxygen as a byproduct of 

photosynthesis. Oxygen is essential for the respiration of all 

organisms.

The majority of photosynthesis takes place in the foliage; 

chlorophyll, which gives plants their green color, is contained in 

chloroplasts in the leaves. Water is absorbed in the roots and 

transported to the leaves where it is combined with carbon dioxide 

in the presence of light to synthesize sugar. The products of 

photosynthesis are transported throughout the plant and are used 

in respiration, which is essentially the reverse chemical reaction of 

photosynthesis (fig. 3.3.5). Respiration, which occurs in the dark as 

well as the light, releases the chemical energy from the 

photosynthate for a wide variety of growth and maintenance 

functions (Kramer and Kozlowski 1979).

Net photosynthesis is measured by determining the amount of 

carbon dioxide that is removed from the air around a leaf or plant. 

The total photosynthetic rate is calculated by adding the amount of 

carbon dioxide that is evolved during respiration to that absorbed 

from the air. Plant growth is determined by the amount of 

photosynthate that is left over after respiration, and so the rate of 

net photosynthesis is more relevant to horticulturists than is the 

total rate (Mastalerz 1977). The rate of net photosynthesis is 

affected by several environmental variables (including availability 

of water and carbon dioxide) but is strongly controlled by 

temperature, which determines the rate at which photosynthate is 

used up in respiration (fig. 3.3.6)
Effect of light intensity. Photosynthesis increases curvilinearly 

with the intensity of the light (fig. 3.3.7). The response varies 

between species and is subject to the levels of other limiting 

factors, especially temperature, carbon dioxide (C02), and water. If 

the light intensity is too low (0 to 20 μmol/sec/m2, or approximately 

0 to 1,100 Ix),

the rate of photosynthesis will be less than respiration, and there 

will be a net loss of photosynthate (A, fig. 3.3.7). As light is 

increased to 20 to 50 μmol/sec/m2 (~1,100 to 2,700 Ix),

photosynthesis will equal respiration, and there will be no net 

exchange of C02. This is known as the light compensation point 

(B, fig. 3.3.7), which is important because it determines the 

absolute minimum amount of light that must be supplied to keep a 

seedling crop alive.

81
As the light intensity is increased beyond the compensation point, 

photosynthesis increases rapidly up to approximately 180 

μmol/sec/m2 (~9,700 Ix); above this point (C, fig. 3.3.7), 

photosynthesis continues but at a slower rate. If light intensity 

continues to increase, the plant will reach a point where the 

photosynthetic response curve flattens, the light saturation point 

(D, fig. 3.3.7). Beyond this light intensity level, there is no value in 

providing more light; in fact, it is likely to be detrimental due to the 

extra heat that it produces.

Light saturation occurs at different light levels for different species 

(fig. 3.3.8A; table 3.3.4) and at higher light intensities under high 

temperatures and enriched C02 concentrations (Kramer and 

Kozlowski 1979). The light saturation point also varies between 

different leaves on the same plant; leaves that develop in the 

shade ("shade leaves") reach light saturation at much lower light 

intensities than those that develop in full sunlight ("sun leaves") 

(fig. 3.3.813). Saturation light intensities can be as much as 3 

times higher for an entire plant as for an individual leaf because of

mutual shading (Nelson 1985). Considering that container tree 

nurseries typically grow seedlings at high densities, the saturation 

light intensity would be much higher for a block of seedlings than 

for a single plant.

Extremely high light intensities can also cause "sunburn' (fig. 

3.3.9), which is technically termed photodamage or solarization 

(Levitt 1980). Plants normally adapt to higher light intensity by a 

variety of physiological adjustments. Under stressful conditions, 

these mechanisms may become ineffective and the foliage can be 

photodamaged. Susceptibility to photodamage varies with seedling 

hardiness: dormant stock is least susceptible, and young succulent 

tissue is particularly sensitive (Gillies and Vidaver 1990). But even 

dormant, hardy plants can suffer photodamage. Excessive light 

intensity was found to cause chlorosis of Engelmann spruce 

seedlings that were shipped from the nursery to high elevation 

outplanting sites (Ronco 1970).

82
Use of artificial light. It is possible to compensate culturally for 

lack of sunlight with supplemental lighting, but high-intensity 

lighting systems are expensive to install and operate (fig. 3.3.10A). 

Supplemental photosynthetic lighting was used in only 5.4% of the 

forest nurseries surveyed in the United States and Canada. It is 

usually considered to

be uneconomical, except under the following cir

circumstances:

• During the winter at high latitudes, when sun light intensities 

are low and days are short. For horticultural crops, 

supplemental photosynthetic lighting is considered to be 

practical in nurseries above 40° latitude (Nelson 1985).

83
• When the crop is unusually valuable, as with tree improvement 

stock (Bongarten and Hanover 1985).

• When light-demanding species must be grown

where overcast or foggy conditions are common.

(Specifications and other application information for photosynthetic 

lighting are provided in section 3.3.4.4.)

3.3.2.2 Photomorphogenesis

There are many different photomorphogenic responses by plants, 

but by far the most important is photoperiodism, which is the 

response of plants to the relative length of day and night. In 

horticulture, the terms photoperiod and daylength mean 

essentially the same thing and are used interchangeably. The 

ability to sense relative daylength is controlled by a light- sensitive 

pigment called phytochrome (Smith and Whitelam 1990).

84
The phytochrome system. Phytochrome is a blue-green pigment 

existing in two forms that respond to the ratio of red (R = 660 nm) 

to far-red (FR = 735 nm) light. Red light converts the inactive to the 

active form, and far-red light reverses the reaction. Daylight, which 

contains a high R: FR ratio, converts phytochrome to the active 

form; in darkness the active form slowly reverts to the inactive form.

Photoperiodic lighting contains a high R: FR ratio, and so 

effectively shortcuts the natural system (fig. 3.3.11).

Phytochrome is present throughout the plant but is most abundant 

in meristematic tissues, such as seeds and vegetative or flower 

buds (Kramer and Kozlowski 1979). It functions at all stages of the 

life cycle and provides plants with invaluable information about the 

light environment. In addition to sensing daylength, the 

phytochrome system controls other important physiological 

functions such as seed germination, phototropism, and shadeinduced responses (Smith and Whitelam 1990).

Horticulturists have identified a critical daylength for each of their 

commercial species, and traditionally divide their crops into 

long-day, short-day, and day-neutral types. Different cultural 

regimes have been developed for each group (Hartmann and 

Kester 1983). The situation is much simpler in container tree 

nursery culture because we are not interested in flowering, only in 

prolonging or terminating shoot growth.

Extending photoperiod. Photoperiodic lighting (fig. 3.3.10B) is 

the most common use of artificial lighting in horticulture (Bickford 

and Dunn 1972). Container seedling growers use photoperiodic 

lighting to extend daylength in all types of growing environments, 

from open compounds to fully controlled greenhouses. In a recent 

survey, 82% of the forest nurseries in the United States and 

Canada used artificial lighting to regulate photoperiod.

Photoperiod extension is critical to achieving the accelerated 

growth rates that are possible in container tree nurseries, 

especially when seedlings from more northern latitudes or higher 

elevations are grown at more southerly or lower elevation 

nurseries (Arnott and Mitchell 1982). Under natural conditions, 

shoots grow only during the long days of summer and set buds 

and become dormant when the days become shorter in the fall. 

Extending photoperiod is operationally important when seedlings 

cannot be reared to the desired size during the normal spring to 

fall growing season, and becomes particularly valuable when 

multiple crops must be produced in one season.

Species show variable responses to photoperiod extension. 

