47
Contents

3.2.1 Introduction 49

3.2.1.1 Biophysics of water vapor 49

3.2.1.2 Definitions and units 50

Water vapor pressure 50

Water vapor pressure deficit 50

Absolute humidity 50

Relative humidity 51

Dew point temperature 52

3.2.2 Role of Humidity in Tree Seedling

Growth and Development 54

3.2.2.1 Seedling growth 54

3.2.2.2 Vegetative propagation 55

3.2.2.3 Managing pathogens 55

3.2.3 Optimum Humidity Levels 58

3.2.3.1 Seedlings 58

Establishment phase 58

Rapid growth phase 58

Hardening phase 60

3.2.3.2 Vegetative propagation 60

3.2.4 Modifying Humidity in Container

Tree Nurseries 61

3.2.4.1 Growing structures 61

3.2.4.2 Humidification 61

Steam heat 62

Fog and mist 62

Irrigation 63

Evaporative cooling 64

3.2.4.3 Dehumidification 64

Ventilation and heating 64

Dehumidifying the seedling canopy 64

3.2.5 Humidity Monitoring and Control

Systems 66

3.2.5.1 Humidity 66

3.2.5.2 Fog and mist 68

3.2.6 Conclusions and Recommendations 70

3.2.7 Literature Cited 71

48
3.2.1 Introduction

Maintaining the proper atmospheric humidity in container tree 

nurseries is important biologically for several reasons: low humidity 

subjects seedlings to water stress caused by excessive 

transpiration, proper humidity promotes rapid growth, and 

excessive humidity promotes the growth of fungal pathogens and 

other nursery pests such as moss and liverworts. The challenge to 

the nursery manager is to maintain humidities that are high enough 

for good seedling growth without encouraging pests (fig. 3.2.1).

This chapter introduces the basic concepts of humidity, discusses 

the ways in which humidity affects seedling growth, and presents 

optimum levels for container tree nurseries. Cultural methods for 

modifying humidity and equipment for monitoring humidity levels 

are also included.

3.2.1.1 Biophysics of water vapor

In the container nursery environment, water exists in two of its 

three physical states-invisible water vapor (gas) and liquid. Water 

vapor is subject to the same physical laws as th e other gases that 

compose the air, such as nitrogen and oxygen. Moist air can be 

defined as a two-component mixture of dry air and water vapor. 

The air and the water vapor simultaneously occupy the same 

space, but the water vapor acts independently of the other gases. 

Therefore, the partial pressure of water vapor is solely a function 

of temperature and is unrelated to the total atmospheric pressure 

(Gaffney 1978). Air always contains some water vapor, but at any 

given temperature, it can hold only a finite amount. When this 

physical limit is reached, the air is saturated, and when it is 

exceeded, condensation occurs.

Water has several unique physical properties that affect the 

container nursery environment, including the highest known latent 

heat of evaporation. An extremely large amount of thermal energy 

(540 cal/g) is required to take liquid water through the phase 

change from liquid to gas. This is considerably more than the 

energy required (316 cal/g) to bring 1 g of ice at absolute zero, -273 °C (-460 °F), to the boiling point (Hewitt 1974). The same 

amount of thermal energy that is used when water evaporates is 

released when wa-

49
ter vapor condenses: evaporation is an endothermic process 

whereas condensation is an exothermic process. This high latent 

heat of evaporation is operationally significant because it not only 

affects the heating and cooling of the container nursery 

environment, but also cools plants through transpiration.

3.2.1.2 Definitions and units

Water vapor pressure. The water vapor in a given volume of air 

exerts a partial pressure (e) that depends on the amount of the 

water vapor and its temperature. It is useful to think of humidity in 

terms of a pressure or force that is capable of causing the 

movement of water vapor to and from objects in contact with the air 

(Gaffney 1978). The water vapor pressure in the surrounding air, 

called the ambient water vapor pressure (ea), varies from near 

zero in cold dry air to over about 8 kPa (0.08 atmospheres) in warm 

moist air (table 3.2.1). If the ambient vapor pressure is less than 

the equilibrium vapor pressure of liquid water, evaporation 

occurs. When the atmosphere becomes saturated with water 

vapor, the saturation water vapor pressure (es) becomes 

identical with the vapor pressure of water and net evaporation 

ceases.

