Contents

4.2.1 The Importance of Water in Seedling

Culture 71

4.2.2 The Water Status of Container Tree

Seedlings 72

4.2.2.1 Water content 72

4.2.2.2 Water potential 72

4.2.2.3 Plant water potential 72

4.2.2.4 Growing medium water potential 77

4.2.3 Factors Affecting Water Availability in

Containers 79

4.2.3.1 Unique aspects of artificial growing

media 79

High water-holding capacity 79

Water movement in artificial growing

media 79

4.2.3.2 Container effects 79

4.2.3.3 Water loss from container seedlings 80

4.2.4 Quantity and Quality of Irrigation

Water 83

4.2.4.1 Quantity of water required for container

nurseries 83

4.2.4.2 Water quality 83

Effects of salts on irrigation water

quality 83

Effects of salinity on seedling growth 85

Pests in irrigation water 88

Pesticide contamination 89

Water quality tests 89

Corrective treatments for irrigation

water 90

4.2.4.3 Water temperature 94

4.2.5 Types of Irrigation Systems 95

4.2.5.1 Overhead irrigation systems 95

Mobile irrigation booms 95

Fixed overhead sprinklers 96

Fixed basal sprinklers 98

4.2.5. 2 Fixed irrigation system design

principles 98

4.2.5.:3 Testing the efficiency of irrigation 

systems 100

4.2.5.4 Automatic irrigation systems 103

70

4.2.6 Monitoring Water in Containers---

Determining When To Irrigate 105

4.2.6.1 Visual and tactile examination 105

4.2.6.2 Container weight measurement 105

Developing a container weight scale through

experience 107

Developing a container weight scale using

growing medium matric potential 107

4.2.6.3 Monitoring plant water potential with a pressure 

chamber 108

4.2.7 Irrigation as a Cultural TreatmentDetermining How Much To

Irrigate 110

4.2.7.1 Irrigating during the establishment 

phase 110

4.2.7.2 Irrigating during the rapid growth phase 

110

4.2.7.3 Irrigating during the hardening phase 111

4.2.7.4 Irrigating for frost protection 112

4.2.8 Irrigation Wastewater Disposal 113

4.2.8.1 Sprinkler irrigation efficiency 113

4.2.8.2 Managing nursery wastewater 113

4.2.9 Conclusions and

Recommendations 115

4.2.10 References 116
4.2.1 The Importance of Water in Seedling

Culture

The importance of water to the growth of container tree seedlings 

cannot be overemphasized. Water is considered to be the principal 

growth-limiting factor in natural terrestrial ecosystems and is one of 

the most important growth-promoting factors in artificial 

ecosystems such as greenhouses. The ecological importance of 

water reflects its physiological importance, and almost every plant 

process is directly of indirectly affected by water. As an example, 

photosynthesis decreases drastically as moisture stress increases 

(figure 4.2.1).

Water influences plant growth in four major ways (Kramer 

1983):

1. Water is a major constituent of a plant, composing

80 to 90% of the fresh weight.

2. Water is the "universal solvent;" providing nutrient

transport within the plant.

3. Water is a biochemical reactant in many plant processes, including photosynthesis.

4. Water is essential for maintaining turgidity in plant

cells, promoting cell expansion and plant growth.

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Although the terminology surrounding WP may appear 

complicated at first, it is the best way to describe seedling water 

status because the basic principles and units remain the same 

from the growing medium, through the seedling, and into the 

atmosphere (Spomer 1985) (figure 4.2.2). Components of WP, 

from the matric potential in the soil to the water vapor in the 

atmosphere, can be described in the same units. A practical 

advantage of the water potential system is that the various 

influences that affect water use and availability, such as salinity, 

can all be described in WP terms (osmotic potential in table 4.2.1).

Water potentials are described in terms of energy-the ability to do 

work. The best measurement of the energy status of water is WP 

(often represented by the Greek letter psi ?), which is the energy 

difference between the chemical potential of water in the seedling, 

growing medium, or atmosphere compared to that of free, pure 

water at standard temperature and pressure (Kramer 1983). The 

WP of free, pure water is defined as zero and the potential of the 

water in a seedling or the growing me dium solution is decreased 

by factors that limit its ability to do work; WP's in nature are 

therefore always negative numbers (figure 4.2.2). WP is most

practically expressed in units of pressure such as bars (metric), 

the currently accepted Standard International (SI units called 

megapascals (MPa), or pounds per square inch (English units), 

which are dimensionally equivalent to units of chemical potential.

As mentioned earlier, one of the advantages of using WP is that it 

can be separated into its component parts: osmotic potential (OP), 

pressure potential (PP), matric potential (MP), and gravity potential 

(GP) (table 4.2.1). The components of WP in a seedling or in the 

growing medium differ, however, because of the unique properties 

of each system. Some components, such as the effect of gravity 

(GP), are negligible in small seedlings or in short containers, 

whereas other components, such as PP, are not significant in the 

growing medium (table 4.2.1).

4.2.2.3 Plant water potential

There are two terms commonly used to describe the water status 

of tree seedlings; most nursery scientists prefer plant water 

potential (PWP), whereas many nursery managers and 

reforestation foresters are more familiar with plant moisture 

stress (PMS). The two terms are identical in absolute value: 

PWP is always expressed in negative terms, whereas PMS is 

always a positive number (for example, a PWP value of - 1.5 

MPa equals a

4.2.2 The Water Status of Container Tree

Seedlings

An understanding of the physicochemical aspects of seedling 

water relations is not absolutely necessary for successful 

management of a container tree seedling nursery irrigation 

program. However, nursery managers should be familiar with the 

terms that are universally used to describe plant water relations. 

Seedling water status can be described in several different ways: 

water content, water potential, and water movement. Probably the 

most useful measures of seedling water status are water content 

and water potential (Spomer 1985).

4.2.2.1 Water content

The water content of a seedling can be viewed as an equation. At 

any time, seedling water content reflects an integrated balance 

between water absorption through the roots and transpirational 

losses from the foliage to the atmosphere (McDonald and Running 

1979) (fig. 4.2.2). Ritchie (1984) describes the water cycle as a 

simple input-output system:

W = A – T + S

where: W = seedling water content

A = absorption

T = transpiration

S = storage

Seedling water content is in a constant state of change during the 

day, when transpirational losses through the foliage usually 

exceed the rate of water absorption through the roots. This lag 

between water uptake and water loss creates a condition of 

internal water stress within the seedling. Seedling water stress is 

a normal physiological condition during daylight hours; however, if 

this stress is allowed to reach extreme levels for extended 

periods, the seedling growth rate declines and eventually the 

seedling dies. In container tree seedling nurseries, seedling water 

stress is maintained at low levels during the growing season 

through the use of irrigation to stimulate seedling growth. 

Although plant water stress has been described with many 

different terms, the most useful way to describe seedling water 

status involves water energy or potential.

4.2.2.2 Water potential

A full discussion of water potential (WP) is well beyond the scope 

of this manual, but an understanding of the basic concepts is 

useful for container tree seedling nursery managers. The following 

discussion of WP will help familiarize the reader with the terms 

and units of WP that will be used in this chapter (table 4.2.1).

72
73
PMS value of 1.5 MPa). A comparison of the units and 

descriptive terms for PWP and PMS is provided in table 4.2.2. 

Note that a relatively high WP rating (for example, - 1.0) is "less 

negative" than a relatively lower value (for example, - 3.0).

PWP is actually more practical than PMS for nursery management 

purposes because WP can be used to describe the water 

relationships through the entire soil-plant-air continuum (fig. 4.2.2), 

whereas PMS is only useful in describing the water status within 

plants. Therefore, to maintain consistency and minimize possible 

confusion, WP units and terms will be used throughout this 

publication. The concept of "moisture stress" is well established in 

the nursery literature and everyday jargon. This should not cause a 

problem, however, and the term "stress" can still be used 

operationally because PWP and PMS are dimensionally equivalent 

and only differ in sign (table 4.2.2).

PWP is composed of two major components: osmotic potential 

(OP) and pressure potential (PP) (table 4.2.1). The 

interrelationship of these factors is illustrated in fig74

ure 4.2.3, which shows how PWP changes with seedling water 

content (Hofler 1920, as discussed in Ritchie 1984). When a 

seedling is fully turgid, its PWP is zero
because the PP is positive and equal to the OP, which is negative. 

As the seedling loses water content through transpiration, it also 

loses turgidity (wilts) and the PP declines in value until it reaches 

zero, at which point the seedling is completely flaccid and the 

PWP equals the OP (figure 4.2.3). This point of zero turgidity, 

some times called the "wilting point;' is physiologically dangerous 

for the seedling: growth stops and cellular damage and even death 

may result if this condition exists for too long.

PWP is dynamic and changes with time as soil moisture and 

atmospheric demand change. Typical diurnal patterns for PWP 

are given in figure 4.2.4 for two different combinations of growing 

medium moisture and atmospheric demand. On a typical day in a 

well-irrigated growing medium (A in fig. 4.2.4), a seedling begins 

to transpire as soon as the sun comes up, and its PWP decreases 

(stress increases) until the stomata close, at

which point the PWP levels off. Towards sunset, the PWP begins 

to increase as atmospheric demand declines and the seedling 

replenishes its moisture content from the water in the growing 

medium. The second curve (B in fig. 4.2.4) shows the PWP 

pattern for a seedling under a high evaporative demand and in a 

moderately dry growing medium. The PWP begins at a lower 

initial level, because the seedling has not been able to completely 

recharge its moisture supply overnight and, therefore, its PWP 

declines to lower levels (higher stress) during the afternoon. If this 

pattern continues over time, damaging moisture stress levels can 

develop within the seedling.

PWP can be measured by several techniques, but the most 

practical for nursery operations is the pressure chamber (fig. 

4.2.5). The technique for operating this instrument is 

well-described by McDonald and Running (1979):

Cut a small twig or needle from a seedling and place it in the 

chamber with the cut end protruding from the lid. A useful 

analogy is to imagine the water column in the seedling as a 

rubber band. As the water

75
76
stress increases in the seedling, this rubber band is stretched. 

When the twig or needle is cut from the seedling, the tension 

on the rubber band (water column) causes the water to 

withdraw back from the cut surface. By slowly applying gas 

pressure to the cut twig in the chamber, the water is forced 

back to the cut surface of the twig. When the gas pressure is 

exactly equal to the tension (PWP) that is on the water column, 

water will appear at the cut surface; by reading the gauge on 

the pressure chamber as soon as the water appears, the PWP 

can be measured.

New techniques for measuring PWP are being developed. Dixon 

and Tyree (1984) describe a hygrometer that can be attached to 

the stem of woody plants that will directly measure PWP The 

hygrome ter is a small metal chamber housing tiny thermocouple 

sensors that can remain attached to a woody stem for several 

weeks. One limitation of this technique is that the optimum stem 

diameter is about 8 to 10 mm (0.31-0.39 in), which is larger than 

most reforestation stock. Although primarily a research procedure 

at present, future modification of this technology may yield a 

technique for continuously monitoring PWP in an operational 

nursery.

4.2.2.4 Growing medium water potential

Because of large variations in bulk density, pore space, and pore 

size distribution, water in the growing medium is best described by 

potentials rather than on the traditional weight basis. A 

fine-textured medium usually contains more water by weight than 

a coarse-textured me dium, but less water will actually be available 

for plant uptake and growth (Bunt 1976). Water potential, on the 

other hand, gives an indication of the water available for plant 

growth regardless of growing medium texture.

The potential of water in the growing medium solution is called the 

growing medium water potential (GMWP) and is composed of two 

parts: OP, which reflects the influence of dissolved salts, and MP, 

which measures the attraction of water molecules for the surfaces 

and small pores in the growing medium (table 4.2.1). The OP of 

the growing medium solution increases (becomes more negative) 

as the soil water content decreases due to evaporation or 

transpiration and the salinity of the growing medium solution 

increases-a reduction of 50% in the soil water content will 

approximately double the salt concentration (Bunt 1976). (The 

effects of high salinity on seedling growth are discussed in section 

4.2.4.2.)

The MP reflects the energy with which the water in the growing 

medium is held by matric forces and is related to the size of pores 

in the growing medium. The pore volume of a growing medium is a 

function of particle size and arrangement and is composed of air 

and water, which change in inverse proportion to one another. 

After irrigation, excess water is drained out of the container by 

gravitational forces, leaving the growing medium essentially 

saturated; this point is termed "container capacity;" which differs 

from the traditional field capacity due to the effect of the container 

(see section 4.3.2.2). At container capacity, the MP is very high 

(moisture stress is very low) and water is readily available to the 

seedling. As the growing medium loses water through evaporation 

and transpiration, the large pores drain first and are filled with air. 