The need for photoperiod lighting will depend on the following 

three main factors:

Location of the nursery. Nurseries in tropical and subtropical 

climates usually do not provide photoperiod lighting because most 

of their crops grow year round. Growth chamber experiments have 

demonstrated, however, that some tropical species will exhibit 

accelerated shoot growth when produced under extended 

daylength (Kramer and Kozlowski 1979). In temperate climates, 

container

85
photoperiod control at germination. A few species, such as 

mountain hemlock, are so sensitive that they will set a premature 

bud if they germinate in the absence of photoperiodic lighting 

(Arnott 1991). If the photoperiod lights fail for as little as one 

night, then some species will set a premature terminal bud (Arnott 

and Simmons 1985) (fig. 3.3.12D). When seedlings have been 

growing under photoperiodic lighting for extended periods, the 

critical daylength for continued accelerated growth may need to 

be lengthened. Even with a 24-hour photoperiod, most temperate 

species grown for an extremely long time will eventually set bud 

(Kramer and Kozlowski 1979).

Effects of extended photoperiod are also affected by temperature. 

Spruce seedlings subjected to the same photoperiodic lighting 

grew significantly larger in a heated greenhouse than in an 

unheated shelterhouse (Arnott and Mitchell 1982). The same 

researchers found that cool night temperatures produced smaller 

seedlings and delayed formation of a terminal bud even under 

photoperiodic lighting.

Artificial lighting for extending photoperiod is different from that 

used for photosynthesis in all three aspects: intensity, quality, 

and duration. Light for dormancy control acts as a trigger; it 

requires very little power and need not be continuous. In contrast, 

the light required for photosynthesis is for power, which is why the 

light must be of high intensity and continuous (Cathey and 

Campbell 1977). Most forest species will continue growth when 

supplied with 8 μmol/sec/m2 (~430 Ix) of incandescent light given 

3% of the time, although requirements vary by species (table 

3.3.5). Photoperiodic light intensities as low as 0.24 to 0.60 

μmol/sec/m2 (~ 20 to 50 Ix) are effective with some northern 

conifers (Arnott and Mitchell 1982). Above this critical minimum 

light intensity, seedling height growth increases rapidly with 

intensity and then appears to taper off (figure 3.3.14), and there is 

no additional benefit from providing more light. The critical 

minimum intensity varies

nurseries that produce species from a wide geographic area, and 

especially from high-elevation or high-latitude sites, should plan on 

supplying photoperiodic lighting (fig. 3.3.12A).

Species and ecotype. Species, such as Engelmann spruce, that 

only grow on high-elevation sites will always require photoperiodic 

lighting. The degree of response increases with elevation; for 

example, some subalpine species exhibit premature budset even 

under conventional photoperiodic lighting (fig. 3.3.12B). The 

response of seedlings of species that grow over a wide geographic 

range (for example, Douglas-fir) will depend on the specific seed 

source; seedlings from low-elevation, coastal sources grow 

perfectly well without lights, whereas seedlings of ecotypes from 

high-elevation or interior locations cannot be grown without them. 

It is prudent to use photoperiodic lighting when growing a mixture 

of species and ecotypes, or when the response of all seed lots is 

unknown, because nonresponsive species are not harmed and 

operating costs are minimal. Once seedlings set a dormant 

terminal bud, it is difficult and/or time consuming to get them to 

resume shoot growth.

The cropping cycle. Nurseries that grow more than one crop per 

year will usually need photoperiodic lights to keep their crops from 

setting bud prematurely in the fall; this is even more important for 

late fall or winter crops. On the other hand, nurseries that do not 

have rigid time constraints, and therefore do not need to maximize 

growth rates, can grow perfectly acceptable seedlings without 

photoperiodic lighting. Sensitive seedlots may exhibit an 

unacceptable amount of height variation, however (fig. 3.3.12C).

Other factors. Response to photoperiodic lighting is also a 

function of seedling age. For example, Scotch pine showed no 

response at 6 weeks but exhibited a strong photoperiod 

requirement by 18 weeks of age (fig. 3.3.13). The precise age at 

which different species become sensitive is not known. Most 

container nurseries should begin

86
with seed source: four different interior spruce seedlots had 

critical minimum intensities ranging from 0.24 to 0.96 

μmol/sec/m2 (~20 to 80 Ix) (Arnott and Mitchell 1982). Upper 

limits at which

no additional growth response were noted ranged from 1.0 

μmol/s/m2 (~100 Ix) for Engelmann spruce to 8.6 μmol/s/m2 (~800 

Ix) for white spruce (Arnott and Macey 1985).

87
88

As for light quality, the objective is to keep the phytochrome in the 

active form by using lamps that generate a high ratio of red (660 

nm) to farred (735 nm) wavelengths (fig. 3.3.11). (See section 

3.3.4.5 for operational applications of extending photoperiod.)

Shortening photoperiod. Short-day treatments, also known 

as "blackout," are produced by excluding daylight for several hours 

during the day (fig. 3.3.15). This technique has been used for 

many years in horticulture to induce flowering in short-day plants 

(Mastalerz 1977). During the naturally long days of summer at 

higher latitudes, container seedling growers have found it effective 

to use short-day treatments to stop height growth, induce budset, 

and promote cold hardiness in conifer crops (Colombo and others 

1982). These treatments have afforded much more control over 

growing schedules because nursery managers can rapidly and 

uniformly induce budset at any time of the year (Odium and 

Colombo 1988).

Short-day treatments may be used for several purposes (van 

Steenis 1991):

• Inducing budset in species such as western hemlock, Sitka 

spruce, western redcedar, and ecotypes such as those of 

Douglas-fir from low elevation and coastal climates.

• Forcing premature budset in early season crops that must be 

shipped and outplanted during the summer.

The short-day technique has proven especially valuable at high 

latitudes where termination of photoperiodic lighting or other 

stress treatments may not be sufficient to trigger terminal bud set 

and initiate the hardening phase. In Canada, switching off 

photoperiodic lighting is not particularly effective because of the 

natural long days. Furthermore, moisture stress, which is also 

used to arrest seedling shoot growth, significantly reduces 

seedling biomass and adversely affects the size of the terminal 

bud (O'Reilly and others 1989, Grossnickle and others 1991).
The short-day technique is not normally required in nurseries at 

low latitudes; turning off the lights is usually sufficient. Seedlings 

that have been grown under photoperiodic lighting respond 

quickly to changes in photoperiod. During the rapid growth phase, 

growers can generate a "relative" short-day condition by 

switching off the photoperiodic lighting system when height 

growth reaches 85 to 90% of the target.

Short-day treatments are a relatively new technology in forest 

nurseries, however, and have potential hazards. For example, 

some nurseries have had problems with maintaining a firm bud 

after the short-day treatments have been terminated (fig. 

3.3.16A). This can be caused by not applying the blackout 

treatment for the proper length of time, or by unusually warm 

temperatures that interfere with the dormancy process (Odium 

1991).

89
Not all species may respond favorably to these treatments. Recent 

research with western redcedar has shown that the short-day 

treatments must be applied too early in the season to be practical 

and may be detrimental to root growth (Krasowski and Owens 

1991). The phenological effects of short-day treatments can persist 

for at least 1 year after outplanting. Black spruce seedlings that 

received short-day treatments had greater height growth than the 

controls during the first growing season because they broke bud 

sooner and set bud later (Odium and Colombo 1988). This 

potential advantage can be a liability because many of these 

seedlings suffered cold injury due to spring frosts. At the end of the 

first growing season, however, the short-day seedlings had 

developed new terminal leaders and were actually taller than 

seedlings

90
91

from the natural day treatment (Odlum 1991; fig. 3.3.16C). 

Obviously, more research is needed on both the short-term and 

long-term effects of this technology. (See section 3.3.4.6 for 

more information on short-day treatments.)

Phototropism. Although not nearly as culturally

important as photosynthesis or photoperiodism,

phototropism concerns morphological responses to

differences in light quality. Recent evidence indicates that these 

phototropic responses may actually be controlled by a 

phytochrome response to the ratio of red to far-red light (Smith and 

Whitelam 1990). In enclosed greenhouses, some species ex

hibit undesirable shoot elongation ("stretching") when grown in 

excessive shade. West coast growers have observed that 

Douglas-fir seedlings become tall and spindly when overhead 

structures or equipment cast permanent shadows on the growing 

area or the covering is old or dirty.