The higher the temperature, the higher the equilibrium water vapor 

pressure and, in the range of common nursery temperatures, 

saturation water vapor pressure approximately doubles for each 11 

°C (20 °F) increase in temperature (table 3.2.1).

Water vapor pressure deficit. Another important humidity concept 

is vapor pressure deficit (VPD), which is equal to the difference 

between the saturation water vapor pressure and the ambient 

water vapor pressure at the same temperature:

VPD = es - ea

The VPD is important in horticulture because it represents the 

evapotranspirational demand of the surrounding atmosphere, as 

well as the proximity to the dew point (when ea = es). Therefore, 

growers can use the VPD to determine whether to irrigate 

because seedling transpiration will be high or ventilate to avoid 

condensation.

50

Water vapor pressure and vapor pressure deficit are expressed in 

standard pressure units (table 3.2.2). The metric units are pascals 

(Pa), kilopascals (kPa), and megapascals (MPa), whereas the 

English units are atmospheres (atm). Older units include bars (b), 

inches or millimeters of mercury (in. Hg or mm Hg), and pounds 

per square inch (Ib/in2).

Absolute humidity. The actual amount of water vapor in a given 

volume of air is the absolute humidity; it is expressed as weight per 

volume. A direct relationship exists between the dew point, the 

saturation water vapor pressure, and the absolute humidity 

because each depends only on the actual amount of water in the 

air at a given atmospheric pressure (table 3.2.1). Absolute humidity 

is rarely measured in container tree nurseries, however, and 

relative humidity is used instead.
Relative humidity. The most common descriptor of humidity, 

relative humidity (RH), is also the most practical measurement in 

container tree nurseries. RH can be defined as the ratio of the 

amount of moisture in a volume of air to the total amount of 

moisture that can be held at saturation at a given temperature and 

atmospheric pressure, and is expressed as a percentage. To 

compute RH,

the ambient water vapor pressure is divided by the saturation 

water vapor pressure:

RH (%) = ea X 100

Es

Because both RH and VPD are related to temperature, these 

humidity indexes can be obtained from handy reference charts 

when two of the three values are known (table 3.2.3).

51
Growers routinely measure RH with psychrometers and 

hygrothermographs and use this information to control humidity in 

their growing structures. A psychrometer contains two identical 

thermometers—one that measures the ambient temperature (drybulb temperature) and another that measures the lowered 

temperature caused by evaporative cooling (wet-bulb 

temperature). The difference between the two temperatures is the 

wet-bulb depression. The wet and dry-bulb temperatures are 

used to compute the relative humidity and dew point from 

psychrometric charts (fig. 3.2.2) or tables (table 3.2.4). The 

wet-bulb depression is a handy measurement that can be used for 

several purposes in a greenhouse operation, such as showing the 

theoretical efficiency of evaporative cooling pads (Hanan and other 

1978). (More information on measuring humidity can be found in 

section 3.2.5.)

Dew point temperature. Saturation occurs when the air is cooled 

to the point that the saturation vapor pressure exceeds the ambient 

vapor pressure and condensation occurs-the temperature at which 

this occurs is called the dew point. The practical significance of 

the dew point is that it is directly related to the amount of water 

vapor in a volume of moist air (Gaffney 1978). The dew point 

temperature has direct application to container nursery 

management because of its relationship to condensation, which 

occurs when humid air comes in contact with a cooler surface such 

as the inside of the greenhouse covering or a leaf. Growers 

manage humidity in their nurseries to minimize condensation, 

which can cause disease problems.

52
53
Plants absorb water through their roots from the growing medium 

and lose it through their leaves into the air through a process 

called transpiration, which is essentially bioregulated evaporation. 