The pores never drain completely, however, as an increasingly 

thinner film of water surrounds the growing medium particles. The 

thinner the water film layer, the lower the MP (the higher the 

moisture stress) and the less water that is available to the 

seedling. The smaller pores are the last to lose their water. 

Eventually, the water content of the medium (and the MP) will be 

so low that the seedling is unable to obtain water as fast as it is 

lost to transpiration and the seedling will begin to lose turgor and 

wilt. The permanent wilting point occurs when the seedling is 

unable to recharge its moisture reserves overnight and remains 

flaccid (Bunt 1976).

The GMWP and PWP are closely related; the PWP pattern for a 

seedling that starts out in a moist growing medium (high GMWP) 

but is not irrigated is shown in figure 4.2.6. As the growing medium 

dries out, the GMWP gradually decreases. The normal PWP curve 

occurs each day (fig. 4.2.4A), but the PWP decreases to a lower 

level on each subsequent day because the seedling is not able to 

recharge its moisture supplies from the increasingly drier growing 

medium. The PWP becomes more and more negative until 

ultimately the seedling is unable to recover.

Most traditional soil moisture instruments measure only MP and

ignore the OP, which can be a significant component in heavily 

fertilized soils such as greenhouse media (fig. 4.2.7). The MP can 

be measured directly with a pressure membrane in a testing lab or 

with a tensiometer at the nursery. The best method of measuring 

GMWP appears to be the small-wire thermocouple psychrometer 

technique. These small psychrometers are encased in porous 

bulbs and are attached to electrical

77
78

leads and the GMWP can then be measured directly with a 

microvoltmeter (Kramer 1983). These psychrome ters could be 

placed within the growing medium inside the container and the 

GMWP measured easily and instantaneously. This technique was 

not being used operationally at any of the nurseries that 

responded to the Container Nursery Survey but may hold promise 

as a way to monitor soil moisture in containers. (Current ways of 

operationally monitoring irrigation are discussed in section 4.2.6.)
4.2.3 Factors Affecting Water Availability in

Containers

4.2.3.1 Unique aspects of artificial growing media

The use of "artificial soil" composed of materials like peat 

moss and vermiculite affects many properties of water in the 

growing medium, such as water holding capacity and water 

movement.

High water-holding capacity. Most of the growing media used in 

container tree seedling nurseries, such as a mixture of peat moss 

and vermiculite, have the ability to absorb and retain a much 

higher percentage of water than the mineral soils of bareroot 

nurseries. The most accurate way to illustrate this difference is 

with a soil moisture retention curve that shows the relationship between gravimetric soil moisture and MP. Soil moisture retention 

curves are developed in a soil-testing lab using a pressure plate 

apparatus; separate curves must be developed for each type of 

soil or growing medium because the relationship changes 

significantly based on physical characteristics, particularly texture 

and structure. Figure 4.2.8 shows two soil moisture retention 

curves for a typical silty loam soil for a bareroot nursery soil and a 

1:1 peat-vermiculite growing medium; note that the 

peat-vermiculite growing medium retains considerably more water 

over the entire range of MP The properties of peat-vermiculite 

media are discussed further in the chapter on growing media in 

volume two of this series.

Water movement in artificial growing media. The infiltration 

rate and capillary conductivity of peat-vermiculite growing media 

change with water content. When irrigation water is applied to the 

surface of a container, the rate at which it is absorbed into the 

growing medium is termed the infiltration rate. The infiltration rate 

for peat-vermiculite media is relatively high because most growing 

media have good porosity. If a medium is allowed to dry out, 

however, the infiltration rate can be severely restricted because 

dry peat particles actually repel water and become hydrophobic 

(Furuta 1978). Wetting agents (surfactants) are commercially 

available that can be applied to the growing medium and increase 

the infiltration rate; these chemicals are detergents that break 

down the surface tension of water and cause it to penetrate the 

medium much more easily. Wetting agents are often used during 

the hardening period to resaturate the growing medium after it has

been allowed to dry out to induce seedling water stress (see 

section 4.2.7.3).

Capillary conductivity refers to the rate of water movement in the 

growing medium, especially in response to plant uptake. During 

irrigation, the capillary conductiv ity of peat-vermiculite media is 

high in a downward direction due to gravity, but after irrigation, 

capillary conductivity may be limited in very porous media (Furuta 

1978). Hanan and others (1978) studied the capillary conductivity 

of two growing media with different textures and found that 

seedlings growing in very porous mixes could experience severe 

water stress even though water could still be squeezed from the 

mixture by hand. Water movement in growing media can be 

increased by using components that provide a mixture of particle 

sizes and by insuring that the medium is firmly tamped into the 

container during loading. Use of a growing me dium that is too fine 

or excessive tamping, however, can cause waterlogged conditions 

and seriously reduce seedling growth.

4.2.3.2 Container effects

Water in a container behaves differently than water in an 

unconfined soil; this fact should be understood by container 

nursery managers because it affects water management and 

resultant irrigation practices. The application of a given amount of 

water to a fixed amount of growing medium in a small container 

produces a different water content, different moisture movement, 

and therefore a different plant response than the same amount of 

water applied to the same volume of unconfined soil (Furuta 

1978).

79
4.2.3.3 Water loss from container seedlings

Besides the usual gravitational drainage of excess water, there 

are two pathways for water loss from solid-wall containers in a 

container tree seedling nursery: evaporation from the growing 

medium surface and seedling transpiration. Evapotranspiration 

rates are related to energy input, mainly sunlight in a greenhouse, 

and several studies have correlated water loss to solar input. 

Evapotranspiration increases with higher temperatures, lower

When irrigation is applied to a filled container, the water 

percolates downward under the influence of gravity until it reaches 

the bottom of the container; at this point, water flow ceases 

because the force of gravity is less than the combined forces of 

adhesion and cohesion within the water column. Drainage from 

the bottom of the container only occurs when the force due to the 

height of the water column is sufficient to overcome these 

adhesive and cohesive forces. The biological significance of this 

drainage pattern is that a zone of satu rated growing medium 

always exists above the medium/ air interface at the bottom of the 

container (Furuta 1978). The depth of this saturated layer is a 

function of the growing medium texture as shown in figure 4.2.9. 

The depth of this saturated zone will be greater for a fine-textured 

medium than for a coarse-textured one because capillary forces 

are greater in smaller pores.

The height of the container determines the proportion of freely 

draining growing medium that it contains, assuming that the 

growing medium texture is the same (Whitcomb 1984). A 10.2-cm 

(4 inch)-high container will have the same depth of saturated 

medium as a 25.4-cm (10-inch; container, but the shorter container 

will have proportionately less freely drained growing medium (fig. 

4.2.10). This "capillary fringe" effect is independent of container 

diameter or shape. For a given container, the only way to modify 

the depth of this saturated layer is to change the growing medium 

texture to a coarser mix (Furuta 1978). The presence of this 

saturated layer has serious implications for the aeration of the 

growing medium; Bunt (1976) showed that changing the container 

height from 5 to 20 cm (2 to 7.9 inches) increased the air volume in 

the container from 4 to 8% (fig. 4.2.11).

The containers that are used in many container tree seedling 

nurseries have a relatively small top opening and limited volume. 

The small top opening is important operationally because it is 

extremely difficult to distribute irrigation evenly between 

containers, which leads to considerable variation in growing 

medium water content. This distribution problem becomes even 

more criti cal when the seedlings become larger and their foliage 

begins to intercept irrigation before it can reach the top of the 

container. Foliage interception is particularly serious for 

broadleaved species. Because small containers have a 

corresponding small volume of growing medium, they have limited 

moisture reserves and require frequent irrigation, especially in 

times of high evapotranspirational losses (Furuta 1978).

80
humidities, and increased air movement. Seedling water losses 

are greatest, therefore, when high temperatures cause the vents 

to open in an enclosed greenhouse. High temperatures and 

drying winds can also increase water use in semi-controlled 

structures. Although many equations have been developed to 

predict mathematically the amount of water loss from climatic 

factors for

81

agricultural crops, they are too imprecise to have much practical 

usefulness for irrigation management in operational container 

tree seedling nurseries.

Evapotranspiration losses in container nurseries can be divided 

into two general time periods. Evaporation from the growing 

medium surfa ce is the major loss early in the growing season 

(fig. 4.2.12A). As seedlings become larger, however, foliar 

transpiration is responsible for a greater proportion of water loss 

because the root system
can extract water from throughout the growing medium; at the 

same time, the foliage shades the surface of the growing medium, 

which reduces evaporation (Furuta 1978). This change in type of 

water use affects the water status of the growing medium and 

resultant irrigation practices. During germination and emergence 

the medium dries out only in the surface layer, whereas later in the 

growing season, the growing medium moisture will be depleted 

throughout the container (fig. 4.2.12). Different species of 

seedlings use water at different rates; Ballard and Dosskey (1985) 

found that Douglas-fir seedlings used more water than either of 

two species of hemlock (fig. 4.2.13). Irrigation practices that are 

used to manage these changing patterns of water use are discussed in sections 4.2.6 and 4.2.7.

82
4.2.4 Quantity and Quality of Irrigation

Water

Because water is considered to be the most important biological 

factor controlling plant growth, the quantity and especially the 

quality of irrigation water is the single most important 

environmental factor in the operation of a container tree nursery. A 

successful container nursery must have an ample supply of 

high-quality irrigation water available throughout the growing 

season. Gartner (1981) stresses the importance of water quality in 

nursery site selection and discusses the characteristics of different 

irrigation water sources. Both the quantity and quality of water 

from potential sources should be checked thoroughly prior to the 

establishment of a container nursery. (Site selection criteria are 

discussed in volume one of this series.)

4.2.4.1 Quantity of water required for container

nurseries

There are basically two sources of irrigation water for use in 

container tree nurseries: groundwater and surface water. Both 

sources have been used successfully, although surface water 

from streams, reservoirs, or lakes is more likely to be contaminated with pests such as fungal pathogens or weed seeds.

The primary consideration in selecting an irrigation water source 

is to insure that enough water is available throughout the growing 

season for all the possible uses at the nursery. Water availability 

is often a problem during the early spring and late fall if irrigation 

is needed for frost protection in outside growing compounds or 

overwintering areas. The availability of surface water may be 

more variable than that of well water, although the reliability of 

any irrigation source should be well-researched before the 

nursery is constructed.

The amount of water necessary to produce a crop of container tree 

seedlings depends on many factors, such as climate, type of 

growing structure, type of irrigation system, growing medium, and 

seedling characteristics. Matthews (1983) calculated that 1,000 

conifer seedlings in Styroblock 2® (41 cm3 = 2.5 cubic inch 

capacity) containers require up to 45.4 liters (12 gallons) of water 

per week, depending on type of irrigation system. At Mt. Sopris 

Nursery in Colorado, a crop of 1,000 conifer seedlings in 164- cm3

(10-cubic-inch) Ray Leach Single Cell® containers required from 

56.8 liters (15 gallons) early in the season to over 189.3 liters (50 

gallons) per week during mid-summer. The University of Idaho 

Forest Research Nursery in Moscow, ID, uses 42.6 to 54.7 liters 

(11.2 to 14.4 gallons) of irrigation water per week for 1,000 conifer

seedlings in 65 cm3 (4 cubic inches)

Ray Leach Pine Cell® containers. The Colorado State Forest 

Nursery in Ft. Collins uses 75.8 to 94.6 liters (20 to 25 gallons) of 

irrigation water per week for a thousand of their large 492- cm3

(30-cubic-inch) Styrofoam® containers.

4.2.4.2 Water quality

The quality of irrigation water is another important factor in the 

selection and management of a container tree seedling nursery. 

Water quality should be a primary consideration in the site 

evaluation for a new nursery because there is no inexpensive way 

to improve poor-quality irrigation water.

The term water quality means different things to different people 

because quality is dependent on intended use. For domestic 

purposes, factors such as color, taste, turbidity, odor, and toxic 

ion concentrations determine water quality, whereas for 

irrigation purposes, water quality is determined by two factors:

1. The concentration and composition of dissolved

salts (total salinity and individual toxic ions).

2. The presence of pathogenic fungi, weed seeds, algae, and possible pesticide contamination.

Effects of salts on irrigation water quality. Salinity is 

considered to be the primary factor in the determination of 

agricultural water quality (Richards 1969). For our purposes, a salt 

can be defined as a chemical compound that releases charged 

particles called ions when dissolved in water: for example, 

potassium nitrate (KNO3) releases two ions, one a positively 

charged cation (K+) and the other a negatively charged anion 

(NO3

-

). Salts can be either beneficial or harmful, depending on the 

characteristics of the specific ions involved, as well as the total 

salt concentration. KNO3 is a fertilizer salt and both K+ and NO3

-

are nutrient ions, whereas other salts, such as sodium chloride 

(NaCI), consist of harmful ions (Na+ and CI-) that can damage or 

even kill plant tissue. A list of the major ions that affect irrigation 

water quality is provided in table 4.2.3.