The shoot growth of many species naturally orients toward the sun 

or other predominant light source, and this tropism can become 

excessive in nurseries located at higher latitudes in the fall and 

winter (fig. 3.3.17).
in solar intensity affects crop scheduling. Nurseries in the tropics 

routinely sow their crops without regard to the time of year. The 

potential for multi ple crops decreases at higher latitudes, however.

3.3.3 Optimum Light Levels

Light affects all phases of seedling growth and development, from 

seed germination through the accelerated growth that typifies 

container seedlings, to budset and hardening, and this influence in 

particularly significant in the container nursery environment. The 

intensity and duration of available sunlight should also be 

considered during nursery development, because both crop 

scheduling and the number of crops that can be grown per year will 

be affected.

3.3.3.1 Crop scheduling

Not only does daylength change with the season at high latitudes, 

but so does the intensity of solar radiation. In fact at higher 

latitudes, seasonal variation of solar intensity is almost as extreme 

as that of photoperiod (fig. 3.3.18). This seasonal change

92
93

3.3.3.2 Establishment phase

The germination of most tree seeds is not dependent on light intensity 

levels, but both light quality and photoperiod can have significant 

effects on some species. For example, red light (660 nm) promoted 

germination of loblolly pine seed, whereas far-red light (730 nm) 

inhibited it (McLemore 1971). Both germination rate and total 

germination are promoted under 8- to 12-hour daylengths for most 

species (Kramer and Kozlowski 1979). Some exceptions do occur, 

however, as Douglas-fir seeds germinated better under a 16-hour 

photoperiod than an 8-hour one (Jones 1961).

The cotyledons of germinating conifer seedlings become 

photosynthetically active as soon as they emerge from the growing 

medium, and the development of the primary needles depends on the 

photosynthate from the cotyledons. Most germinants grow best at 

moderate light intensities, around 55 μmol/s/m2 (~3,000 Ix), and some 

reduction in light intensity may therefore be necessary to reach 

optimum germination in environments with high levels of solar energy. 

With most popular greenhouse coverings, a 50% shadecloth should 

create the desired light intensity. A less dense shadecloth (30 to 50%) 

may be necessary in cloudy climates. The proper light intensity varies 

with species, however; with red pine seedlings, primary needle 

development was severely retarded when light levels decreased 

below 120 μmol/sec/m2 (~6,500 Ix) (Kozlowski and Borger 1971). In 

southern pine nurseries, any shade is undesirable, but shadecloth is 

used for protection from hard rains.

Germinants emerge into an environment with high levels of solar 

energy that subject them to stress. Light, temperature, and moisture 

conditions are critical at this time, and growers must be especially 

vigilant to ensure that all environmental factors are kept at optimum 

levels. The type of seed covering can also affect light and 

temperature in the microenvironment around the germinant. LightIn the temperate zones, container nursery managers growing one 

crop per year should schedule around the summer solstice (June 

21 in the Northern Hemisphere) so that there will be sufficient 

sunlight for seedling growth throughout the crop cycle (fig. 3.3.19). 

Multiple crops should also be scheduled around the solstice. For 

container nurseries growing two crops per year, the first crop 

should be sown early enough that it can be removed from the 

greenhouse and the second crop sown by the solstice (fig. 3.3.19). 

Crops sown later in the year have increasingly less sunlight 

available for photosynthesis and this is even more significant in 

very cloudy climates (fig. 3.3.4). Decreasing solar intensity will be 

reflected in slower growth rates during the fall and winter (Downs 

1985) and also affects other nursery production factors, such as 

the incidence of diseases like grey mold (Tuller and Peterson 

1988). So, unless production demands require a second crop, it is 

more economical to raise just one "summer" crop in the temperate 

zone nurseries. (Crop scheduling will be discussed further in 

volume six of this series.)
agers are removing the polyethylene coverings of their 

greenhouses in midsummer to maximize light intensity (fig. 

3.3.20B). This practice has been shown to improve seedling quality 

(Tuller and Peterson 1988) (see section 3.3.4.1 for more 

information).

colored grit or perlite covering the growing medium reflects a 

considerable amount of solar radiation. On the other hand, dark 

seed mulches absorb solar radiation and damaging temperatures 

can develop around the succulent stem. (Heat injury to container 

seedlings is discussed in more detail in volume five of this 

series.)

3.3.3.3 Rapid growth phase

After container seedlings become established, the light intensity 

should be gradually raised to the saturation point to maximize 

photosynthesis. If seedlings were shaded during the establishment 

phase, then the covering should be normally removed unless 

daytime temperatures would become damaging without it. This is 

often the case in the tropics or in arid climates during the summer.

The light saturation point varies with different species (table 3.3.4), 

making it best to grow crops with different light requirements in 

separate growing environments. If separate environments are not 

available, growers must design a light environment that is suitable 

for the majority of the crop species. Shading one section of a 

growing structure for shade-loving species is an option. It is most 

practical for greenhouses that are oriented perpendicular to the 

solar arc (that is, north to south), but even then the shaded area 

will gradually change during the growing season. Ideally, shading 

should be adjusted throughout the day and from one day to the 

next to optimize light intensity but, if the shading can only be 

applied and removed manually, this is impractical. Motorized 

shading systems are now available that can achieve this result 

(Vollebregt 1990) (see section 3.3.4.6).

Because light intensity is reduced by any type of covering, some 

growers move their crops out of fully enclosed greenhouses as 

soon as the seedlings are established in the containers (fig. 

3.3.20A). Of course, this is only an option when ambient 

temperatures are mild. Brissette and others (1990) reported that 

even 30% shade seriously reduced growth of longleaf pine 

seedlings and recommended growing them in open compounds in 

full sunlight during the summer months. Even as far north as 

British Columbia, some container nursery man94
Photosynthetic efficiency also varies with the type of foliage. For 

example, primary needles of loblolly pine seedlings reached 

maximum photosynthesis at lower light intensities than did older 

seedlings with mature secondary needles (Bormann 1956). Mutual 

shading decreases photosynthetic rates when seedling crowns 

begin to overlap; this can occur relatively early in the growing 

season with large-leafed species or seedlings grown at close 

spacing. Large trees develop sun and shade leaves in response to 

light intensity (Kramer and Kozlowski 1979). Seedlings of 

shade-tolerant species, or any seedlings that have been raised 

under shade, can develop foliage that is not tolerant of high light 

intensity. These seedlings can become "sunburned" if exposed to 

full sunlight (see section 3.3.2.1).

Most nurseries keep their seedlings under extended photoperiod 

for the entire rapid growth phase to promote shoot growth and keep 

them from setting a terminal bud. Even a brief interruption during 

this period can result in bud set and have serious economic 

consequences (fig. 3.3.12D). When seedlings reach approximately 

80 to 90% of their target height, growers shut off the photoperiodic 

lights to initiate the hardening phase.

3.3.3.4 Hardening phase

The objective of the hardening phase is to terminate height growth, 

set full buds, and stimulate caliper and root growth while gradually 

hardening the seedling to tolerate stress. Control of photoperiod is 

the predominant trigger that causes tree seedlings to cease height 

growth and set a terminal bud. Most forest species are very 

sensitive to changes in daylength, although species and ecotypes 

from southern latitudes and mild coastal environments are less 

affected (see section 3.3.2.2). In the United States, most container 

nursery managers shut off the photoperiodic lights to initiate 

terminal budset and induce dormancy. Because of the naturally 

long days in summer, some Canadian growers use blackout 

curtains to shorten the photoperiod. (This technique is discussed in 

section 3.3.4.6), and other cultural techniques for inducing dormany 

are discussed in detail in volume six of this series.)