Although excessive transpirational losses can result in damaging 

moisture stress, a small amount of transpiration is necessary to 

move mineral nutrients in the xylem sap from the roots to the 

leaves (Kramer and Kozlowski 1979). Some transpiration usually 

occurs as long as water is available to the roots. In intense light, 

leaves will absorb enough radiant energy to cause a transpirational 

gradient from the leaf to the air, even at high humidities.

3.2.2 Role of Humidity in Tree Seedling

Growth and Development

Atmospheric humidity can affect container tree seedlings directly 

through its effects on seedling water relations. Controlling humidity 

is even more critical when plants are being propagated 

vegetatively by cuttings or grafting. There is also an indirect effect 

of humidity: many nursery pests thrive in the high-humidity 

environment that often exists in greenhouses.

3.2.2.1 Seedling growth

Humidity principally affects evapotranspiration rates. Under still 

conditions, the rate of evaporation from a wet surface is a function 

of the relative humidity and temperature and is proportional to the 

vapor pressure deficit. At a constant temperature, the higher the 

relative humidity, the lower the vapor pressure deficit (table 3.2.3). 

Under operational conditions, increasing temperature is more of a 

controlling factor than humidity in determining evapotranspirational 

demand. For example, when the RH of the air decreases 30% 

(from 80 to 50%) and the temperature stays at 30 °C (86 °F), the 

VPD increases 2.5 times; however, if the absolute humidity 

remains constant and the leaf temperature increases just 10 °C, 

from 10 to 20 °C (50 to 68 °F), then the VPD increases over 5 

times (Kramer 1983).

Under calm conditions, water vapor collects near an evaporating 

surface, forming a boundary layer. If the humidity in the boundary 

layer approaches saturation, the evaporation rate will almost 

cease, even though the surrounding air is much drier. Wind 

removes the boundary layer and replaces it with drier air, thus 

increasing the evaporation rate (Schroeder and Buck 1970). For 

example, if the air in the boundary layer was 20 °C (68 °F) with 

90% RH, and it was replaced by air at the same temperature and 

60% RH, the VPD would increase over fourfold, from 0.23 to 0.93 

kPa (table 3.2.3).

Container tree seedlings develop significant boundary layers (fig. 

3.2.3) that can significantly lower the evapotranspiration rate in 

the dense seedling canopy in the typical aggregated containers of 

a forest tree nursery. Boundary layers are particularly significant 

in the sheltered environment of an enclosed growing structure 

where air movement is restricted.

54
Maintenance of the proper humidity is of particular concern in 

vegetative propagation. The transpiration rate of cuttings must be 

kept low for several weeks or even months so that they can 

maintain enough turgor to produce new roots. Grafted seedlings 

are often kept under greenhouse conditions because the high 

humidities reduce the moisture stress on grafted scions (Hartmann 

and Kester 1983). Special rooting environments are constructed to 

maintain these higher humidities (fig. 3.2.5).

3.2.2.3 Managing pathogens

It would seem that very high humidities would be desirable in 

container tree nurseries, but this is not always the case. Nursery 

pests such as pathogenic fungi, moss, and liverworts are 

stimulated by high humidities, especially if there is free water 

present. Cryptogams (moss, algae, and liverworts) thrive in the 

container nursery environment and can completely cover the top of 

the container and interfere with seedling growth. In extreme cases, 

these pests can form a thick plug that completely prevents the 

infiltration of water and liquid fertilizers (fig. 3.2.6A). Even some 

insect pests can be related to high humidity environments. 

Dark-winged fungus gnats can build up damaging populations in 

greenhouses that have excessive amounts of moss and algae.

55

Most of the transpirational water loss occurs through the stomata 

on the leaves, which must also remain open long enough to absorb 

sufficient carbon dioxide (C02) for photosynthesis (fig. 3.1.2). 