Many terms and units have been used to describe salinity (table 

4.2.4). Because an aqueous solution of dissolved ions conducts 

electricity, salinity is traditionally expressed as electrical 

conductivity (EC); the higher the salt concentration, the higher the 

EC reading. EC is measured in units of electrical conductance over 

a specific distance (usually 1 cm) and at a standard temperature

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84

[25 °C (77 °F)]. The most commonly used units in irrigation water 

quality are micromhos per centimeter (abbreviated mho and 

pronounced "micromows") and the SI units of microsiemens per 

centimeter, which are equivalent. Microsiemens per centimeter 

(abbreviated as /μS/cm) will be used as the standard EC unit in 

this publication.

Another older system for reporting the total salt content of 

irrigation water is total dissolved solids (TDS), which can be 

determined by evaporating a known weight of water and 

weighing the resultant salt deposit (California Fertilizer 

Association 1985). TDS in parts per million can be estimated by 

multiplying the EC (in microsiemens per centimeter) by 0.64 

(table 4.2.4).

Specific ions are generally described in units of milliequivalents 

per liter (meq/I) or parts per million (ppm) (table 4.2.4). The former 

are the preferred units for water quality purposes, whereas the 

latter are more practical for fertilizer calculations, so both should 

be requested in irrigation water quality tests. Although the exact 

conver-
sion will vary with the ions involved, 1,000 μS/cm of salinity 

equals approximately 640 ppm of total salts and the meq/I of total 

salts can be estimated by dividing the EC in /AS/cm by 100 (table 

4.2.4) (California Fertilizer Association 1985).

The topographic location of a container nursery can have an effect 

on irrigation water quality because of local climatic or geologic 

influences. Irrigation water may be contaminated by saltwater 

intrusion in coastal areas. In arid or semi-arid climates where 

evapotranspiration exceeds precipitation, salts naturally 

accumulate in the soil, and geologic deposits and groundwater 

irrigation sources are often high in salt content. Matthews (1983a) 

reported that irrigation water quality was much better in coastal 

container nurseries in British Columbia than in nurseries in the 

interior of the province. Hallett (1987) tested 32 irrigation water 

sources in the relatively humid Maritime Provinces of Canada and 

found an average EC of 170 1tS/cm compared to an average EC 

value of 507 μS/cm for 15 nurseries in the semi-arid interior West 

of the United States (Landis 1982). There is a significant amount 

of local variation, however, because the EC values ranged from 19 

to 787 μS/cm in the Maritimes and from 58 to 1,460 μS/cm in the 

western United States.

Effects of salinity on seedling growth. Soluble salts can affect 

plant growth in several different ways. Fuller and Halderman 

(1975) discuss four effects: 1) total salinity can reduce water 

availability, 2) sodium ions decrease soil permeability, 3) certain 

specific ions are directly toxic, and 4) availability of other mineral 

nutrients is altered. Another, nonbiological effect of dissolved salts 

is the unsightly effect of salt crusts on the foliage, which may affect 

the salability of seedlings. The specific ions involved and the 

evaluation criteria for these five categories of seedling injury are 

listed in table 4.2.5.

Effect of salinity on water availability. The total salinity of a 

water source, as measured by the EC, exerts an osmotic effect 

in the growing medium solution, which can reduce the water 

available for plant growth. Salts decrease the osmotic potential 

and therefore the water potential in the growing medium solution: 

a value of 3,000 μS/cm represents an osmotic force of 0.1 MPa. 

Although this may not seem particularly high, it may become 

significant during critical times such as seed germination, 

especially if the growing medium is allowed to dry out, which can 

greatly increase the osmotic concentration. Whitcomb (1984) 

reported that the soluble salt level doubles when growing 

medium

dries from 50 to 25% moisture content. The high fertilization rates 

used in container nurseries can add to the salinity problem. The 

OP becomes an increasingly more significant factor of the total 

GMWP as the water content of the growing medium decreases 

(fig. 4.2.7).

Most woody plant seedlings, and conifers in particular, are 

extremely sensitive to salinity damage. The principal damage of 

high salinity is reduced growth rate, which usually develops before 

more visible foliar symptoms become evident. Very sensitive 

species, such as blue spruce and Douglas-fir, can suffer growth 

reductions as high as 50% when the EC of the growing medium is 

as low as 2,500 μS/cm (table 4.2.6). The symptoms of salt injury 

vary with species and may include one or more of the following: 

foliar tip burn (fig. 4.2.14A), scorching, or bluish color; stunting; 

patchy growth; and eventual mortality. Water with high salt levels 

can also cause whitish deposits on the leaf surfaces (fig. 4.2.14B) 

that, although not directly damaging, may reduce the sale-ability 

of the crop. Salt deposits may also accumulate on sprinkler 

nozzles and reduce their efficiency.

Salinity rating guidelines for irrigation water for container tree 

nurseries are provided in table 4.2.7. The EC

85
can easily be checked at the nursery using a conductivity meter, 

and salinity tests should be run at least every month because 

water quality can change significantly over the course of a year. 

The EC and pH of the irrigation water at a California nursery was 

monitored monthly, and the values were found to vary considerably 

86

because of differences in water quality between different irrigation 

wells (fig. 4.2.15). Although there is no inexpensive way to remove 

salts from irrigation water, cultural practices such as increasing the 

porosity of the growing medium and leaching more frequently can 

help alleviate the effects of saline water.
Although highly saline irrigation water is not desirable, very pure 

irrigation water can also cause problems; in fact, distilled or 

deionized water is not recomme nded for irrigating growing tree 

seedlings. Water with a very low level of dissolved salts will leach 

fertilizer ions as it passes through the growing medium. 

Vetanovetz and Knauss (1988) define "too pure" irrigation water as 

having a low total soluble salt level (EC < 200 μS/cm), and low 

calcium and magnesium contents. They state that use of very pure 

water with artificial growing media may

leach out the nutrients from incorporated fertilizer amendments, 

such as limestone and dolomite, resulting in a calcium or 

magnesium deficiency. This will not be a serious problem, 

however, if container tree seedling nurseries add fertilizer salts to 

the irrigation water through liquid fertilizer injection.

Sodium effect on growing medium permeability. Although 

sodium (Na+) is directly toxic to seedlings, this ion has an equally 

serious effect on growing medium structure. An excess of Na+

ions relative to the concentration of calcium (Ca2+) and 

magnesium (Mg2+) ions can cause clay particles to disperse and 

seal up the pores, which seriously reduces permeability and gas 

exchange. This sodium effect is usually measured in terms of the 

sodium adsorption ratio (SAR), which gives a relative index of the 

concentration of Na+ to Ca 2+ and Mg2+ ions. Actually a revision of 

the SAR index, called the adjusted sodium adsorption ratio 

(ASAR), is row preferred because it also considers the effect of 

carbonate (CO3

2-

) and bicarbonate (HCO3

-

) ions (table 4.2.5). 

ASAR standards for nursery irrigation water are provided in table 

4.2.7. Fortunately, sodium-induced permeability problems only 

affect natural soils and should therefore not be a problem in the 

"artificial" growing media (those that do not contain soil) that are 

commonly used in most container tree seedling nurseries.

Toxicity of specific ions. The third effect of salinity is the direct 

toxicity of certain ions, especially sodium, chloride, and boron, to 

seedling growth (table 4.2.5). Individual plant species vary 

considerably in their sensitivity to these ions, but all container 

seedlings should be considered very susceptible because of their 

small size and succulence. Ayers (1977) provides water quality 

guidelines for either root or foliar absorption of these three 

potentially toxic ions, and Bunt (1976) also published water quality 

standards (table 4.2.7). Other ions, including any of the heavy 

metal ions such as manganese or zinc, can also be toxic if present 

in high enough concentrations. These problems should be 

identified in the initial irrigation water quality tests, however.

Mineral nutrient availability. Excesses of certain ions for 

example, calcium and magnesium-in the irrigation water can 

produce nutrient imbalances in the growing medium solution and 

lead to problems with nutrient uptake and utilization in some 

plants (Fitzpatrick and Verkade 1987) (table 4.2.5). Vetanovetz 

and Knauss (1988) state that, if irrigation water contains over 100 

ppm Ca 2+, this mineral nutrient may accumulate in the

87
growing medium and cause deficiencies of Mg 2+ or iron. 

Greater than 50 ppm of Mg 2+ in the water can also cause 

deficiencies of other mineral nutrients, such as Ca2+ and 

potassium (table 4.2.7). These nutrient imbalances are 

complicated and particularly difficult to diagnose, but should not 

be a problem in container tree seedling nurseries that use 

artificial growing media and a well-balanced fertilization 

regimen.

Foliar residues. Certain compounds, such as iron and calcium 

carbonate, can cause discoloration or spotting of seedling 

foliage (table 4.2.5, fig. 4.2.14B). Although these cosmetic 

blemishes do not directly affect seedling growth, they do affect 

customer appeal. Some well waters contain dissolved iron in 

the ferrous form; when these waters are applied through 

sprinkler irrigation, the ferrous iron is converted to the ferric 

form, which has a typical rust color that can stain foliage 

(Swanson 1984) or clog irrigation nozzles (Fitzpatrick and 

Verkade 1987). "Hard" irrigation water contains high levels of 

calcium, magnesium, carbonate, or bicarbonate ions. When this 

irrigation water is sprinkled on plant foliage, it produces white 

deposits of calcium carbonate or magnesium carbonate upon 

evaporation. These carbonate deposits can also build up on 

irrigation nozzles. Water

88

quality standards for foliar staining ions are listed in ta ble 4.2.7.

Pests in irrigation water. Container nurseries that use irrigation 

water from surface water sources such as ponds (fig. 4.2.16), 

lakes, or rivers may encounter problems with biotic pests, that is, 

weeds, pathogenic fungi, moss, algae, or liverworts (Baker and 

Matkin 1978). Surface water that originates from other nurseries or 

farmland is particularly likely to be contaminated with water-mold 

fungi, such as Pythium and Phytophthora, that can cause 

damping-off (Whitcomb 1984). Recycled nursery irrigation water 

should also be suspect. Water-mold fungi can be isolated from 

nursery irrigation water sources using "baits" of green fruit such as 

pears or apples (McIntosh 1966). Many weed seeds and moss 

and algal spores are small enough to pass through the irrigation 

system and can cause real problems in container nurseries. 

Irrigation water was considered to be the source of a blue-green 

algae problem in a British Columbia container nursery (Vance 

1975).

Water-borne pests can be killed with chlorination, and some 

specialized filtration systems can remove many disease 

organisms from irrigation water. (See the follow-
ing section for more information on chlorination and filtration.)

Pesticide contamination. Irrigation water, especially in 

agricultural areas, may have become contaminated with residual 

pesticides. Urbano (1987) reports that pesticide contamination of 

groundwater is becoming an increasingly serious problem in the 

United States. In 1979, the active ingredient (aldicarb) of the 

herbicide Temik® was identified in well water in Long Island, NY, 

and groundwater contamination is considered a potential threat in 

many other areas. Herbicides applied to adjacent cropland or to 

control aquatic weeds in reservoirs can affect irrigation water 

quality. Vance (1975) reported substantial losses of container 

seedlings due to herbicidal control of aquatic weeds in a nursery 

irrigation reservoir. Potential sources of irrigation water should be 

tested for pesticide contamination when a site is being evaluated 

prior to development.

Water quality tests. Ideally, water quality tests are performed 

during nursery site selection and at regular intervals thereafter, but 

many container tree seedling nurseries have never had a detailed 

water analysis performed. A complete analysis of irrigation water 

quality should consist of a salinity evaluation listing the 

concentrations of eight specific ions that should be reported in 

milliequivalents per liter (meq/liter) and parts per million (ppm) 

(table 4.2.8). For a small additional fee, it is possible to test for the 

remaining nutrient ions at the same time. In addition to the ion 

concentrations, the testing

lab should report three standard water quality indices: electrical 

conductivity, adjusted sodium adsorption ratio, and pH. EC and 

pH can be measured directly from the water sample and the 

ASAR can be computed from the specific ion concentrations in 

milliequivalents per liter (table 4.2.9).

Irrigation water should also be tested for the presence of 

pathogenic fungi, preferably during the site selection process but 

also if a problem is observed at a later date. Most plant pathology 

laboratories can conduct bioassays of irrigation water. Testing for 

residual herbicides is also possible but can be expensive because 

of the sophisticated analytical procedures required. Due to the 

different chemical structures of the various pesticides, a separate 

analysis for each suspected pesticide is usually required. 