95

Many container nurseries begin the hardening phase by removing 

the seedlings from the greenhouse and exposing them to ambient 

conditions; other nurseries simply remove the covering of the 

greenhouse. Growers should be cautious, however, when moving 

shade-tolerant species from a shaded growing area to full sunlight 

because they can become "sunburned" (fig. 3.3.9). Growers can 

prevent photodamage by minimizing plant stress and by making 

certain that sensitive species are gradually acclimated to high light 

intensity. Succulent foliage becomes hardened as it matures by 

developing a thicker cuticle and undergoing other morphological 

changes in the epidermis (Levitt 1980). Susceptible species should 

be moved from full shade gradually; first to an area with 

intermediate light intensity for several weeks, then finally to full 

sunlight.
The degree of solar modification varies with the characteristics of 

the growing structure and the type, age, and cleanliness of the 

covering (Hanan and others 1978). Solar radiation in the PAR 

wavelengths was measured in two greenhouses in a forest nursery 

in British Columbia, one constructed of fiberglass panels and one 

of polyethylene film. The polyethylene covering transmitted all the 

measured spectra better than the fiberglass covering (fig. 3.3.23), 

and the average difference in the all PAR wavelengths was 12.2% 

(Tuller and Peterson 1988). Both the coverings reflected more of 

the shorter wavelengths, thus producing a light environment 

relatively higher in the red and infrared bands.

McMahon and others (1990), in a detailed survey of spectral 

transmittance of various greenhouse coverings and shading 

materials, found considerable variation between products (table 

3.3.6). Both the intensity and quality of sunlight varied, not only 

between different structural and shade coverings, but also between 

similar products, such as brands of polyethylene film. Total light 

transmission ranged from 95 to 52% for the structural materials and 

from 45 to 21% for the shading materials. Although basically 

unaffected by the structural materials, the blue and red 

wavelengths were significantly changed by the shading materials. 

Both blue light and the ratio of far-red to red light can affect

96

3.3.4 Modifying Light in Container Tree

Nurseries

Light is one of the most important cultural tools available to the 

container nursery manager, who can modify the intensity of natural 

sunlight or supplement it with artificial lights. The location and 

orientation of the growing structure is also important because 

persistent shadows can decrease seedling growth rates (fig. 

3.3.21). (Locating container tree nurseries is discussed in more 

detail in volume one of this series.)

3.3.4.1 Effect of structural coverings

Solar radiation is modified, both in intensity and quality, from the 

time it enters the earth's atmosphere. The type of greenhouse and 

its physical location also affect the nature of incident sunlight. In 

spite of reductions due to solar angle and seasonal cloud cover, 

the solar radiation measured inside a greenhouse (fig. 3.3.22) still 

roughly follows the normal distribution pattern for solar energy 

over the growing season (fig. 3.3.19).
solar angle at 37° latitude on the winter solstice in the Northern 

Hemisphere. Under these simulated winter conditions, light 

transmission decreased from 8.0 to 14.6% for common 

greenhouse coverings. (Properties of greenhouse coverings are 

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

3.3.4.2 Shading the growing area

Heat buildup becomes more of a problem during the summer 

months, and many growers modify the light transmission 

characteristics of their greenhouses by applying a light-reflecting 

whitewash material or by installing shadecloth (fig. 3.1.21). 

Whitewash and shadecloth are considered "perma nent" shading 

because they are typically left in place for most of the growing 

season. Shadecloth is preferable to whitewash because it can be 

removed seasonally without leaving a residue. Commercial 

shadecloths can produce from 20 to 90% shade and also are 

available in different colors. The transmission of both total PAR 

radiation and specific wavelengths is affected by the type of 

shading material (table 3.3.6), and the selective filtering of some 

shadecloth will affect plant growth (McMahon and others 1990).

97

plant morphology. Light with a high far-red to red ratio tends to 

produce larger leaves and longer internodes ("stretching") 

(McMahon and others 1990).

The condition and cleanliness of the greenhouse covering can 

have a profound effect on light transmission (fig. 3.3.24); Nelson 

(1985) estimates that accumulation of dust alone on the 

greenhouse coverings can reduce sunlight transmissions by as 

much as 20%. Hard plastic or fiberglass coverings should be 

cleaned regularly, and even polyethylene tarps can be hosed off 

with water. Cleaning or replacing the covering will affect not only 

the intensity of sunlight but also the temperature regime within the 

structure, and so cultural practices must be adjusted accordingly.

The season is also a factor because light transmission through 

greenhouse coverings varies with the angle of the sun. Garzoli 

(1988) measured the solar radiation transmission through a 

variety of greenhouse coverings with the sun directly overhead 

and at a 60° angle, which is the maximum
The proper choice of shading becomes an averaging process, 

however, because sunlight intensity changes during the day with 

the angle of the sun and the degree of cloud cover. Because of the 

labor involved, it is uneconomical to adjust the shadecloth 

manually to prevailing light conditions. With the advent of 

automatic retractable shading systems, however, growers have 

the option of adjusting the light intensity within the growing area to 

maximize photosynthesis or lower temperature several times a 

day (fig. 3.3.25). Although relatively expensive to install, automatic 

shading systems can greatly increase the amount of sunlight 

reaching the crop and therefore affect seedling growth rates. In an 

operational greenhouse trial, an automated shading system 

allowed the crop to receive 50% more hours of PAR than the crop 

in a house with permanent shade (Vollebregt 1990).

98

3.3.4.3 Types of lamps

The two most important biological factors to consider during 

selection of a horticultural lighting system are light intensity and 

light quality, together known as spectral energy distribution, 

because they will determine whether the system will be effective. 

Once these two criteria have been addressed, then a number of 

other operational aspects must be considered: energy efficiency, 

uniformity of light distribution, initial cost of the lamp and fixture, 

average lamp life, replacement cost of lamps, and resistance to 

corrosion. For horticultural applications, artificial lighting can be 

divided into incandescent, fluorescent, and high-intensity discharge lighting:

Incandescent lamps. A standard incandescent lamp 

contains a tungsten filament enclosed in a glass bulb that is filled 

with nitrogen gas to prevent oxidation and evaporation of the 

tungsten at high temperatures (Bickford and Dunn 1972). A 

typical incandescent lamp produces radiation that peaks in the 

infrared band (fig. 3.3.26A) and so generates a relatively high 

amount of heat.
Incandescent lights are not recommended for photosynthetic 

lighting because they generate so much heat and are too expensive 

to operate at the required intensities. They can be balanced with 

fluorescent lights to better cover the PAR wavelengths, but this 

combination is only used in growth chambers. Incandescent lamps 

are the most widely used type of photoperiodic lighting in U.S. and 

Canadian nurseries, however (table 3.3.7), because they provide a 

usable spectral energy distribution, are cheap to install, and can be 

turned on and off frequently without loss of bulb life. Incandescent 

lamps gradually deposit tungsten on the inside of the bulb (a 

process called "blackening") which decreases the light output and 

their useful service life (Aldrich and Bartok 1989). Standard 

incandescent bulbs are the least efficient and have the shortest 

average life of all the common light sources used in horticultural 

lighting (table 3.3.8), which means that bulbs must be replaced 

frequently.

A tremendous variety of standard and specialty incandescent lamps 

are available for many different applications (Kaufman and 

Christensen 1984). The standard incandescent bulb requires an 

external reflector to direct the light downward and provide more 

even distribution (fig. 3.3.27A); external reflectors cast unwanted 

shadows and may collect moisture, however. This problem can be 

avoided with specialized incandescent lamps that have internal 

reflectors to concentrate the light and focus it forward (fig. 

3.3.27B-D). These flood lamps direct all wavelengths onto the crop. 

Both standard incandescent and tungsten-halogen reflectors can be 

coated with a transparent, slightly iridescent external ("dichroic") 

coating that reflects less of the infrared wavelengths forward (fig. 

3.3.28). This is sometimes an advantage, because it raises the ratio 

of red to far-red light and makes these lamps more efficient for 

photoperiodic lighting.