Although stomata occupy only about 1 % of the leaf surface area, 

transpiration rates can approach 50% of the evaporation rate from 

a free water surface (Kramer and Kozlowski 1979). Stomata 

remain open as long as their turgor pressure remains high and light 

levels remain adequate. However, if the rate of water loss exceeds 

the rate of water uptake, internal moisture stress builds up. If the 

stress becomes severe, metabolic processes such as 

photosynthesis are adversely affected. When the moisture stress 

reaches a critical level, the stomata begin to close and net 

photosynthesis decreases. With further increase in moisture stress, 

stomata close further and eventually photosynthesis stops. This 

series of events may occur regularly on sunny days in many 

species, even when the seedlings are well watered, because the 

roots cannot absorb water as fast as the leaves can lose it. For 

maximum growth, the stomata must remain open as long as 

possible, and so growers can promote growth by maintaining 

relatively high humidities in the growing area (fig. 3.2.4). (A more 

detailed discussion of seedling water relations can be found in 

volume four of this series.)

A moderate transpiration rate is also beneficial because it cools 

the leaf and keeps it near the optimum temperature for 

photosynthesis and other metabolic processes (Clawson and 

others 1989). Other seedling growth processes, such as cell 

enlargement, also depend on positive turgor pressure.

3.2.2.2 Vegetative propagation

Although most forest tree seedlings are currently produced from 

seed, many nurseries also practice some form of vegetative 

propagation. Tree improvement stock, in particular, is often 

propagated vegetatively so that the desired genotypes can be 

maintained. Seed orchard stock is propagated by cuttings or 

grafting, and genetic test stock can often be produced easier and 

cheaper with cuttings. Many species that are used for forestry or 

conservation purposes, such as poplars and willows, are 

propagated vegetatively.
56
57

Although most fungi thrive under high humidity, certain plant 

pathogens particularly favor this environment; the fungal pathogen 

that causes grey mold is a notable example. The spores of Botrytis 

cinerea require free moisture to germinate and penetrate seedling 

foliage (fig. 3.2.6B), and high humidities are conducive to the 

subsequent spread of the fungus. In fact, only 3 hours at 

temperatures of 15 to 20 °C (59 to 68 °F) and 98% RH promote 

infection if there is free moisture present. Peterson and others 

(1988) consider RH values over 90% to be ideal for the germination 

of B. cinerea spores and found that RH within the canopy of 

Douglas-fir seedlings typically exceeded this threshold at night. 

Grey mold becomes serious in the fall when cooler temperatures 

cause moisture to condense on seedling foliage, especially in overly 

dense seedling canopies. The percentage of time when the RH 

exceeded 90% in a fiberglass greenhouse increased from 59% in 

August to 85% in October. (Nursery pests are discussed in detail in 

volume five of this series.)
3.2.3 Optimum Humidity Levels

It is extremely difficult to set ideal humidity levels for container tree 

nurseries because relative humidity varies so much with 

temperature. Optimum humidity levels will change during the 

growing season for seedlings and will differ for seedlings and 

cuttings.

3.2.3.1 Seedlings

There have been few controlled experiments to determine optimum 

humidity levels for plant growth. In one growth-chamber experiment 

with temperatures of 18 to 24 °C (65 to 75 °F), Krizek and others 

(1971) showed that 40% RH severely reduced the growth of 

seedlings of three species of garden flowers (ageratum, petunia, 

and marigold). Raising the RH to 65% resulted in striking increases 

in fresh weight, dry weight, leaf area, and height; increasing the RH 

to 90% produced no further benefits. Similar responses have been 

reported for loblolly pine (Seiler and Johnson 1984) and cucumber 

plants (van de Sanden 1985). Besides these few examples, 

research on the effects of humidity on plant growth is not 

extensive.

Most of our current knowledge has been obtained through 

experience and observation in operational container nurseries. In 

response to a recent survey, container growers reported that their 

target RH values decreased during the growing season, ranging 

from 60 to 80% in the establishment phase to 45 to 50% during the 

hardening phase (table 3.2.5). Several nurseries stated that they 

did not really have targets for humidity because it is so difficult to 

control, especially in growing structures that are not fully enclosed, 

such as shadehouses. Most agreed, however, that high humidities 

were definitely important during seed germination and emergence.