Therefore, specialized pesticide tests are generally considered 

only when a definite problem is suspected.

Fitzpatrick and Verkade (1987) discuss the proper procedure for 

collecting a sample for irrigation water testing: use a clean plastic 

bottle with a firm lid, let the water

89
90

run for several minutes, and rinse the sample bottle well before 

collecting the sample. Chu (1986) states that a 500-ml (about 1-pint) sample is sufficient for most water tests but that 1,000 ml 

(about 1 quart) is needed for pesticide te sting. Label the sample 

bottle properly with a waterproof marker before sending it to the 

analytical laboratory. The sample should be tested as quickly as 

possible but can be stored under refrigeration for short periods, if 

necessary. Water quality parameters can change over time, 

however, and Chu (1986) reports that the values for some test 

parameters, such as pH and chlorine, can change after only a few 

hours (table 4.2.10).

Corrective treatments for irrigation water. Any irrigation water 

treatment system requires significant initial expense, making it 

prudent to determine irrigation water quality before a nursery is 

developed. Sometimes, however, there is no other option because 

it is less expensive to treat the irrigation water than to attempt to 

relocate an already established nursery. There are six standard 

water treatment procedures.

Acidification. Irrigation water is often treated with acid to lower 

the pH level to the ideal range of 5.5 to 6.5. Phosphoric and 

sulphuric acid are most commonly used, although other acids 

such as nitric or acetic acid have also been utilized. Acidification 

will not change the salinity of irrigation water, but it does remove 

the carbonate and bicarbonate ions, as reflected by the lower pH. 

Phosphoric acid was used to acidify the irrigation water in a 

Canadian container tree seedling nursery; the pH was lowered 

from 8.8 to 6.1 but the total salinity, as reflected by the EC 

readings, was not changed appreciably (table 4.2.11). 

(Acidification is also discussed in section 4.1.3.3.)

Reverse osmosis. This treatment consists of forcing irrigation 

water through a semipermeable membrane so ,hat the salt ions 

are left behind. The process is relatively expensive, but systems 

are available for nurseries. As an example of the costs involved, a 

California container tree seedling nursery recently purchased a 

reverse osmosis system, at an initial cost of over $50,000, de-

signed to produce 40,000 gallons per day of treated water at an 

operating cost of approximately $0.80 per thousand gallons. This 

system is designed to signifi cantly improve irrigation water quality 

from an initial pH of 8.1 and EC of 2,218 μS/cm to a pH 5.8 and 

an EC of 312 /μS/cm. Reverse osmosis systems require regular 

maintenance but are a practical irrigation water treat-
ment system for container tree seedling nurseries if the initial 

cost can be justified.

Deionization. Deionization is an effective yet costly method to 

remove unwanted salts from water and would only be practical for 

crops of very high value (Furuta

1978). The process consists of passing water over ion-exchange 

resins that are charged with either H+ or OH- ions; these ions are 

exchanged for the Ca 2+, CI-, or other charged ions in the irrigation 

water, resulting in chemically pure water. Boron salts are not 

removed by deionization, although all ions can be removed by reverse osmosis (Hartnrann and Kester 1983). Another drawback to 

this process, in addition to the high cost, is that it is relatively 

slow, and treated water must generally be accumulated and 

stored to supply the volume needed by large container tree 

seedling nurseries.

Water softeners. This water treatment is included only for 

completeness and should never be used for treating irrigation 

water at container tree seedling nurseries. Wa ter softeners do not 

improve the salinity level of water, but merely convert "hard" 

water, which contains an abundance of Ca 2+ and Mg 2+ ions, to 

"soft" water, which is predominated by Na+ ions; this additional 

Na is much more injurious to plants than the Ca and Mg that was 

replaced (Whitcomb 1984). The primary benefit of softened water 

is that it makes soaps and detergents clean more effectively.

Chlorination. Chlorination is a viable water treatment for 

nurseries that have a problem with fungi, bacteria, algae, or 

liverworts that are introduced through the irrigation system. The 

two most common ways of introducing chlorine into the irrigation 

water are:

91
1. liquid sodium hypochlorite (household bleach, NaOCI) 

or powdered calcium hypochlorite [Ca(OCI)2] can be 

added to the water, and

2. pressurized chlorine gas (CI2) can be injected into

the irrigation system.

Chlorine gas is the most common and cheapest way to chlorinate 

water but is relatively dangerous to use, compared to sodium or 

calcium hypochlorite. When chlorine is added to water, it interacts 

to produce hypochlorite (HOCI) and chlorite ions (OCI-), which are 

powerful oxidizing agents and responsible for the disinfectant 

action of the solution. In actuality, only part of the applied chlorine 

(called the free residual chlorine) is effective because some of the 

chlorine ions combine with organic substances (combined 

chlorine) and are essentially inactivated (Tchobanoglous and 

Schroeder 1985). The various forms of chlorine used in chlorination should not be confused with the chloride ion (CI-

), which is an 

important irrigation water quality ion, but has no disinfectant 

properties (Green 1987).

There are five aspects of operational chlorination: 1) initial contact, 

2) contact time, 3) form and concentration of disinfectant, 4) 

species and concentration of pathogens, and 5) environmental 

factors, particularly pH and temperature (Tchobanoglous and 

Schroeder 1985). For example, when exposed to warm 

temperatures and sunlight, hypochlorite decomposes and loses its 

disinfectant properties (Green 1987). The engineering aspects of a 

chlorination system are too complicated to be discussed here but 

suffice it to say that, to be effective as a disinfectant, a critical 

concentration of free residual chlorine must be maintained for a 

specified time period. Baker and Matkin (1978) report that 1 ppm 

free residual chlorine will kill zoospores of Phytophthora 

cinnamomi in 1 minute but that the mycelium of the fungus is 

much more resistant.

A chlorination system, whether chlorine gas injection or 

hypochlorite addition, should be designed so that the chemical is 

thoroughly mixed with the irrigation water and has enough time to 

act. Because chlorine is deactivated by suspended organic matter, 

irrigation water should be filtered before treatment. Chlorine gas is 

also very hazardous and corrosive and, therefore, a chlorination 

expert should be consulted when designing a chlorine injection 

system. The chlorine level in the applied irrigation water should be 

periodically monitored to make certain that the chlorination system 

is working

92

properly; this can be done with a commercially available 

swimming pool test kit or with a Hach® kit (Frink and Bugbee 

1987).

Chlorination is routinely being used in some forest and ornamental 

nurseries. Standard household bleach (5.25% sodium 

hypochlorite) at a rate of 20.4 cm3 per 1,000 liters of water (2.6 

fluid ounces per 1,000 gallons) will produce approximately 1 ppm 

chlorine (Baker and Matkin 1978), and the authors recommend a 

minimum contact time of 4 minutes. Bunt (1976) recommends 

adding enough sodium hypochlorite to produce a chlorine content 

of 5 to 20 ppm, but states that a chlorine concentration as low as 

0.5 ppm can be effective if the treated water is stored. Handreck 

and Black (1984) recommend adding enough bleach to give 2 

ppm chlorine. Daughtry (1984) discusses an operational 

chlorination system using injected chlorine gas that produces a 

concentration of 0.3 ppm residual free chlorine and a contact time 

of about 25 seconds within the irrigation line.

Many plants are sensitive to chlorine, and Bunt (1976) reported 

no problems with water containing 5 ppm of chlorine. Injected 

chlorine at 5 to 10 ppm was not found to be phytotoxic to a wide 

range of plant species and the higher level partially controlled 

liverwort (Scott 1980, as reported in Whitcomb 1984). Many 

domestic water supplies are chlorinated to control human 

pathogens but this treatment usually produces a relatively low 

chlorine level (about 1 ppm) that is not injurious to most plants 

(Frink and Bugbee 1987).

Filtration. Filters can be used to remove suspended or colloidal 

particles, such as very fine sand, that can damage irrigation or 

fertilization equipment or plug irrigation nozzles. In addition to 

removing suspended inorganic particles, filters can be used to 

remove some unwanted pests, such as weed seeds or algae, 

from the irrigation system (figure 4.2.17).

Two general types of filters are commonly used in water treatment: 

granular medium filters and surface filters. Granular medium filters 

consist of beds of granular particles that trap suspended material 

in the pores between the particles, whereas surface filters use a 

porous screen or mesh to strain the suspended material from the 

irrigation water (Tchobanoglous and Schroeder 1985). Granular 

medium filters can be used to remove fine sand or organic matter 

and are constructed so that they can be backflushed for cleaning. 

Surface filters include screens or cartridges of various mesh sizes 

to remove suspended
material; screens must be physically removed and cleaned 

whereas cartridge filters are not reusable and must be regularly 

replaced (Sprinkler Irrigation Association 1983).

Filtration is recommended for container tree seedling nursery 

irrigation water, and filters should be installed before the water 

passes through the nutrient injector to intercept sand particles that 

can cause excessive wear or plug valves. )ones (1983) 

recommends cartridge filters because they are easy to change. 

Back flushing of screens or granular medium filters is not practical 

with many nursery irrigation systems. The installation of a 25-/Am 

cartridge filter is recommended for the Smith Measuremix® liquid 

fertilizer injector. Handreck and

Black (1984) recommend using filters small enough to remove 

particles greater than 5 μm in diameter, which will take care of 

most suspended materials (fig. 4.2.17). The types of filters used 

for swimming pools are capable of removing suspended particles 

greater than 50 μm in diameter. Specialized filtration systems, 

such as the Millipore©, can remove particles around 1 /μm in 

diame ter; such a system is therefore capable of removing some 

disease organisms as well as most suspended solids (fig. 4.2.17). 

Unfortunately, these sophisticated filtration systems are relatively 

expensive and require frequent maintenance (Jones 1983).

93
94

4.2.4.3 Water temperature

Another aspect of water quality that can be controlled in container 

tree seedling nurseries is the temperature of the irrigation water. 

Cold irrigation water can significantly lower the temperature of the 

growing medium and has been shown to reduce plant growth in 

ornamental container nurseries. Seeley and Steiner (1965) applied 

irrigation water of different temperatures to a soil-based medium 

and found that 4.5 °C (40 °F) water lowered the growing medium 

temperatures about 4 °C (7 °F) in the surface layer and 1.7 °C (3 

°F) at the 7.6cm (3-inch) level. Carpenter and Rasmussen (1970) 

studied the effect of cold irrigation water on the growth of two 

flower crops and recorded lower growing medium temperatures. 

These lower temperatures resulted in reduced plant height and 

weight during the winter season. Hanan and others (1978) 

recommend that ornamental growers in cold climates consider 

warming irrigation water to the normal ambient temperature range 

during the day.

The effect of using cold irrigation water on tree seedling crops has 

not been studied in detail, but cold soil can definitely reduce water 

uptake by trees. Kramer and Kozlowski (1979) reported that the resistance to water flow through 

tree roots doubles as the temperature decreases from 25 to 5 °C 

(77 to 41 °F). Kozlowski (1943) reported that pine seedlings 

suffered a 50% reduction in the transpiration rate and began to 

wilt at soil temperatures of 5 °C (41 °F), compared to a control 

temperature of 30 °C (86 °F). Woody plant seedlings differ, 

however, in their response to growing medium temperatures. 

Kozlowski (1943) found that loblolly pine was affected more than 

eastern white pine, and Kaufmann (1975) observed that root 

resistance for water absorption became limiting for citrus 

seedlings at 13.5 °C (58 °F), compared to 7.5 °C (45 °F) for 

Engelmann spruce.

On an operational basis, the effect of cold irrigation water on tree 

seedling growth has not been studied enough to make specific 

recommendations, but this effect may be significant for winter 

crops in northern climates. Cold irrigation water may be most 

damaging during seed germination and seedling emergence. One 

of the most practical ways of maintaining growing media at ideal 

temperatures is to provide under-the-bench heating, which may be 

more economical than direct heating of the entire volume of 

applied irrigation water.
4.2.5 Types of Irrigation Systems

The method of applying irrigation water in container tree seedling 

nurseries depends on the size of the operation and the 

characteristics of the crop species. Most large nurseries use some 

sort of mechanical irrigation system; based on the Container 

Nursery Survey, only 2% of the respondents used hand watering. 

Hand watering, however, is often necessary for small container 

tree seedling nurseries or for nurseries producing species with 

radically different water requirements (fig. 4.2.18).

Furuta (1978) lists three major types of irrigation systems used in 

ornamental container nurseries: overhead sprinkling, individual 

container (including drip irrigation), and subirrigation. Overhead 

sprinkling is the only one used in container tree seedling nurseries 

because the containers are too small for drip irrigation of individual 

containers, and subirrigation would prohibit air root pruning at the 

bottom of the containers.