99
100
101
102
103

A relatively new addition to the incandescent class is the 

tungsten-halogen (also called "halogen quartz") lamp, which has 

the same spectral energy distribution as the standard 

incandescent lamp (fig. 3.2.26A) but has several distinct 

advantages, including longer bulb life and higher efficiency at the 

same wattage (Kaufman and Christensen 1984; table 3.3.8). The 

thin, tubular bulb is made of a quartz-silica glass, and a halogen 

gas (iodine or bromine) surrounds the tungsten filament and prevents the blackening that is typical of an incandescent lamp 

(Bickford and Dunn 1972). The small size of tungsten-halogen 

lamps enables them to be mounted in more efficient reflector 

fixtures (fig. 3.3.29). Like other incandescent bulbs, however, their 

energy efficiency is still relatively low compared to other types of 

lamps (table 3.3.8).

Fluorescent lamps. Fluorescent lamps produce light when a 

mercury arc excites fluorescent powders ("phosphors") that coat 

the inner walls of the bulb. Electrical current flowing between the 

electrodes on both ends of the lamp produces an arc in the 

mercury vapor, generating ultraviolet radiation and exciting the 

phosphors to emit light (Kaufman and Christensen 1984). 

Fluorescent lamps contain a complex electric circuit including a 

ballast, which provides enough voltage to start the electric 

discharge. Because the ignitor in the bulb wears out with use, 

lamp life is shortened if they are turned on and off frequently 

(Bickford and Dunn 1972).
Fluorescent lamps are available with widely different spectral 

energy distributions, depending on the specific coatings on the 

bulb; "cool white" bulbs are useful for most horticultural 

applications (fig. 3.3.26B). The initial installation cost of 

fluorescent lamps is twice that of incandescent, but they are over 

three times more efficient in producing visible light. They may last 

up to 12 times longer than standard incandescent lamps, provided 

they are not cycled on and off frequently (table 3.3.8). Fluorescent 

lamps are widely used in growth chambers because they are a 

linear light source that produces relatively little infrared radiation, 

which is important in a closed environment (AIdrich and Bartok 

1989). Fluorescent lamps have several disadvantages, however. 

Light intensity per unit area is generally low, although special 

high-output bulbs are available (table 3.3.8).

The relatively large fixtures (fig. 3.3.30A-B) cast undesirable 

shadows, but this can be reduced by orienting them north to south 

so that their shadows move. Waterproof fixtures are 

recommended for greenhouses. In spite of these restrictions, 

fluorescent lamps are being used for both photosynthetic and 

photoperiodic lighting in container tree nurseries in the United 

States and Canada (table 3.3.7). If they must be used, they are 

best mounted on an irrigation boom (fig. 3.3.30B).

High-intensity discharge lamps. These compact, high-output 

lamps are becoming popular for horticultural lighting because they 

efficiently produce light in the PAR wavelengths and require little 

maintenance. High-intensity discharge (HID) lamps generate light 

by passing an electric current through a pressurized gas at high 

temperature, causing the gas to glow. Several different gases are 

used, including sodium and mercury-iodide (metal halide), which 

determine both the intensity and quality of the emitted light (fig. 

3.3.26C-D). Because it takes several minutes to generate the 

necessary gas temperature and pressure, there is a time lag 

before the lamps are fully operational. Lamp fixtures are relatively 

large because they require a ballast to regulate the electrical 

current (fig. 3.3.31); some models are equipped for remote 

ballasting (Aldrich and Bartok 1989). All HID lamps are 

significantly more energy efficient than other sources of artificial 

light (table 3.3.8).

104

High-pressure sodium. High-pressure sodium (HPS) lamps 

generate light by passing an electric current through sodium 

vapor in an elongated arc tube (fig. 3.3.31A), which contains a 

double glass envelope to contain the corrosive sodium vapor 

(Kaufman and Christensen 1984). The spectral energy distribution 

extends across the visible spectrum but peaks sharply in the 

yellow bands, very close to optimum for both photosynthesis and 

photoperiod lengthening. Very low levels of infrared light are 

produced (fig. 3.3.26C). HPS lamps emit a very intense light, so 

the fixtures can be spaced widely enough to avoid blocking much 

sunlight. Both 400W and 1,000W lamps are used
105
in tree nurseries. The larger lamps are more efficient if they can be 

mounted high enough in the growing structure. Development of a 

new ceramic arc tube has increased lamp life to over 20,000 hours 

(table 3.3.8). HPS lamps are excellent for both photosynthetic and 

photoperiodic lighting in container tree nurseries (table 3.3.7).

Low-pressure sodium. Low-pressure sodium (LPS) lamps are 

the most energy efficient of all types of horticultural lighting (table 

3.3.8). Their spectral energy distribution is more restricted than 

HPS lamps (Aldrich and Bartok 1989), but this is not a 

disadvantage. They are less intense and take up more space, 

thus creating more shadows, and they have been used for 

photoperiodic lighting in only a few container tree nurseries (table 

3.3.7).

Metal halide. Metal halide lamps are a modification of the older 

mercury lamps; the arc tube contains various metal halides (for 

example, dysprosium and thallium iodides) as well as 

mercury-argon vapor (Kaufman and Christensen 1984). Compared 

to mercury lamps, metal halide lamps produce 50% more light 

output, in a well-balanced spectral energy distribution (fig. 

3.3.26D). Metal halide lamps are available in wattages of 400W 

and 1,OOOW and have a life expectancy of 10,000 to 15,000 

hours. They are rarely used in container tree nurseries (table 

3.3.7), because HPS lamps are more efficient and have a much 

longer life (table 3.3.8).

Reflectors. All types of lamps used in horticulture require some 

sort of reflector to focus the light and distribute it evenly over the 

crop area. External reflectors are often purchased as part of the 

lighting fixture (fig. 3.3.29 to 3.3.31) but can also be homemade 

(fig. 3.3.27A). Specialized types of incandescent lamps have 

built-in reflectors, such as the parabolic aluminized reflector (the 

established acronym PAR is unfortunately the same as that for 

photosynthetically active radiation). Of particular interest is the 

dichroic reflector, which reflects the PAR wavelengths forward and 

transmits infrared radiation out the rear of the lamp (fig. 3.3.28).

106

3.3.4.4 Photosynthetic lighting

Because of high energy costs, it is rarely practical to provide 

enough artificial light to increase photosynthesis in container tree 

nurseries. The exception is at higher latitudes, where it is

sometimes economical to use photosynthetic lighting to 

supplement natural daylight during the fall, winter, and spring 

(Bickford and Dunn 1972). At present, only about 5% of the forest 

nurseries in the United States and Canada supply photosynthetic 

lighti ng (table 3.3.7).

Light intensity. If artificial lights are the only source of 

illumination, as in a growth chamber, the minimum requirement for 

commercial plant production is considered to be about 250 

μmol/s/m2 (--20 klx), which is about one-eighth the intensity of 

normal sunshine (ASHRAE 1989). The lights must also be kept on 

for at least 12 hours per day to permit reasonable rates of growth. 

Highintensity lights generate a tremendous amount of heat, but 

this is less of a problem when photosynthetic lighting is used at 

night, for the heat may be useful. Such lighting systems are 

expensive to install and operate unless electricity is quite 

inexpensive.

Recent horticultural experiments with supplemental photosynthetic 

lighting, however, have shown that significant increases in plant 

growth are possible, especially in terms of dry weight. At least 

some of this gain may be due to the incidental warming of the 

growing medium (Downs 1985). Supplemental lighting is 

sometimes needed to compensate for cloudy weather, shading 

from greenhouse structures or equipment, or during the winter at 

higher latitudes. When 122 μmol/s/m2 (-10 klx) of PAR light is 

added for 8 to 16 hours per day, growth rates can approach those 

obtained in growth chambers (ASHRAE 1989). In Alberta, where 

normal December sunlight is only 70 μmol/s/m2 (~3.8 klx), 12 

conifer species responded strikingly to supplemental winter 

lighting. Two different lighting systems were effective-60 to 100 

μmol/s/m2 (~4.7 to 7.9 klx) of fluorescent lighting or 210 to 220 

μmol/s/m2 (~17.6 to 18.4 klx) of high-pressure sodium lighting, 

supplied for 18 or 24 hours per
Most of the major lamps have the desired red wavelengths (fig. 