Establishment phase. Managing the humidity is most critical 

during the germination period. Seeds are sown on top of the 

growing medium under a thin covering that must be kept moist so 

that the seed never dries out. Many nurseries use special mist 

nozzles during this period to keep the growing medium "moist but 

not wet" (fig. 3.2.7). Maintaining high relative humidities of 60 to 

90% (table 3.2.6) eliminates the need for frequent irrigation, which 

would keep the growing medium too wet and promote damping-off.

58

Because relative humidity varies so much with temperature, it is 

more effective to manage for vapor pressure deficit. VPD values 

corresponding to the RH targets (table 3.2.6) should only be 

considered approximate guidelines, considering the difficulty in 

precise adjustment of humidity. VPD can be easily calculated by 

using the RH and temperature measurements from a 

psychrometer or hygrothermograph (see section 3.2.5 for more 

information).

Rapid growth phase. As soon as the seedlings have established 

root systems, the relative humidity should be reduced to 50 to 80% 

(table 3.2.6). This will keep evapotranspiration low, but the surface 

of the growing medium and the seedling foliage will remain dry. A 

VPD of approximately 1.00 kPa is a reasonable target for this 

phase; the colored line in table 3.2.3 delineates the combinations 

of temperature and RH that will keep the evapotranspirational 

demand below that target.
59
Hardening phase. The cultural objectives of this phase are 

slowing height growth, promoting bud set, and hardening the 

seedlings to environmental stresses. Lowering humidity to ambient 

levels during this period (table 3.2.6) causes seedlings to undergo 

a mild moisture stress. This can be difficult in completely enclosed 

greenhouses, however, because the cooler temperatures during 

late summer and fall promote high humidities and often 

condensation, especially at night. For this reason, many nurseries 

move their seedlings from the greenhouse at the beginning of the 

hardening phase, and others remove the covering, unless outside 

conditions are too stressful (table 3.2.5). Shelterhouses are 

particularly beneficial during this period because their sides can be 

raised to promote good cross ventilation (fig. 3.1.12B).

3.2.3.2 Vegetative propagation

Significantly higher humidities are required for all types of 

vegetative propagation than for seedling culture. With all types of 

cuttings, the normal water supply has been completely severed 

and water stress can quickly become severe. The problem is 

critical with softwood cuttings, which have leaves that are still 

transpiring, and hardwood cuttings, which root slowly. Because 

the production of new roots requires positive turgor pressure, 

plant moisture stress must be minimized by keeping the ambient 

vapor pressure at nearly the same level as that in the plant 

(Hartmann and Kester 1983). Maintaining relative humidity values 

as close to 100% as possible is desirable (fig. 3.2AA); once 

cuttings have rooted, they are gradually hardened to ambient 

conditions by allowing humidities to decrease. Newly grafted 

plants also benefit from highly humid environments until the grafts 

have taken and normal internal water relations have resumed.

When temperatures in the growing area become excessive, many 

nurseries apply a fine mist, often in combination with shade, to cool 

the seedlings (fig. 3.2.8). Some of the mist evaporates before 

reaching the ground, thus lowering the air temperature. These mist 

applications should be relatively brief, however, or the mist will 

accumulate on the seedling foliage and encourage nursery pests. 

This warning is particularly important to heed immediately after 

routine irrigation. Free surface moisture keeps the air nearly 

saturated within the seedling crown and promotes foliar diseases 

such as grey mold. Scheduling irrigations early in the day allows 

time for the moisture on the seedling foliage to evaporate. A critical 

period for humidity control in a greenhouse is when the seedling 

crowns close. During this period, adequate air circulation must be 

maintained throughout the greenhouse to lower humidity around 

the seedlings; air circulation is even effective during periods of high 

humidity because the moving air creates a vapor pressure gradient 

from the foliage to the atmosphere.

60
3.2.4.2 Humidification

Humidification is used operationally to retard evapotranspiration 

under the following conditions:

1. During the establishment phase, when seeds,

germinants, cuttings, and new grafts require

conditions that are "moist but not wet."