4.2.5.1 Overhead irrigation systems

The basic design consideration of any irrigation system used in 

container tree seedling nurseries is that water must be applied 

evenly to many individual containers that have a relatively small 

top opening in relation to their volume. To complicate matters, 

each container houses a growing seedling that will eventually 

produce enough foliage to intercept a significant proportion of 

the applied irrigation and prevent it from reaching the growing 

medium surface. Overhead systems are also very wasteful of 

water, for only a limited percentage of the applied water ever 

reaches the roots of the seedlings.

Weatherspoon and Harrell (1980) studied the efficiency of different 

irrigation methods for ornamental container plants and found that 

only 13 to 26% of the irrigation water applied through overhead 

sprinklers was retained by the growing medium. On the positive 

side, the artifi cial growing media used in container tree seedling 

nurseries have high infiltration capacities compared to most natural 

agricultural soils, and wind drift is not a problem in completely 

enclosed greenhouses.

There are many types of overhead irrigation systems, but in 

container tree seedling nurseries they can be divided into mobile 

and fixed systems. Mobile systems consist of a traveling irrigation 

boom, and 59% of the nurseries surveyed in the Container Nursery 

Survey used this type of irrigation system. Fixed irrigation systems 

consist of regularly spaced irrigation nozzles and were used in 31 

% of the surveyed nurseries, whereas the remaining 10% used 

some combination of watering systems.

Mobile irrigation booms. This popular irrigation system consists 

of a horizontally oriented boom that carries a distribution pipe 

containing a series of regularly spaced nozzles. The boom is 

mechanically drawn along the length of the growing area by an 

electric motor, usually covering one or more benches (fig. 4.2.19). 

The boom is reversed mechanically when it reaches the end of the 

bench and the seedlings are irrigated from the opposite direction. A 

number of passes is usually required to completely saturate the 

containers. The booms can be supported from a ceiling track, or on 

a floor-mounter cart; the connecting hose is pulled along underneath in a supporting carriage or trails on the floor.

Irrigation boom systems distribute water very evenly compared to 

fixed systems because they supply a mov ing curtain of water with 

none of the distribution problems inherent with circular pattern 

sprinklers. They are relatively expensive, however, and because 

they are me chanical systems, are susceptible to breakdown. 

Shearer (1981) points out that moving sprinklers are not efficient 

for frost protection because they move too slowly to provide 

continuous coverage of all the growing areas.

A variety of nozzle types has been used on irrigation booms 

including circular, cone, and flat fan, although the majority of 

nurseries use some variety of the flat fan nozzle (fig. 4.2.20A). If 

possible, a relatively coarser spray nozzle than that used for 

pesticides should be used for irrigation to minimize misting, insure 

good penetration through seedling foliage, and apply the re95
quired amount of irrigation water in a short time. Some nurseries 

have three different types of irrigation nozzles mounted on parallel 

booms: a flat fan nozzle for irrigation (fig. 4.2.20A), a misting 

nozzle (fig. 4.2.20B), and a hollow cone nozzle for applying 

pesticides. Once the irrigation nozzle has been selected, irrigation 

coverage is dependent on: 1) the spacing of the nozzles on the 

boom, 2) the distance of the nozzle above the containers, 3) the 

water pressure at the nozzle, and 4) the speed of the irrigation 

boom. The spacing of the nozzles along the boom and the proper 

distance above the bench can be determined from the nozzle 

performance specifications supplied by the manufacturer. Hallett 

(1982a) recommended Teejet 8003® nozzles mounted on 31-cm 

(12.2-inch) centers on a boom set at 40 to 50 cm (15.7 to 19.7 

inches) above the seedlings. Willingdon (1987) states that Teejet 

8008® are the most commonly used irrigation boom nozzles in 

British Columbia nurseries. Nozzle pressure can be directly measured from the irrigation nozzle with a pressure gauge equipped 

with a pitot tube. Be aware that the water pressure reading at the 

nutrient injector or in the headhouse can be significantly different 

from the actual delivered pressure at the nozzle; to monitor 

irrigation nozzle efficiency, therefore, water pressure must be 

measured at or near the nozzles. The final factor controlling 

irrigation coverage is the speed of the irrigation boom itself. A rate 

of approximately 2.4 to 3.1 m/min (8 to 10 feet per minute) has 

proven effective in an Idaho container tree seedling nursery 

(Myers 1987).

96

Many commercial irrigation booms are equipped with variable 

speed motors so that the speed can be adjusted for each nursery 

situation.

Fixed overhead sprinklers. This type of irrigation system 

consists of a series of parallel irrigation lines, usually constructed 

of plastic PVC pipe, with sprinklers spaced at uniform intervals to 

form a regular grid pattern. Overhead sprinklers apply water at a 

fairly rapid rate compared to the irrigation boom system and are 

relatively inexpensive to install and operate (Davidson and 

Mecklenburg 1981). Fixed irrigation systems generally do not 

apply water as evenly as mobile systems but will do an 

acceptable job if properly designed and maintained.

Generally, the growing structure is divided into irrigation "bays" 

depending on the number of nozzles that the pump can operate at 

one time at the desired water pressure. Ideal operating pressures 

vary with the type of sprinkler, and specifications are available 

from the manufacturer. Some sprinklers come in different 

coverages such as full-circle, half-circle, and quarter-circle, so that 

full overlap coverage can be obtained by placing irrigation lines 

around the perimeter of the irrigation bay. Each bay should be 

able to be separately controlled with a solenoid valve, which can 

be connected to an irrigation timer so that the duration and 

sequence of irrigation can be programmed. The size of each 

irrigation bay can be designed so that species of differing wa-
ter requirements can be grown within a larger growing structure. 

When designing a new irrigation system, it is a good idea to 

obtain the help of an irrigation specialist to insure that the system 

is balanced in terms of coverage and water pressure.

Several types of irrigation nozzles are used for fixed overhead 

irrigation systems. Spinner sprinklers, which have offset nozzles 

at the end of a rotating arm, spin in a circle when water pressure 

is applied (figure 4.2.21A). Stationary nozzles (fig. 4.2.21 B) have 

no moving parts but distribute water in a circular pattern; these 

nozzles also come in half-circle and quarter-circle patterns. Mist 

nozzles are also sometimes installed on overhead irrigation lines. 

Mist nozzles are primarily used during the germination period and 

for cooling and humidity con-

97
trol because they do not supply water at a fast enough rate for 

normal irrigation.

Some drawbacks of fixed overhead irrigation systems are:

1. Lack of uniform coverage: This problem can be avoided by 

proper design of the irrigation system, ensuring proper 

overlap between nozzles (see section 4.2.5.2 for irrigation 

design considerations).

2. Waste of irrigation water: Because irrigation water is applied 

to the entire growing area, including aisles and walls, 

considerably more water is wasted than with boom systems. 

This is less of a problem in arid climates, however, because 

evaporation of this water adds to the humidity in the growing 

structure.

3. Nozzle drip: A quantity of residual water remains in the lines 

after the solenoid valve has closed and drips out the nozzles 

onto the production benches, washing seed out of containers 

or causing disease problems by keeping the growing medium 

saturated and the foliage constantly wet. The irrigation lines 

can be located over the aisles so that the nozzles drain on to 

the floor. Special anti-drip valves (fig. 4.2.21A) are available 

that close when the water pressure drops, or drip lines can be 

attached to the bottom of the nozzle to drain off the excess 

water. Residual water remaining in the irrigation lines also 

can be a problem during the winter, when freezing can break 

the pipes.

Furuta (1978), Langhans (1980), and Davidson and 

Mecklenburg (1981) present good discussions of the design 

and operation of overhead irrigation systems.

Fixed basal sprinklers. Basal irrigation systems are commonly 

used in large outdoor growing or holding areas; they are similar to 

overhead systems in design and operation in that they use a 

regular grid of permanent or movable irrigation lines with regularly 

spaced sprinklers (fig. 4.2.22A). The main type of sprinkler nozzle 

is the rotating-impact type (fig. 4.2.22B); these sprinklers rotate 

slowly due to the impact of a spring-loaded arm that moves in and 

out of the nozzle stream. Rotating-impact sprinklers are available 

from several manufacturers in a variety of nozzle sizes and 

coverages. Because the impact arm is driven by the water 

pressure out of the nozzle jet, the water distribution pattern of 

these sprinklers is particularly dependent on proper water 

pressure. Sta98

tionary sprinklers can also be used on basal irrigation systems.

One major advantage of basal irrigation systems is that impact 

sprinklers have relatively large coverage areas, which means that 

fewer nozzles and less irrigation pipe are required. Impact 

sprinklers typically have larger droplet sizes compared to boom or 

overhead systems and can result in splash damage on 

germinating seed. Residual water drains back down the irrigation 

riser, although nozzle splash can still be a problem (fig. 4.2.22B). 

All of the irrigation lines must run along or under the floor, 

however, creating obstacles for workers and making it difficult to 

operate machinery. Basal irrigations systems also are typically not 

as effective as mov ing systems in applying water uniformly; proper 

design and maintenance can produce an acceptable system, 

however.

4.2.5.2 Fixed irrigation system design principles

The efficiency of an irrigation system is primarily dependent on its 

original design, and there are few operational procedures that can 

improve a poorly designed system. Basic engineering 

considerations, such as friction loss in pipes or fittings and the 

effect of water pressure on sprinkler function, must be incorporated 

into the irrigation system design. Therefore, it is important to consult 

an irrigation engineer during the planning stages. An excellent 

general reference for sprinkler irrigation system design is available 

from the Irrigation Association (Pair and others 1983).

Uniform water application is a function of five factors:

1. Design of the nozzle

2. Size of the nozzle orifice

3. Water pressure at the nozzle

4. Spacing and pattern of the irrigation nozzles

5. Wind

The size of the sprinkler nozzle and its resultant coverage pattern 

can be determined by consulting the performance specifications 

provided by the sprinkler manufacturer. Container nursery 

managers should select a nozzle size that is coarse enough to 

penetrate seedling foliage and minimize wind drift but not large 

enough to create splash problems.
The water pressure at the nozzle has a major effect on sprinkler 

function and efficiency and must be considered during initial 

planning. Performance specifications for each sprinkler type can 

be obtained from the manufacturer. The water pressure should be 

regularly monitored with a gauge permanently mounted near the 

nozzles (fig. 4.2.23) or with a pressure gauge equipped with a pitot 

tube directly from the sprinkler nozzle orifice. The pressure should 

be checked at several different nozzles including the nozzle 

furthest from the pump. The importance of regular water pressure 

checks cannot be overemphasized, because many factors can 

cause a change in nozzle pressure. Water pressure that is either 

too high or too low can cause erratic distribution patterns such as 

the "doughnuts" or strips shown in figure 4.2.24. (Compare the 

doughnut pattern in figure 4.2.24A with the computer diagram of 

water distribution in figure 4.2.28A.)

Both the type of nozzle and the water pressure affect the irrigation 

droplet size, and the average droplet size decreases as nozzle 

size decreases and water pressure increases. Droplets that are 

too large can cause physical splash damage to germinating seeds, 

whereas very small droplets from "misting" nozzles are subject to 

wind drift and evaporation losses in exposed growing areas. Handreck and Black (1984) estimate that as much as 90% of the water 

from a misting nozzle can be lost on a hot

windy day. Misting irrigation nozzles can be corrected by reducing 

the water pressure or changing the size of the nozzle orifice. A 

simple procedure for measuring droplet size involves exposing a 

dish of SAE 90 oil under the irrigation shower and measuring the 

size of the droplets. An irrigation droplet with a diameter in the 

range of 1.0 to 1.5 mm (0.04 to 0.06 inches) is recommended for 

most container nursery situations (Handreck and Black 1984).

The spacing and pattern of the sprinklers in fixed irrigation 

systems is related to sprinkler function and the effect of wind. 

Regardless of the type of sprinkler used, water distribution is never 

completely uniform over the stated coverage area, and so 

irrigation systems should be designed to provide adequate overlap 

between sprinklers. This is especially important in shadehouses or 

outdoor growing areas where wind drift can be a problem (figs. 

4.2.19B and 4.2.25). Furuta (1978) states that the maximum 

spacing between rotating impact sprinklers should range from 65% 

of the spray diameter with no wind to 30% in winds of greater than 

8 miles per hour. Fixed sprinklers should not be spaced any 

further than 50 to 65% of the sprinkler coverage diameter. Spacing 

of a spinner sprinkler should be no more than 40% of the spray 

diameter in the row and 60% of the diameter between rows (fig. 