3.3.26) for photoperiod control, and incandescent, fluorescent, and 

high-intensity discharge types have been used successfully. The 

choice of the best lighting system, therefore, is one of economics 

and the layout of the nursery. If the growing area is already 

equipped with photosynthetic lighting, the same equipment can be 

generally used to extend the daylength.

For photoperiodic lighting to be fully effective, other environmental 

factors, such as temperature, humidity, water, and mineral 

nutrition, should be optimal. Stressed seedlings do not respond 

normally to light stimuli.

Types of photoperiodic lighting. A variety of terms have been 

used to describe photoperiodic lighting systems, and confusion 

has resulted because a useful term must denote both duration 

(how long the lights are left on), and timing (when the lights are 

activated).

The duration of photoperiodic lighting has traditionally been 

described as either continuous or intermittent, and both types 

have been used successfully. In continuous lighting, lights are 

turned on for prolonged periods. Although many types of lamps 

can be used for continuous lighting, the HID types are the only 

cost-effective option and HPS lamps are recommended.

In intermittent lighting, the night is interrupted with short bursts 

of light at regular time intervals during the prescribed period. The 

lights can be left on for as little as 3% of the time, provided no dark 

period is longer than 30 min (Tinus and McDonald 1979). Arnott 

and Mitchell (1982) reported that supplying light for 2 out of every 

30 min was effective. Intermittent lighting can be achieved in two 

ways, depending on the type of lamps used. Lights that will 

tolerate repeated switching, such as standard incandescent lamps, 

are mounted in fixed arrays, while those that require ballasting 

must be mounted on a moving irrigation boom because of their 

slow start-up time. In this application, the lamps must remain on 

continuously and intermittent lighting is achieved by moving the 

boom back and forth. Obviously, this lighting option is only 

practical in nurseries with boom irrigation systems.

107

day (Dymock and Wilson 1986). In Wisconsin, the addition of 

between 18 and 63 μmol/s/m2 (~1.4 to 5.0 klx) of PAR during the 

winter significantly increased the growth of white spruce, jack 

pine, and hybrid poplars (Roberts and Zavitkovski 1981).

Light quality. Because not all wavelengths are equally effective 

for photosynthesis, artificial lighting should be high in the PAR 

bands (fig. 3.3.1). The most efficient wavelengths of light are 600 

to 700 nm, in the red part of the visible spectrum; blue, green, and 

yellow light (400 to 600 nm) have only one-half to two-thirds the 

efficiency of red light. Infrared light (> 700 nm) is not effective for 

photosynthesis but is merely absorbed and increases leaf 

temperature. Ultraviolet light (< 400 nm) is also ineffective and can 

be damaging.

High-pressure sodium lights are recommended for photosynthetic 

lighting because they are the most efficient and cost effective. 

Metal halide and fluorescent lamps can also be used but are less 

desirable in terms of PAR output per watt and life expectancy 

(table 3.3.8).

Lamp fixtures and positioning. Because of the high light 

intensity that is required, lamps for photosynthetic lighting must be 

positioned close to the crop. Mounting photosynthetic lights on the 

irrigation boom would not produce continuous lighting, and so 

fixtures must be mounted in a regular, fixed pattern over the crop.

3.3.4.5 Photoperiodic lighting

The greatest use of artificial lighting in horticulture is for extending 

the photoperiod (Bickford and Dunn 1972). The majority of the 

plants grown for forestry and conservation purposes require 

extended photoperiods to produce rapid growth rates, and so most 

growing areas are equipped with some type of lighting for 

extending daylength (table 3.3.7). Nurseries in southern climates 

and coastal areas with mild maritime climates that grow ecotypes 

native to those areas may not need photoperiodic lighting. (See 

section 3.3.2.2 for more information on when photoperiodic 

lighting is needed.)
All types of lamps have been used on irrigation booms: 

incandescent (fig. 3.3.27A), fluorescent (fig. 3.3.30B), and 

high-pressure sodium (fig.3.3.31 D).

Continuous and intermittent lighting can be applied in three 

different time sequences (fig. 3.3.32)

• Night-break lighting is a technique in which the lights are 

turned on for a few hours in the middle of the night, breaking it 

into two shorter dark periods (Vince-Prue 1975). The 

night-break can be either continuous or intermittent, although 

the former is more common.

• Daylength extension lighting involves supplying 2 to 4 hours 

of artificial light after dusk or before dawn (Nitsch 1957). 

Continuous lighting is normally used for daylength extension.

• All-night lighting is a technique in which the lighting system is 

activated all night long. Intermittent lighting is the most

economical option.

Therefore, to be completely accurate, photoperiodic lighting 

systems should be described with a term that denotes both 

duration and timing, for example, "continuous night-break 

lighting" or "intermittent all-night lighting" (fig. 3.3.32).

Many different photoperiodic lighting systems have been used in 

container tree nurseries, and the best system will depend on the 

type of greenhouse and type of lamps that are available. Either 

continuous or intermittent night-break lighting can be just as 

effective as a longer light period (Arnott 1989). Continuous 

daylength extension is sometimes used but is less economical than 

other systems. Electrical consumption was reduced by 75% and 

lamp life increased considerably when HPS lamps were used to 

provide continuous night-break lighting in a British Columbia 

container nursery (Forestry Canada 1991). Standard incandescent 

lamps are commonly used to produce intermittent all-night lighting, 

and energy savings of 60 to 80% over continuous lighting are 

possible (Bickford and Dunn 1972). Regardless of the economics, 

providing continuous light for the entire night (that is, a 24-hour 

photoperiod) is not desirable and can even be detrimental. 

Continuous all-night lighting may reduce height growth of container 

tree seedlings by as much as 30% (Tinus and McDonald 1979).

108

Light intensity and quality. Research trials have determined 

photoperiodic lighting intensities for many species and ecotypes 

(table 3.3.5), and these recommendations have been validated in 

operational container nurseries (table 3.3.7). To ensure that 

photoperiodic lighting will be effective, the light intensity should be 

at least 8 μmol/s/m2 (~430 Ix), and should be increased to 16 

μmol/s/m2 (~860 Ix) when the crop has a higher light requirement. 

Some species can be grown with even less light intensity (Arnott 

and Macey 1985). If the exact light intensity is unknown for a 

species or ecotype, it is always better to supply more light than to 

risk providing too little. Some species that set bud and become 

dormant require chilling before they will initiate new shoot growth. 

This can be economically disastrous because the crop will not 

meet height specifications within the allotted crop schedule.

Because phytochrome is triggered by the relative amounts of red: 

far-red light, the light output should be high in the red (600 to 700 

nm) band and low in the far-red (700 to 800 nm) band (see 

section 3.3.2.2 for more details).

Nurseries should have a backup source of electrical power for 

their photoperiodic lighting systems because loss of extended 

daylength, even for a short time, can be damaging. Research 

studies have shown that light failure for a period of one night 

(Arnott 1984), or three nights (Arnott and Simmons 1985), caused 

significant height reduction of white spruce and mountain hemlock 

seedlings, respectively. The authors have observed a similar 

response with Engelmann spruce (fig. 3.3.12D), blue spruce, and 

green ash.

Lamp fixtures and positioning. In container tree nurseries, 

photoperiodic lighting is typically installed in either fixed or mobile 

systems. Although some lamps can be installed in either manner, 

others are best suited for one specific type.