2. At times during the growing season when the

outside air is much colder than in the green

house because cold air contains less moisture.

3. In arid climates, where the outside air is often

hot and dry.

Humidification is most commonly needed in arid climates during 

cold weather when relatively dry air is brought into the greenhouse 

and heated, which further lowers the RH. Whereas 

dehumidification relies on the heating and ventilation systems to 

dissipate atmospheric moisture, humidification requires 

conservation of moisture and addition of water vapor to the 

greenhouse atmosphere.

61

3.2.4 Modifying Humidity in Container

Nurseries

Most container tree nurseries are not designed with specific 

equipment for controlling humidity, but utilize existing heating, 

ventilation, and irrigation equipment to maintain humidities 

within the desired range. The type of growing structure has an 

overriding effect because some greenhouses hold humidity 

better than others.

3.2.4.1 Growing structures

Fully enclosed structures are better for maintaining a given 

humidity level because they inhibit air exchange with the outside 

environment. All greenhouses leak air to some degree, and so the 

tighter the structure, the less the variation in humidity can be 

expected (Hanan and others 1978). It is difficult to keep humidity 

high in semi-enclosed greenhouses (such as shelterhouses), which 

have roll-up sides that cannot be tightly sealed. This design feature 

is a definite advantage, however, when the objective is to 

dehumidify the environment rapidly.

The type of greenhouse covering is also important. Plastic tarp 

(polyethylene or "poly") coverings fit more snugly and have fewer 

seams than rigid panels, and so they allow less air exchange. Wellinsulated growing structures, such as those with 

double-polyethylene coverings and thermal blankets, will have 

higher humidities (Aldrich and Bartok 1989). However, because of 

their poor insulation, single-layer polyethylene-covered 

greenhouses often develop condensation on their inside surfaces, 

which can lead to drip problems (Mastalerz 1977). There are also 

differences in transparency to sunlight between different coverings, 

which would affect internal temperatures and therefore relative 

humidity levels. For a 4-month period in the late summer and fall in 

British Columbia, the RH in fiberglass growing structures was 

significantly higher than that in plastic-covered structures, and this 

variation was not solely due to differences in temperature (fig. 

3.2.9). These differences were culturally important because 

humidity -related disease losses were 8 times higher in the 

fiberglass house than in the polyethylene-covered structure 

(Peterson and others 1988).
Humidity is conserved by keeping the greenhouse closed 

whenever possible. Because transpiration from the seedlings 

adds moisture to the air, it is much easier to maintain humidity in 

a full greenhouse compared to one that is only partially full. In 

cold weather, water vapor condenses on the inside of uninsulated 

coverings, drips to the floor and drains away, removing moisture 

from the greenhouse atmosphere. Condensation is reduced on 

double-walled, well-insulated coverings. In poorly insulated 

greenhouses, maintaining humidity is difficult in cold weather 

regardless of the humidification system.

Steam heat. The easiest way to humidify a greenhouse is with 

steam, because the water is already a vapor. Steam-heated

greenhouses can be equipped with vents in the steam line that 

are controlled by a humidistat. These vents must be located in a 

safe place where no one can be scalded and where the water 

vapor will be quickly distributed throughout the greenhouse.

Fog and mist. Humidity can also be added by spraying fine 

droplets of water into the air (fig. 3.2.10). The difference between 

fog and mist is in the size of the droplet. Mist droplets are large 

enough to settle out in a few seconds and will wet the surfaces 

on which they land. Fog droplets are almost invisibly small and 

will remain suspended for several minutes, during which most 

will evaporate. Properly applied, fog will not wet foliage. With 

either fog or mist, it may be necessary to shade the greenhouse 

to maintain the desired high humidity.

There are two basic types of nozzle used to produce fog or mist. 