4.2.26) and should produce effective irrigation coverage in the 

wind conditions normally en99
countered in container tree seedling nurseries. Very often 

sprinklers are spaced at greater intervals in a cost- saving effort, 

but this is false economy considering the profound effect of water 

and injected nutrients on seedling growth.

The two common sprinkler patterns for fixed irrigation systems are 

the box (rectangular) and diamond (triangular) pattern. Shearer 

(1981) concluded that there is no real difference between the 

standard rectangular sprinkler pattern and the triangular pattern 

under normal conditions.

4.2.5.3 Testing the efficiency of irrigation systems

Both new, and existing irrigation systems should be tested 

periodically to see if they are performing properly. Many nursery 

managers assume that a new system will perform according to the 

engineering specifications, but

100
this should be checked under normal operating conditions. Fischer 

(1987) found that theoretical irrigation patterns differed from 

operational patterns and attributes this discrepancy to two factors:

1. The theoretical patterns assume that the water pressure will 

be identical at each nozzle, which is impossible because of 

pressure losses within the lateral distribution lines.

2. Droplet collision between adjacent sprinklers will

affect distribution.

Existing irrigation systems need to be checked every few months 

because nozzles can become plugged or wear

down to the point that they are no longer operating properly.

Irrigation systems can be easily checked by running a "cup test;" 

which involves measuring the irrigation water caught in a series of 

cups laid out on a regular grid system throughout the growing area 

(fig. 4.2.27). Containers for cup tests should have a circular 

opening that has a narrow rim; the shape of the container below 

the opening is not important as long as the cup is stable and deep 

enough to hold several centimeters of water without any splashing 

losses. For any such container, the amount of water collected can 

be converted to precipita tion rate in inches per hour by the 

following formula (Furuta 1978):

101
102
103

P = (C x 7620) / (D2 x T)

Where: P = irrigation water applied per hour

(centimeters)

C = water "caught" in can (milliliters)

D = inside diameter of can opening 

(millimeters)

T = time of irrigation period (minutes).

A classic method for determining the variability in irrigation 

application was developed by Christensen (1942) by calculating a 

numerical index that he called the coeffi cient of uniformity (CU). 

The CU is calculated by running a cup test and using the data in 

the following formula (Furuta 1978):

CU = 100 [1.0 - (B / A)]

Where: CU = coefficient of uniformity (%)

B = sum of deviations of individual values from 

the mean value

A = sum of the individual values

A completely uniform irrigation pattern will produce a CU of 

100%, and the lower the CU, the more variable the irrigation. 

The standard target for most agricultural irrigation systems is a 

CU of 85% (Zimmerman 1966), which also is the minimum 

acceptable value that Shearer (1981) suggests for tree nursery 

crops.

A computer program for modeling sprinkler irrigation distribution 

patterns has recently been developed at the University of 

California at Davis, using water depth informa tion from actual cup tests to generate three-dimensional 

graphs (Fischer 1987). Using this informa tion, water distribution 

problems can be easily identified, such as "doughnuts" from low 

water pressure at a single nozzle (fig. 4.2.28A) or the "breadloaf" 

pattern (fig. 4.2.28B) that is characteristic of poor water 

distribution around the perimeter of the growing area.

4.2.5.4 Automatic irrigation systems

Several types of automatic controllers are available, some using 

time clocks and one using container weight, so that irrigation can 

be automatically applied (Hanan et al. 1978). This equipment 

allows the nursery manager to preprogram periods of irrigation 

and is a great time-saver in terms of labor. The prudent grower, 

however, will never become completely reliant on automatic 

systems and will continue to directly monitor irrigation efficiency 

and its effect on seedling growth on a regular basis.
104
4.2.6 Monitoring Water in Containers

Determining When to Irrigate

Determining the moisture status of the growing medium in most of 

the containers used in container tree seedling nurseries is a 

problem because it is difficult to observe or sample the medium in 

small containers. Some containers, such as the "book" type, can 

be opened up to allow direct observation of the moisture content of 

the medium, a definite advantage. In spite of the operational 

difficulties, it is absolutely necessary to regularly monitor the 

moisture status of container growing media because the limited 

volume of moisture reserves in small containers means that critical 

moisture stresses can develop quickly.

Hanan et al. (1978) and Furuta (1978) discuss several moisture 

monitoring techniques that can be used in ornamental container 

nurseries. White (1964) evaluated two standard methods, 

tensiometers and electrometric instruments (gypsum blocks), for 

container crops and concluded that they were not appropriate for 

three reasons: 1) lack of response above field capacity, 2) deterioration due to frequent fertilization, and 3) their relatively large size, 

which prohibited their use in small containers. Hanan et al. (1978) 

concluded that there is no inexpensive, yet accurate, instrument 

that can measure growing media moisture content in containers; 

any method must be supported by actual observation and the 

grower's experience.

Two surveys of irrigation monitoring have been conducted in recent 

years. McDonald (1978) surveyed container tree seedling 

nurseries in the western United States and found that 60% used a 

visual and tactile examination of the growing medium and seedling 

condition, 22% monitored container weight, and the remaining 18% 

used other methods, including the pressure chamber. The 

Container Nursery Survey found that most container tree seedling 

nurseries in the United States and Canada used some sort of 

irrigation monitoring system including: container weight (48%), 

visual and tactile methods (33%), commercial moisture meters 

(8%), and a combination of methods, including the pressure chamber (11 %); 13% of the nurseries reported no irrigation monitoring 

system.

4.2.6.1 Visual and tactile examination

This technique consists of direct observations of the growing 

medium and seedling condition to determine irrigation need. 

McDonald and Running (1979) describe a system to estimate the 

moisture content of bareroot nursery soils using the "feel or 

appearance" of the soil

but these guidelines are not applicable for artificial growing 

media. The best technique is to observe the relative ease with 

which water can be squeezed from the medium, and attempt to 

correlate this moisture condition with seedling appearance and 

growth; this process requires a lot of experience and is very 

subjective. In spite of its obvious limitations, the visual and tactile 

technique is still widely used and can be very effective when used 

by a knowledgeable, experienced nursery manager.

4.2.6.2 Container weight measurement

Based on the results of the Container Nursery Survey, measuring 

container weight is now the most popular moisture monitoring 

technique in container tree seedling nurseries. The basic principle 

behind this technique is simple: because water is relatively heavy 

in relation to the other container components, the moisture content 

of a tray or block of containers can be monitored by weight. The 

weight of the container decreases between irrigations as the water 

in the growing medium is lost through evaporation and 

transpiration, and the seedling crop is irrigated when the container 

weight reaches some predetermined level (fig. 4.2.29).

The weight of a tray of containers varies with many factors such as 

container type, type of growing medium, degree of medium 

compaction, crop species, and stage of seedling development, but 

one of the most significant is moisture content. Matthews (1983b) 

reported that a saturated Styroblock 2A®, containing growing 

medium compacted to an average density of 0.1 g/ml (ovendry 

weight), will weigh between 7.00 and 8.25 kg. By developing a 

series of container weights that correspond to available water

content and seedling condition, a container nursery manager can 

use these predetermined weights to determine when to irrigate 

and even to ma nipulate seedling growth and development.

The only piece of equipment needed to determine container 

weights is an accurate scale (fig. 4.2.30); some nurseries use 

more than one scale, leaving the containers on the scale in the 

growing area so that they can be read quickly. Containers should 

be weighed at approximately the same time after irrigation so that 

the results can be accurately compared. The container weight 

procedure must also adjust for seedling weight. As the seedlings 

grow larger, they will have an increasingly more significant 

influence on container weight (as much as 10 to 15% of the total 

container weight). New con105
tainer wet weights that take seedling weights into consideration 

should be calculated at regular intervals during the growing 

season.

Container irrigation weights will vary significantly between seedling 

species due the physiological response of different species to 

moisture stress. Matthews (1983b) developed a detailed container 

weight scale for British Columbia container tree seedling nurseries 

for three different "seedling groups" (table 4.2.12). He further 

distinguished between three different types of containers and three 

moisture stress "levels": low stress for the rapid growth phase, 

moderate stress for slowing growth, and drought stress for 

inducing dormancy during the hardening phase. Drought-stressing 

is effective on some species, such as Douglas-fir, Sitka spruce, 

western larch, and western redcedar (group 3), that are very 

sensitive to over-irrigation and will produce excessive top growth at

106

the expense of caliper and root growth. Other species, such as 

western hemlock, mountain hemlock, true firs, and 

white/Engelmann spruce (group 1), may be perma nently injured 

by indiscriminate drought stressing (table 4.2.12). Krizek (1985) 

concluded that the procedure of withholding water to induce plant 

moisture stress is very difficult to control operationally because of 

the difficulty of reaching and maintaining a specified level of 

growing medium moisture availability.

There are two ways to develop a scale for relating container 

weights to irrigation demand: 1) through experience and 2) using 

measurements of growing medium matric potential. Before a 

container weight scale can be developed, however, certain terms 

need to be defined:
wet weight = the weight of a filled and sown container after it has 

been completely saturated and allowed to drain freely. The 

medium will be at "container capacity;" which is conceptually 

the same as field capacity in a natural soil

irrigation weight = the container weight when irrigation is needed. 

This value will vary with seedling size and cultural objectives.

Developing a container weight scale through experience. The 

first step is to measure container weights at several times following 

irrigation and attempt to correlate these weights with the available 

moisture content of the growing medium, and seedling condition. 

The visual and tactile method of directly observing the amount of 

moisture in the growing medium can be used to estimate available 

moisture levels, and the wilting point can be established by 

observing seedling turgor during these measurement periods.

When enough information has been gathered on container 

weights, the data can be converted into a container weight scale, 

which shows the irrigation weight as a percentage of the wet 

weight. McDonald and Running (1979) suggest that the irrigation 

weight be set at around 75 to 80% of the wet weight. With enough 

experience, a comprehensive scale can be developed so that 

irrigation timing can be scheduled at various growth stages during 

the growing season and even into packing and storage (table 

4.2.13). Each nursery should develop its

own container weight scale, however, because there will be 

variation between types of growing medium, container 

characteristics, and individual seedling response.

Developing a container weight scale using growing medium 

matric potential. A more scientific, yet time consuming way to 

determine the irrigation weight of a container is based on the 

growing medium matric potential (GMMP), which is a measure of 

the energy required to absorb moisture from the growing medium. 

GMMP values can be obtained from soil moisture retention curves 

that depict the relationship between percent soil moisture by 

weight and GMMP (See section 4.2.2 for further discussion on 

water potential terminology and section 4.2.3.1 for more 

information on soil moisture retention curves.) Although this 

technique is unable to adjust for media compaction and the effect 

of the container, it is

107
the most empirical method of estimating GMMP in small 

containers at the present time. Future technology may produce 

sensors small enough to directly monitor GMMP within the 

container.

A typical soil moisture retention curve for a growing medium of 

50% peat moss and 50% vermiculite is shown in figure 4.2.8. 

Once the curve is established, the GMMP values having biological 

significance ("target values") can be read from the curve, along 

with the corresponding growing medium water contents; the exact 

water contents for specific GMMP values can also be determined 

by the testing laboratory during the development of the curve. The 

target GMMP values given in table 4.2.14 were developed from 

recommendations in the literature. Day (1980) gives an excellent 

discussion of irrigation monitoring in bareroot nurseries and 

recommends maintaining a GMMP of between - 0.010 to - 0.0.'5 

MPa to ensure a good balance of aeration and moisture. 

McDonald and Running (1979) recommend that a growing 

medium be irrigated between - 0.050 and - 0.080 MPa, compared 

to a recommendation of less than - 0.055 MPa for a medium 

coarse peat growing medium (Puustjarvi and others 1972, as 

reported in Hallett 1982b). Based on these recommendations, the 

target irrigation limits were set at - 0.010 MPa for the wet weight 

and - 0.050 MPa for the irrigation weight (table 4.2.14).

Once the target GMMP values and growing medium water 

contents have been set (table 4.2.14), the water weight losses of 

the container at each of the points can be calculated using the 

following relationship, which is also the basis for the soil moisture 

retention curve:

% Water content = ___weight of water______

weight of ovendry medium

Once the water weight losses have been calculated for each of the 

target values, a sensitive container weight scale can be developed 

that is specific for each container type (table 4.2.14). For the Super 

Cells, seedlings in the rapid growth phase should be maintained 

between - 0.01 and - 0.05 MPa of growing medium MP, which 

converts to a water weight loss of between 4.04 and 5.77 kg. To 

induce moisture stress (-0.10 MPa) during the hardening period, 

the container weights can be allowed to drop below the ideal 

medium moisture conditions, approximately 6.26 kg for the Super 

Cells (table 4.2.14). It is impossible to precisely

108

prescribe a growing medium MP for hardening, however, because 

seedling moisture stress is a function of evaporative demand and 

seedling physiology in addition to soil moisture levels (see section 

4.2.7.3).