Fixed systems. Photoperiodic lighting, installed either overhead in 

a regular grid pattern or mounted at an oblique angle on side walls 

or posts, can be used for either intermittent or continuous lighting. 

With either type of mounting, the placement of the individual lamps 

is critical because the light intensity can vary in three dimensions.
109
Because they can be switched on and off easily, standard 

incandescent lights are often mounted in fixed overhead patterns 

to produce intermittent lighting (fig. 3.3.27C-D). Different lines or

zones of lights can be controlled by an electrical time clock that 

turns the lamps on and off at prescribed intervals. In smaller 

growing structures, all of the lamps can be programmed to come 

on at once. In most larger installations, different lighting stages 

are designed to switch on sequentially, to reduce the total 

electrical demand and allow lighter duty electrical service to be 

used. High-intensity discharge lamps, such as HPS lamps, are 

also mounted in fixed overhead patterns, and their higher light

output permits greater spacing (fig. 3.3.31). Fluorescent lamps 

are rarely used in this manner. Both of these types of lamps 

require ballasts, which means that they have long warm-up 

times, and their usable life will be shortened by frequent 

switching on and off.

Photoperiodic lamps can also be obliquely mounted around the 

perimeter of a greenhouse or outdoor growing area to produce 

continuous lighting. High-wattage tungsten halogen lamps ("flood 

lights") are commonly used in fixed oblique mountings because 

their fixtures contain wide-angle reflectors and can be mounted at 

any angle (fig. 3.3.29). In British Columbia container nurseries, 

400W HPS lamps are sometimes obliquely mounted on the 

sidewalls of growing structures. In another unique fixed lighting 

system, two HPS lamps are mounted on an irrigation boom that is 

moved to the middle of the growing area and pointed in opposite 

directions. The boom is moved to the middle of the growing area 

each night (Arnott and Mitchell 1982). As a result of their irregular 

positioning, the intensity of fixed oblique lighting should be 

carefully checked to insure that all seedlings are receiving enough 

light. Shadows from vertical structural supports can also be a 

problem.

Mobile systems. The second type of photoperiodic lighting 

systems uses lamps mounted on a mobile irrigation boom, 

which produces intermittent lighting by moving back and forth. 

Fewer lamps are required and their pattern of overlap is only 

critical in one dimension. Mobile lighting is

110

practical in nurseries that already have mobile irrigation booms but 

probably would not be cost effective if the booms are only used for 

lighting.

Many different types of lamps have been used in mobile lighting 

systems, including incandescent (fig. 3.3.27A) and fluorescent 

(fig.3.3.30B). Arnott (1974, 1976) recommends the high-pressure 

sodium lamp (fig. 3.3.31 D). In British Columbia container 

nurseries, irrigation booms with HPS lamps are passed over the 

seedlings every 25 to 30 minutes (Arnott and Mitchell 1982). In 

one nursery, where irrigation booms were used for night-break 

lighting, the boom traveled the length of a 60-m (200-foot) 

greenhouse in 8 minutes (Arnott 1989). Hallett (1982) reported 

that nurseries in the Maritime Provinces also use mobile lighting to 

extend photoperiod, with the irrigation carts traveling down the 

benches at a speed of 4 to 10 m/min (13 to 33 feet per minute).

Mobile lighting systems have the advantage of requiring only a few 

lamps to achieve the same result as a much larger fixed array. 

They have several disadvantages, however. If the boom should 

malfunction, even for a short time, the seedlings could cease shoot 

growth and set bud (Arnott and Mitchell 1982). The close proximity 

of the irrigation nozzles and the photoperiod lamps could produce 

problems because liquid fertilizer solutions are corrosive to 

electrical machinery and fixtures. Mobile lighting systems were 

being used in around 20% of the container tree nurseries surveyed 

in the United States and Canada.

A third, newly developed option is to use a centrally located lamp 

mounted in an oscillating parabolic mirror (fig. 3.3.33A) to sweep a 

beam of light over a large growing area (fig. 3.3.33B). This system 

uses the energy-efficient HSP lamp, burning continuously, to 

provide intermittent lighting (fig. 3.3.33C).

When designing a photoperiodic lighting system, container 

nursery managers should consult with a horticultural lighting 

expert. Applying the lighting information from manufacturers' 

specifications or another local nursery can cause problems, and it 

is strongly recommended that growers test any potential 

arrangement under operational conditions.
All photoperiodic lighting systems should be checked immediately 

after installation for uniformi ty and adequate light intensity. In 

particular, light intensity is never completely uniform from fixed 

overhead installations. Lamps laid out in grids produce circular 

intensity patterns similar to the watering patterns from fixed 

overhead irrigation systems. The magnitude may not always be 

evident to the human eye, but the variation in light intensity that is 

measured directly under a lamp and between lamps can be 

significant (fig. 3.3.34). Oblique installations produce a fan-shaped 

light pattern, the intensity of which decreases with distance. The 

overall intensity pattern should be carefully mapped, therefore, to 

ensure that there are no areas with below-minimum light levels. If 

the installed lighting is found to be inadequate in intensity, the 

problem can sometimes be solved by

leaving the lights on for a longer time; there is evidence for a 

reciprocal relation between light duration and light intensity, at 

least for blue spruce (Young and Hanover 1977).

3.3.4.6 Short-day treatments

Whereas shortening the dark period with photo periodic lighting 

promotes shoot growth and inhibits dormancy, lengthening the 

dark period triggers cessation of height growth and budset. And 

just as intermittent photoperiodic lighting is as effective as longer 

light periods, a relatively short period of darkness has worked well. 

These short-day or "blackout" treatments have been used for 

years in the floriculture industry to promote flowering of short-day 

plants but have only recently been adapt111
ed to forest nurseries. At first, nursery managers had to install and 

remove the curtains manually each day or use homemade curtain 

handling systems. In the past 10 years, however, reliable automatic 

curtain systems have been developed (Vollebregt 1989a, b). To 

increase their cost effectiveness, blackout curtains can also be used 

to reduce heat loss at night (Heacox 1989). (See section 3.1.4.4 for 

more discussion on heat curtains.)

Blackout curtains can be constructed of several different materials. 

Originally black shadecloth or polyethylene tarp was used. Initial trials 

revealed some problems: dark shadecloth absorbs solar radiation and 

impermeable sheeting retards ventilation, which may result in 

damaging heat buildup in enclosed structures. The following criteria 

should be considered when purchasing a blackout curtain (Vollebregt 

1990):

• Reflective upper surface-Curtains with a white or aluminized 

covering (fig. 3.3.35A) that reflects both visible and infrared 

radiation will remain cooler than other materials.

• Porosity -A permeable curtain will not allow water from 

condensation or leaks to accumulate on the fabric. Prolonged 

exposure to water could damage the fabric or the mechanical de112

ployment system. A porous material will also encourage air 

exchange and lower the humidity of the air below the cover. 9 

Pliability -Blackout curtains should furl or bundle easily and 

compactly so that they will not interfere with normal sunlight 

transmission during the day (fig.3.3.35B).

Automatic blackout curtains (fig. 3.3.35D) are available to fit most

types of growing structures, and it is much easier and more 

economical to incorporate a blackout curtain into the original 

greenhouse design than to retrofit an existing structure. Gabled 

structures can be fitted with curtains that roll up and down along 

the side walls. For the overhead portion there are two basic 

installations: gutter-to-gutter systems, which unfold like an 

accordion horizontally across the house, and truss-to-truss 

systems, which unfold from one truss to the next, often following 

the roof line. For
either of these systems to be installed it is important that no 

machinery be suspended from the purlins or go vertically through 

the roof (like a flue pipe). In gutter-to-gutter systems the cloth lies 

flat on wire along the bottom of the trusses. In truss-to-truss 

systems this may also be the case, but the cloth can also be 

suspended against part or all of the roof in a peaked configuration. 