The impact nozzle directs a stream of water against a surface, 

breaking the water into droplets. The centrifugal nozzle spins the 

water into an orifice, which achieves the same thing. Because of 

the high surface tension of water, the smaller the droplet, the more 

energy is required to produce it. This energy may come from 

several sources. Mist nozzles generally operate satisfactorily at 

the standard domestic water pressure of 300 to 450 kPa (45 to 65 

pounds per square inch). Fog nozzles require a booster pump

62
to raise the pressure to 2,700 to 10,000 kPa (400 to 1,500 pounds 

per square inch). Another fogging system uses an electric motor 

to spin a wheel that has orifices on its rim. Centrifugal force raises 

the water pressure at the orifice, and a fan is often used to 

distribute the fog. A third type uses com- air to shear the water 

into fog.

The choice of system will depend on the type of crop, climate, 

greenhouse ventilation system, cultural objectives, and water 

quality (Weed 1989). Mist systems are cheaper to operate and will 

wet the foliage beneath them. This may be beneficial because leaf 

temperatures will be reduced and the mist can deliver mineral 

nutrients or pesticides to the crop. However, excessive misting 

can leach nutrients, leave mineral deposits, encourage growth of 

algae, and promote fungal diseases (Hartmann and Kester 1983). 

Fog systems cost more to install and operate but have proven 

superior for control of humidity. They are especially useful in 

vegetative propagation and can be used outdoors for frost 

protection (Gordon 1989).

Because fog and mist nozzles have tiny orifices, the water must 

pass through very fine pore filters to remove any suspended 

particles that might clog the orifice (fig. 3.2.1 1 A). Fogging or 

misting with saline water will deposit salts on the foliage (fig. 3.2.1 

1 B). These deposits not only are unsightly, but also may cause 

salt burn and inhibit photosynthesis. This problem can be 

particularly serious in vegetative propagation systems because of 

the frequent applications. Fertilizers and pesticides that contain 

suspended particles should not be applied with a fog system. 

(Irrigation water quality and filtration are discussed in detail in 

volume four of this series.)

Irrigation. Standard irrigation nozzles can also be

used to humidify a greenhouse if they are turned

on for brief intervals. However, this must be monitored carefully, 

because over-irrigation can result in suboptimal leaf temperatures, 

wet foliage, and satu rated growing media- conditions that can 

promote fungal diseases. Overhead mobile irrigation booms

are particularly effective for humidification because

they provide even coverage and irrigation intervals can be easily 

controlled (Garzoli 1988). Some growers have outfitted mobile 

booms with multiple spray heads, one of which is a misting nozzle 

(fig. 3.1.22B). The effectiveness of cooling with irrigation was found 

to be short-lived, however, as the increase in RH lasted less than 1 

hour (Mastalerz 1977). No matter what type of irrigation system is 

used, the water should be filtered to remove suspended solids that 

can cause problems.

63
Dehumidifying the seedling canopy. Perforated ventilation or 

heating tubes are often located under raised benches so that air is 

forced up through the seedlings, effectively dehumidifying the 

microenvironment within the canopy (fig. 3.1.27B). The warm air 

not only dries the foliage but also raises the temperature of the root 

plug, which can be beneficial, especially during winter months 

(Hallett 1982). Peterson and Sutherland (1989) found that 

underbench ventilation with cool air took 11 hours longer to dry the 

seedlings than ventilation with heated air, but concluded that cool 

air ventilation was better for lowering the germination potential of 

the grey mold fungus.

64

Evaporative cooling. In arid climates, an evaporative cooling 

system (fig. 3.1.19) can be an effective means of humidification 

during warm weather. Hanan and others (1978) report that 

evaporative cooling will typically raise the RH to about 70 to 80% 

and that the cool, humid air flow will also reduce the VPD. 

Evaporative cooling systems should not be used as a principal 

source of humidity, however, but rather as a beneficial effect of 

temperature control.

3.2.4.3 Dehumidification

Dehumidification is necessary to reduce high atmospheric humidity 

and prevent problems such as excessive condensation. High 

humidity most often occurs under the following conditions:

1. After irrigation, especially when the growing

area cannot be immediately ventilated.