Developing a container weight scale requires a significant amount 

of effort and record keeping, but container weight is one of the few 

objective, nondestructive, repeatable techniques for monitoring 

irrigation in container nurseries. Container weight is also the best 

way to determine irrigation needs early in the growing season 

before the seedlings are large enough to show moisture stress or 

use in the pressure chamber.

4.2.6.3 Monitoring plant water potential with a pressure 

chamber. Plant water potential (PWP) readings are the most 

accurate way to determine the water status of a tree seedling, and 

predawn measurements with a pressure chamber (fig. 4.2.5) can 

give an excellent indication of the need for irrigation (see Section 

4.2.2.3 for basic information on PWP). Day (1980) recommends 

that every container nursery obtain and use a pressure chamber; 

McDonald (1978) reported that pressure chambers were being 

used in 10 to 15% of forest nurseries in the Western United States.

The major restriction to this technique is that it is destructive, and 

seedlings must be large enough to fit easily in the pressure 

chamber, which means that it cannot be used early in the growing 

season. With larger seedlings, individual needles or needle 

fascicles can be used; Ritchie (1984) illustrates a pressure 

chamber modification that accommodates single conifer needles 

that would permit several measurements of PWP on the same 

seedling. McDonald and Running (1979) describe a method for 

relating pressure chamber readings to seedling water 

requirements and the difficulty of interpreting midday PWP values.

Day and Walsh (1980) developed a comprehensive manual for the 

use of pressure chambers in nursery and reforestation work, which 

includes procedures for handling sample material and operating a 

pressure chamber:

1. Seedling stems or other plant parts to be tested

should be cut cleanly, and should not be recut.

2. The bark around the seedling stem should be removed 

prior to testing; a standard length of 2 cm (0.8 inch) is 

recommended.
3. The plant part should not protrude through the pressure 

chamber lid more than 1 to 2 cm (0.4 to 0.8 inch).

4. PWP measurements should be made within 5 min-

utes of cutting the seedling stem.

5. The compressed gas should be applied at a constant

rate of 0.04 to 0.07% MPa (5 to 10 psi) per second.

6. Plant parts should be only measured once and then

discarded.

Despite the suitability of the pressure chamber technique for 

monitoring irrigation in container tree seedling nurseries, there are 

few published standards for nursery managers to use. A 

generalized chart of seedling response to a range of predawn 

PWP values is given in table 4.2.15. Predawn pressure chamber 

readings should always be used whenever possible because they 

represent the most stable indication of the moisture status of the 

seedling. Some scientists recommend midday readings, but these 

values are too variable to be useful on an operational basis (see 

figure 4.2.4 for typical diurnal PWP patterns). Although there are 

significant differences between species, a general rule would be to 

irrigate when predawn PWP values exceed - 0.5 MPa. Cleary et al. 

(1978) state that periods of slight moisture stress (around - 0.5 

MPa) promote sturdier and better- conditioned seedlings compared 

to over-irrigated stock. Seedling moisture stress should never be 

allowed to ex ceed - 1.0 MPa unless reduced growth or dormancy 

induction is desired.

109
4.2.7 Irrigation as a Cultural Treatment

Determining How Much to Irrigate

Once the decision has been made that irrigation is required, the 

next step is to determine how much water should be applied per 

irrigation event. The amount of water can be described in terms of 

water depth (centimeters) or water volume (liters) per unit area, 

and estimates of how much water to apply is given in section 

4.2.4.1. On an operational basis, however, irrigation events are 

normally controlled by clock timers for fixed irrigation systems or 

number of passes with the irrigation boom in mobile systems. Both 

of these need to be determined empirically for individual nursery 

conditions.

The most important concept in container irrigation is to apply 

enough water during each event to more than saturate the 

medium so that a small amount of leaching occurs. Due to the 

unique properties of artificial growing media in containers (see 

section 4.2.3.2), enough water must be applied to the surface to 

force the air out of the medium pores. Because irrigation in 

container tree seedling nurseries is only applied from overhead, 

this "front" of water moves downward through the growing medium 

as long as the irrigation continues; if the irrigation period is too 

short, the water will never reach the bottom of the container and 

result in a perched water table with a layer of dry growing medium 

underneath. Because of this, it is important not to partially irrigate 

a crop of container seedlings because only the top part of the 

medium will be wetted (Nelson 1978).

If the growing medium throughout the container is not completely 

saturated after every irrigation, the tree seedling will never develop 

roots in the dry medium at the bottom of the container, resulting in 

a poorly formed plug. Another hazard is that fertilizer salts will 

accumu late in the medium and cause salinity damage or "fertilizer 

burn." The general rule of thumb is to apply approximately 10% 

more water than is needed to completely saturate the entire 

growing medium profile at each irrigation. The best procedure is to 

actually check to make sure that drainage is occurring during or 

immediately after irrigation by direct inspection, or attaching a vial 

or plastic bag to the bottom of the container itself to collect the 

leachate. (More information on salinity and a procedure for 

determining proper leaching, see sections 4.1.3.4 and 4.1.9.2.)

The amount of irrigation to apply varies during the growing season 

in a container tree seedling nursery due to the stages of seedling 

development and the cultural objectives of the nursery manager. 

Because water is so

110

essential to plant growth, the irrigation regime can be 

manipulated to control seedling growth.

4.2.7.1 Irrigating during the establishment phase

Immediately after the sown containers are placed in the growing 

area, the growing medium should be completely saturated. Prior 

to seed germination, the major water loss is by evaporation from 

the top of the container (fig. 4.2.12A). Irrigation during this period, 

therefore, must be applied so as to replenish the moisture in this 

thin surface layer, which is best accomplished by frequent 

mistings or light irrigation. Too-infrequent irrigations will allow the 

seed to dry out and may decrease germination, whereas 

over-irrigation may cause excessively wet conditions around the 

seed and promote damping-off.

Irrigation can also be used to control the temperature around 

the germinating seed. Germinants, particularly those with 

dark-colored seed coverings, can be injured by high growing 

medium temperatures. Matthews (1983b) recommends 

irrigation if surface temperatures exceed 30 °C (86 °F).

4.2.7.2 Irrigating during the rapid growth phase

Once the seedling root system becomes established, the pattern 

of water use changes and transpiration gradually replaces 

evaporation as the major source of water loss (fig. 4.2.12B). When 

the seedling becomes large enough to completely shade the top 

of the container, surface evaporation declines and becomes 

insignificant compared to transpiration. Due to the "container 

effect" (see section 4.2.3.2), a small zone of saturated medium 

may exist at the bottom of the container, the depth of which will 

depend on the porosity of the growing medium (fig. 4.2.9) and 

container height (fig. 4.2.10). Because the available water will be 

removed from the top of the container first, the salinity will become 

higher in the small reservoir of growing medium solution at the 

bottom, which underscores the need for frequent leaching. In fact, 

one of the apparent signs of a salinity problem is a salt crust 

around the drainage hole of the container.

Most of the existing container manuals (Carlson 1983, Tinus and 

McDonald 1979) recommend maintaining the growing medium at 

"field capacity" to maximize growth rates. Some nursery scientists, 

however, believe that regular periods of slight moisture stress will 

result in sturdier seedling growth. Cleary and others (1978) recommend inducing periods of a "mild moisture stress" between 

waterings for coastal Douglas-fir seedlings, let-
ting the PWP reach up to 0.5 MPa before irrigation. This concept 

is reflected in the recommended PWP ranges in table 4.2.15. 

Matthews (1983b) advocates an irrigation program of alternative 

wet-dry periods to minimize the buildup of moss, algae, and 

liverworts. Nursery managers should be aware, however, that 

each seedling species reacts differently to any cultural practice, so 

operational tests should be conducted before moisture stressing is 

adopted as a standard practice.

One other factor that must be considered when scheduling 

irrigation is the effect of foliage interception. Although the foliage of 

a young seedling is limited in coverage, the leaves of larger 

seedlings, especially broadleaf species, can cause a significant 

reduction in the amount of irrigation that reaches the growing medium surface. The duration of the irrigation period must, therefore, 

be adjusted periodically during the growing season to account for 

interception losses.

4.2.7.3 Irrigating during the hardening phase

Manipulation of the irrigation regimen in container tree seedling 

nurseries has been found to be one of the most effective ways to 

initiate the hardening of seedlings prior to storage or shipment. 

Because seedling growth is so critically tied to moisture stress 

levels, a grower can reduce height growth, induce bud set, or 

initiate development of cold hardiness in many species of container 

seedlings by culturally inducing water stress (fig. 4.2.31). This 

"drought stressing" procedure consists of withholding irrigation for 

short period of time until the seedlings can be seen to wilt or some 

predetermined moisture stress is reached. After this stress 

treatment, the crop is returned to a maintenance irrigation 

schedule. Matthews (1983b) recommends a drought stress 

treatment to induce bud set after adequate height growth is 

attained (table 4.2.12); this drought stress period may last up to 14 

days depending on species. Timmis and Tanaka (1976) found that 

moisture-stressed seedlings were smaller in diameter and had 

lower root and shoot dry weights compared to unstressed 

seedlings but had significantly more terminal buds, which were 

also formed earlier in the season. They also concluded that mild 

moisture stress levels increased the ability of the seedling to 

cold-harden.

In a recent review of the effects of water stress on seedling 

quality, Joly (1985) listed two physiological and morphological 

effects of water deficit on bareroot seedlings that may be useful 

to container nursery managers:

1. Increasing moisture stress can be used to induce seedling 

dormancy during early to mid-summer (Zaerr et al. 1981).

2. Mild PWP values (-0.5 to - 1.0 MPa) during midsummer will 

initiate the sequence of events leading to cold hardiness 

(Blake et al. 1979).

Two more effects can be added:

3. Moderate moisture stress levels (-1.0 to -1.5 MPa) can be 

used to retard unwanted late-season shoot growth (lammas or 

proleptic growth), although this treatment reduced cold 

hardiness (Blake et al. 1979).

4. Christersson (1976) demonstrated that both pine and spruce 

container seedlings could be drought-hardened by imposing a 

period of moisture stress and that these hardened seedlings 

could tolerate a more severe drought stress (-3.5 MPa) than 

unhardened seedlings (-2.5 MPa).

Moisture stress as a cultural treatment can be affected by other 

environmental conditions. Blake et al. (1979) pointed out that the

effects of moisture stress treatments on seedling hardening are 

affected by photoperiod. Mild moisture stress was only effective 

under the long days of summer or the equivalent extended 

photoperiod in a greenhouse environment.

111
of cold hardiness and should always be insulated if seedlings 

are to be stored under exposed conditions. (More information 

on hardening procedures can be found in the hardening 

chapter in volume six of this series.)

An excellent discussion of using sprinkler irrigation for frost 

protection in bareroot tree seedling nurseries is provided by 

McDonald (1984), and the same basic principles apply in container 

nurseries. Sprinkler irrigation protects against cold injury because 

heat is released when water freezes on the seedling foliage, and 

the ice layer provides some degree of insulation value. The main 

protection comes from the heat released from the freezing water, 

however, and so this protective effect lasts only as long as 

irrigation continues to be applied. Irrigation should begin as soon 

as the temperature drops below freezing and continue until the ice 

is melted. Some nurseries test their seedlings for frost hardiness 

and base their determination of when frost protection should begin 

on these tests. Frost protection with sprinkler irrigation cannot 

protect against severe "hard" freezes, but agricultural crops have 

been saved in temperatures as low as -8 °C (17 °F). The amount 

of water to apply for frost protection varies with temperature and 

wind velocity, however. Some suggested irrigation application 

rates are provided in table 4.2.16 (Hansen and others 1979).

It is also important to realize that irrigation cultural treatments can 

have detrimental results. Continued frequent irrigation into late 

summer can delay the normal development of frost hardiness 

(Lavender and Cleary 1974). Applying a moisture stress treatment 

under a short photo period may actually inhibit the development of 

frost hardiness (van den Driessche 1969).

One problem with operationally implementing moisture stress as a 

cultural practice is that there can be considerable variation in 

growing medium moisture between adjacent containers. Due to 

differences in irrigation application and seedling water use, it is 

hard to achieve a uniform average level of seedling water potential 

in a greenhouse. Another operational problem is that, if the 

growing medium is allowed to dry too far, it can become 

hydrophobic and difficult to rewet even if wetting agents are used.