Peaked truss-to-truss curtains are more expensive to install, but 

have the advantage that climate control equipment mounted above 

the purlins, such as a carbon dioxide generator, can be used when 

the curtain is in

113
place. Because of the lack of a vertical wall, blackout cloth is 

harder to deploy in quonset-style greenhouses, but it is possible.

Deployment can be controlled by a timeclock or photocell, but 

should also have an independent manual override. Automatic 

control is very useful to reduce labor requirements and increase 

reliability (Vollebregt 1989ab, 1990; Weed 1990).

In Canadian container nurseries, the conventional treatment is to 

apply blackout for a period of 10 days to 6 weeks beginning in July 

(Matthews 1983, Odlum 1991). The actual length of the short-day 

period will depend on the species and ecotype of the seedlings 

and the objective of the treatment. Short periods of less than 2 

weeks tend to predispose the crop to flush again if growing 

conditions are ideal, whereas longer treatments, such as 6 weeks, 

cause a significant reduction in dry matter accumulation. Hawkins 

and Draper (1988) treated spruce seedlings of 49 to 55° N latitude 

origin with "dynamic" blackout periods that simulated naturally 

decreasing daylengths. Although periods as short as 2 weeks 

were effective, they found that 4 weeks at a 13-hour photoperiod 

produced the best quality stock (fig. 3.3.36).

Because the relatively thick curtains trap heat and high humidity 

around the crop, short-day treatment periods should be kept as 

brief as possible. Applying the short-day treatment in the early 

morning instead of later in the day can minimize this problem (van 

Steenis 1991). An under-the-bench air circulation system would 

also help. Several nurseries have had problems with lammas 

shoot production after the blackout curtains have been removed 

(fig. 3.3.16A). It is important to make sure that no light from 

adjacent sources, such as security lights, reaches the crop or the 

seedlings may break bud (Colombo and Smith 1984).

Recent research has shown that the phenological effects of 

short-day treatments can persist for at least 1 year after 

outplanting. (See section 3.3.2.2 for more discussion.)

114
3.3.5 Light Monitoring and Control

Systems

3.3.5.1 Measuring light levels

The proper light measuring equipment will depend on the 

objectives of the individual and the particular application.

Photometers. Photometers measure radiant ener

gy in the visible spectrum; they include the common "light meters" 

that are sold through camera shops or laboratory supply companies

(fig. 3.3.37A). They are portable, inexpensive ($50 to 100), and 

give readings in lux or foot-candles (Aldrich and Bartok 1989). 

Photometers will provide a relative measure of solar intensity and 

can also be used to check the intensity patterns ofphotoperiodic or 

photosynthetic lights. Because the quality of light varies with the 

type of lamp, conversion tables must be used to compensate for 

the difference between the sensitivity spectrum of the meter and 

the light spectrum of the lamp (table 3.3.3). Even the light meters 

built into cameras are usable, because values in lux or foot candles 

can be calculated from film speed, shutter speed, and lens opening 

(table 3.3.9).

115
the night. For certain sensitive species, if the lights fail for even a 

single night, the seedlings can terminate height growth. Such a 

failure can be economically disastrous because some species and 

ecotypes will not resume growth until a chilling requirement has 

been met, or will exhibit bud break in irregular patterns (fig. 3.3.12). 

For this reason it is advantageous to have photoperiodic lights 

wired into the greenhouse alarm system.

There are two basic control devices: timers and photocells. 

Timers-electromechanical timeclocks or solid-state 

microprocessors-switch the lights on and off at preselected times. In 

the simplest system, a 24-hour timer turns lights on at dusk and off 

at dawn or for a shorter period during the middle of the night. This 

would provide a long day and could also augment photosynthesis, 

depending on the light intensity. For intermittent photoperiodic 

lighting a 60-division timer with a 6to 30-minute cycle would operate 

in series with the 24-hour timer to provide short bursts of light 

throughout the night. Photocells (fig. 3.3.38) respond to changes in 

light intensity and can be used in conjunction with timers, 

eliminating the need to reset the clock after power outages or as the 

natural daylength changes throughout the year. For photosynthetic 

lighting, photocells can turn on lights when sunlight intensity drops 

below a critical level (Aldrich and Bartok 1989).

Radiometers. Radiometers measure radiant energy

over a wide range of wavelengths. Portable, moderately expensive 

($450) meters have separate sensors ($300) that can measure all 

three aspects ofelectromagnetic radiation: energy (μmol/s/m2), 

radiation (W/m2), and illumination (Ix) (fig. 3.3.37B and C). Data 

recorders ($1,100) are also available to provide continuous 

readings of light as well as temperature and relative humidity.

A light sensor must be properly used; it should be held at crop 

level, facing the light source. Readings should be taken on a 

regular grid pattern throughout the growing area, especially in 

locations that appear to be shaded. When checking the intensity 

pattern of photoperiodic lights, take readings both directly under 

and between the lamps to determine the minimum intensity.

3.3.5.2 Control systems

Horticultural lighting systems must be precisely regulated to 

achieve the desired cultural objective. Reliability is crucial with 

photoperiodic lighting because the system must operate 

unattended during

116
117

The photoperiod can also be shortened by excluding light and 

creating short-day conditions. Short-day treatments are used to 

stop height growth, set full buds, and induce cold hardiness in 

sensitive species; this technique is primarily used in nurseries at 

higher latitudes. In most other nurseries, simply turning off the 

photoperiodic lighting is effective. Because blackout curtains affect 

other environmental conditions, especially temperature and 

humidity, they must be used properly. Growers should use light 

meters to check the amount of sunlight transmitted through their 

growing structures. Coverings with poor light transmission should 

be cleaned or replaced. Photoperiodic lighting systems should be 

checked regularly to make sure that the critical minimum intensity is 

exceeded throughout the growing area. Sensitive species will stop 

height growth and set a terminal bud after only a brief interruption of 

photoperiodic lighting, which can have disastrous economic 

consequences.

3.3.6 Conclusions and Recommendations

Light is the most complex and variable of the limiting factors that 

affect the growth of container tree seedlings. Light affects all the 

different phases of seedling growth and development, from seed 

germination to bud set and hardening. The amount and timing of 

solar radiation affects both crop scheduling and the number of 

crops that can be grown per year. The location of the nursery and 

the type of growing structure affect the amount of light that will be 

available for seedling growth.

Container nursery managers manage light for two reasons: to 

increase photosynthesis and to control seedling dormancy by 

modifying the photoperiod, or daylength. Growers can stimulate

high photosynthetic rates by supplementing sunlight in areas of 

low natural light intensity. But, because of the high light intensities 

that are required, it is usually not considered economical to 

provide enough artificial lighting for photosynthesis, although 

some nurseries supplement natural sunlight during the fall or 

winter. If photosynthetic lighting is required, high-pressure sodium 

lamps should be arranged to produce about 85 μmol/s/m2 (~7.0 

klx) at plant level. They must operate continuously for several 

hours to produce a measurable effect on seedling growth.

Many forest species are very sensitive to photoperiod, and so 

growers can extend the growing season by providing photoperiodic 

lighting. Because it is neither intense nor continuous, photoperiodic 

lighting is a relatively inexpensive cultural tool for growing uniform 

crops of container tree seedlings. Although some species and 

ecotypes from lower latitudes and mild climates do not respond to 

photoperiod extension, ecotypes from high latitude, high elevation, 

or continental climates require it. Photoperiodic lighting systems 

should be described with a term that denotes both duration and 

timing. Many different photoperiodic lighting systems have been 

used in container tree nurseries, and the best system will depend 

on many factors, including the type of greenhouse structure and 

the types of lamps that are available. Intermittent light is often 

cheaper, although continuous lighting has also been used 

effectively. Photoperiodic light intensities should be at least 8 

μmol/s/m2 (~430 Ix), and should be increased to 16 μmol/s/m2

(~860 Ix) where the species or ecotype has a higher light 

requirement.
tainerized Tree Seedling Symposium; 1981 Sept. 14-16; 

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