Most of the research on these techniques has been done with 

coastal Douglas-fir or other coastal species, so nursery managers 

should interpret these findings accordingly. Matthews (1986) 

recently reported that drought stressing does not work well with 

interior species of spruce in British Columbia nurseries, and 

concluded that this cultural practice should still be considered 

"more of an art than a science." Growers should conduct their own 

trials of operational moisture stressing to determine the effect on 

their own species in their respective growing environments. In 

spite of these caveats, induction of mild moisture stresses such as 

those in ta ble 4.2.15 should be considered as a cultural technique 

to manipulate seedling physiology and morphology. A further 

discussion of the hardening process, including moisture stress, is 

provided in the chapter on hardening in volume six of this series.

4.2.7.4 Irrigating for frost protection

Container seedlings that are raised in outdoor growing areas or 

stored in sheltered storage may require protection against freezing 

temperatures in the fall or spring in climates with cold overwinter 

temperatures. Proper hardening procedures will help protect the 

shoot against frost injury, but unusually cold weather can 

sometimes occur suddenly before the seedlings have had time to 

harden sufficiently. Roots do not achieve a high degree

112
Two fertilizer elements, nitrogen and phosphorus, have been 

identified as particularly harmful to the environment or human 

health. Nitrate-nitrogen (N03-) is particularly harmful: water 

containing more than 10 ppm N03- can cause a disease of infants 

called methemo globinemia, which involves an inability to use 

oxygen (Rosen and others 1986). Wastewater from container tree 

seedling nurseries will undoubtedly contain significant amounts 

from excess nitrogen fertilizer, as the anion N03- is not adsorbed 

by the growing medium and leaches from the containers with the 

irrigation water. Much of the applied liquid fertilization solution is 

shed by the foliage of large container seedlings or falls on noncrop 

area and goes directly into the wastewater. Urbano (1987) reports 

that one-third to half of the nitrates applied as fertilizers are ending 

up in groundwater supplies. Phosphorus leached into lakes can 

cause a buildup of algae or water weeds, a process called eutrophication. Whether the amount of fertilizer P leaching from 

container tree seedling nurseries is a serious problem is unknown, 

because P becomes fixed and immobilized in the soil. A study of 

storm runoff into lakes in Minneapolis, MN, found that the P 

content did not increase with P fertilization (Rosen and others 

1986).

Urbano (1987) reports that the amount of herbicides and 

pesticides reaching the groundwater is much smaller (0.1 %) than 

the amount of fertilizer nutrients, but the toxicity of many of these 

chemicals is unknown. The problem is not hypothetical because, 

in 1979, irrigation well contamination with the herbicide Temik© 

(aldicarb) was found to be extensive in Long Island, NY, and 

groundwater contamination with this pesticide has been 

suspected in other states (Urbano 1987).

The implications of excess fertilizer and pesticides in irrigation 

wastewater are obvious, but no acceptable standards have been 

established for many pesticides. Some quality standards for 

discharge waters have been established in southern California 

(table 4.2.17). Florida has one of the most stringent groundwater 

monitoring programs in the United States, testing for 129 different 

chemicals. The most recent state action that will affect nursery 

managers is California's Proposition 65 (the "toxics initiative"), 

which will require regulation of more than 200 potentially harmful 

chemicals (Urbano 1987).

Accepting the fact that considerable irrigation waste ex ists, 

container nursery managers are beginning to consider ways of 

dealing with the problem. The focus of any

113

4.2.8 Irrigation Wastewater Disposal

4.2.8.1 Sprinkler irrigation efficiency

The sprinkler irrigation that is typically used in most container tree 

seedling nurseries is very inefficient. Handreck and Black (1984) 

estimated that less than a third of the sprinkler irrigation water 

applied to containers actually reaches the growing medium. No 

published data exist for forest nurseries, but Weatherspoon and 

Harrell (1980) compared overhead sprinkler and drip irrigation in 

an ornamental nursery: impulse sprinklers had an irrigation 

efficiency (that is, the percentage of applied water retained by the 

growing medium) of 26%, and spinner sprinklers had an even less 

efficient rate of 13%. Boom irrigation may be more efficient than 

fixed irrigation systems because the water is applied only to the 

seedlings, rather than to the entire growing area. Irrigation 

efficiency will gradually decrease during the growing season as 

the seedling crowns intercept and shed an increasingly greater 

proportion of the applied irrigation water. Many ornamental 

container nurseries have converted to drip or subirrigation to 

increase their irrigation efficiency, but the small containers used in 

forest nurseries and the need to air prune roots make these 

irrigation systems impractical.

4.2.8.2 Managing nursery wastewater

The problem of poor irrigation efficiency involves more than simply 

wasted water, however, because many container nurseries apply 

some or all of their fertilizer and pesticides through the irrigation 

system. Liquid fertilizer is usually applied in excess of the actual 

amount needed to saturate the growing medium to stimulate 

leaching of excess salts. Most pesticides are applied in a water 

carrier through the irrigation system, and some of these chemicals 

inevitably end up in the wastewater runoff; growing medium 

drenches are particularly serious in this regard.

Recent testing has revealed that excess fertilizer nutrients and 

pesticides are leaching downward and contaminating groundwater 

supplies. Originally it was thought that the soil filtered out 

impurities and therefore underground water supplies were pure, 

but this theory has recently been refuted. A Government 

Accounting Organization (GAO) survey found that 29% of the 

65,000 community water systems in the United States are unable 

to meet federal minimum drinking water standards. The 

Environmental Protection Agency (EPA) is currently conducting a 

national survey of pesticides in drinking water that should provide 

some good data on the scope of the problem (Urbano 1987).
control program should be on prevention, because there is no way 

to remove the chemicals once they have contaminated the 

groundwater (Urbano 1987). Skimina (1986) studied three 

alternatives for handling irrigation wastewater in a California 

container nursery: discharging into sewers, denitrification, and 

recycling. He found that the recycling was the only practical option 

and designed a water treatment facility (fig. 4.2.32) that produces 

water "pure enough to drink." The recycled water was tested on 

106 ornamental plants and most of the test plants had better 

growth than the plants grown on the original irrigation water; it is 

not, however, used operationally on cuttings and some sensitive 

species. The treatment plant cost $1.3 million to build and 

operating costs were $0.08 per kl ($0.30 per gallon) (Skimina 

1986). The treatment program has also produced several side 

benefits, such as fertilizer reclamation, but one of the most 

significant benefits has been the good community relations that 

have resulted from the recycling project (Urbano 1987).

Most container tree seedling nurseries are not located in heavily 

populated areas and wastewater disposal may not be a serious 

concern at the present time. Undoubtedly, however, the problem of 

irrigation wastewater disposal will become more significant in the 

future, and nursery managers should be prepared to deal with the 

wastewater problem.

114
4.2.9 Conclusions and Recommendations

Because of the overriding importance of water to plant growth, 

water management through irrigation is one of the most critical 

cultural operations in container tree seedling nurseries. An 

understanding of basic seedling-water relations is helpful in 

container nursery management; the water status of a tree seedling 

can best be described in terms of water potential. Water potential is 

the most practical way to describe seedling water status because 

the principles and units remain the same from the growing medium, 

through the seedling, and into the atmosphere. The water potential 

in the growing medium, or the seedling itself, can be separated into 

components that the grower can manage to achieve the proper 

quantity and quality of seedling growth.

Water must be managed differently in container tree seedling 

nurseries, than in a bareroot operation. 'Arti ficial soil;' composed of 

materials such as peat moss and vermiculite, has different 

physicochemical properties than native soil, including a higher 

water-holding capacity. The container also has an effect on the 

water properties of the growing medium because water does not 

drain completely out of the container, which results in a layer of 

saturated media at the bottom. The depth of this layer is a function 

of container height and the properties of the growing medium.

The quantity and quality of the irrigation water is probably the most 

important consideration in the selection and operation of a container 

tree seedling nursery. Suffi cient quantities of water must be 

available throughout the year to supply all the various uses at the 

nursery. The quality of the nursery irrigation water is primarily a 

function of the concentration and composition of dissolved salts, 

although the presence of pathogenic fungi, weed seeds, algae, and 

pesticides must also be considered. Because water treatment is 

impractical and costly in most instances, irrigation water sources 

should be thoroughly tested during nursery site selection. Tree 

seedlings are very sensitive to soluble salts, and so water should be 

tested at all stages of the irrigation process at regular intervals 

during the growing season.

Container tree seedlings are typically irrigated with some type of 

overhead sprinkler irrigation system, from either fixed sprinklers or 

sprinklers on a moving boom. Mobile irrigation booms provide more 

uniform coverage but are subject to mechanical failure. Several 

different types of stationary sprinklers are available and will per115

form satisfactorily if properly designed and maintained. Any 

irrigation system must be tested periodically to insure that it is 

working properly.

Determining both when and how much to irrigate is one of the 

most important day-to-day decisions of the nursery manager. 

Because of the physical limitations of the small containers used in 

container tree seedling nurseries, there is currently no way to 

directly monitor the water potential of the growing medium within 

the container. Experienced growers develop an intuitive skill for 

determining when irrigation is required, using the appearance and 

feel of the growing medium and the relative weight of the 

container. When the seedlings become larger, a pressure 

chamber can be used to directly measure plant water potential. 

Due to the restrictive drainage characteristics of containers, 

growers must apply enough water during each irrigation event to 

completely saturate the entire volume of growing medium and 

flush excess salts out the bottom of the container. The amount of 

water supplied at each irrigation is a function of the growth stage 

of the seedlings and the environmental conditions. In addition to 

promoting rapid germination and seedling growth, water can be 

used as a cultural tool to help harden the seedlings and induce 

dormancy. In cold climates, irrigation can also be used for frost 

protection of seedlings in open growing compounds.

Because of the excess amounts of irrigation required and the poor 

efficiency of most sprinkler systems, disposal of wastewater is a 

important consideration in container nursery management. 

Injected fertilizer nutrients, such as nitrate-nitrogen and 

phosphorus, and pesticides applied through the irrigation system 

may affect groundwater quality and could become a problem in 

nurseries located in urban areas.
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Index of Common and Scientific 

Names

Trees

ash

green ash Fraxinus pennsylvanica 86

white ash F. americana L. 10

birch

paper birch Betula papyrifera Marsh. 10, 12

casuarina

river-oak casuarina (beefwood) Casuarina

cunninghamiana Miq. 86

"cedar"/juniper

Alaska-cedar Chamaecyparis nootkatensis

(D. Don) 107

eastern redcedar Juniperus virginiana L. 86

western redcedar Thuja plicata Donn. 106, 107 

citrus Citrus spp. 94

Douglas-fir Pseudotsuga menziesii (Mirb.) Franco

23, 24, 29, 61, 85, 106, 107, 110, 112

fir

balsam fir Abies balsamea (L.) Mill. 61

grand fir A. grandis (Dougl. ex D. Don) Lindl. 107 

Pacific silver fir A. amabilis Dougl. ex Forties 107

hemlock

mountain hemlock Tsuga mertensiana (Bong)

Carr 106, 107

 western hemlock T. heterophylla (Raf.) Sarg. 106, 107

larch

western larch Larix occidentalis Nutt. 24, 106, 107

magnolia

southern magnolia Magnolia grandiflora L. 86

maple

sugar maple Acer saccharum L. 10 

red maple A. rubrum L. 10, 60

pine

Aleppo pine Pinus halepensis Miller 86

eastern white pine P. strobus L. 94

jack pine P. banksiana Lamb. 13, 14, 22, 25, 38, 58, 62

Japanese black pine P. thunbergiana Franco 29 

Monterey pine P, radiata D. Don 16

loblolly pine P. taeda L. 50, 61, 94

lodgepole pine P. contorta Dougl. ex Loud. 61, 107

ponderosa pine P. ponderosa Doug]. ex Laws. 61, 107

Scotch pine P. sylvestris L. 22, 60 

shortleaf pine P. echinata Mill. 61

poplar, cottonwood Populus

quaking aspen Populus tremuloides Michx. 24

Russian olive Elaeagnus angustifolia L. 86

Saltbush Atriplex spp. 86

Spruce

black spruce Picea mariana (Mill.) B.S.P 10, 11, 58, 62

blue spruce P. pungens Engelm. 85

Engelmann spruce P. engelmannii Parry ex Engelm. 94, 106

Norway spruce P. abies (L.) Karst. 22

red spruce P. rubens Sarg. 9

Sitka spruce P. sitchensis (Bong.) Carr. 106, 107

white spruce P. glauca (Moench) Voss 9, 10, 13, 51, 

58, 61, 106

Fungi and Mycorrhizae

Botrytis cinerea Pers.:Fr. 50

Phytophthora cinnamomi Rands. 88, 92

Pythium spp. 88

Other Plants

algae 88, 91, 92, 93, 111, 113

liverworts 88, 91, 92, 111

moss 88, 91, 111

Animals

deer 61

elk 61

119