2. In climates with perennially high atmospheric

humidity.

Ventilation and heating. The simplest and easiest way to 

dehumidify the growing environment is to ventilate with drier or 

warmer air. When the outside air is drier, growers can simply 

activate the ventilation system whenever the greenhouse humidity 

rises above the target level (fig. 3.2.12). If ventilation alone is not 

effective, then a combination of heating and ventilation will be, 

even when the outside air is very humid. Often, when conditions 

require dehumidification, the outside air is cool enough that the 

heating system automatically switches on when the vents open. It 

is also possible to switch the ventilation and heating systems 

jointly to guarantee effective dehumidification. In addition to 

lowering the ambient RH, the flow of warmer air over the foliage 

can effectively prevent condensation and eliminate temperature 

stratification in the greenhouse (Aldrich and Bartok 1989). High 

ventilation rates with very dry air can result in seedling water 

stress, however (Hanan and others 1977).
The seedling canopy can also be dehumidified with fans, which can 

be moved into place after irrigation or directly mounted on the 

irrigation boom (fig. 3.2.13). One grower has successfully used a 

portable leaf blower to dry the foliage after irrigation.

Overhead radiant heaters (fig. 3.1.28) reduce the humidity within the 

seedling canopy and effectively eliminate condensation on foliage. 

The thermal radiation warms objects rather than the surrounding air, 

thus decreasing RH without increasing air flow around the 

seedlings, which would increase evapotranspiration rates.

65
3.2.5 Humidity Monitoring and Control

Systems

It is relatively difficult to measure humidity compared to the other 

atmospheric variables. Relative humidity is the only measure of 

humidity that is routinely monitored in container tree nurseries, 

although new computer systems can calculate vapor pressure 

deficit.

3.2.5.1 Humidity

Any instrument that measures humidity is called a hygrometer. A 

psychrometer is a common type of hygrometer that consists of 

two adjacent temperature sensors: a dry-bulb sensor that 

measures ambient temperature and a wet-bulb sensor that is 

covered with an absorbent cloth. This cloth is wetted with distilled 

water and both sensors are ventilated with air moving at a rate of 

at least 3.5 m/s (12 feet per second) until the wet-bulb temperature 

reaches a steady state. The difference in temperature between 

wet-bulb and dry-bulb sensors is known as the wet-bulb 

depression.

Two types of psychrometers are commonly used in container 

nurseries. The sling psychrometer (fig. 3.2.14) is whirled 

manually in a circular motion until the wet-bulb temperature 

stabilizes. With the aspirated psychrometer, the thermometers 

remain stationary and air is drawn across the bulbs with a small 

fan. Charts and tables are available for converting the wet and 

dry-bulb temperature to relative humidity or dew point (fig. 3.2.2 

and table 3.2.4). Psychrometers have a precision of 0.3 to 3.0% 

and an effective range of -18 to 260 °C (0 to 500 ° F). Because 

the wet-bulb depression is so slight, psychrometers are less 

accurate at low temperatures, and special psychrometric charts 

are necessary under subfreezing conditions (ASHRAE 1989). 

Also, errors caused by a dirty wet-bulb, or less than optimum 

ventilation, always result in a reduced wet-bulb depression 

reading, which in turn, produces an elevated RH reading. 

However, psychrometers remain a useful way of measuring RH 

under greenhouse conditions.

66
67

The basic device for controlling humidity is the humidistat, which 

has a relative humidity sensor connected to an electrical sw itch (fig. 

3.2.15). Humidistats can be wired to close when humidities rise, 

switching on a dehumidification system, or when humidities fall, 

triggering a humidification cycle. Likewise, dehumidification can be 

programmed into a decondensate stage on the environmental 

control panel, which synchronizes the opening of vents and triggers 

the heating system, and can be operated either manually or 

automatically (fig. 3.2.16). Computer-assisted environmental control 

systems are able to monitor electric hygrometers continuously and 

generate permanent records of relative humidity and other related 

climatic variables. (Environmental control systems are discussed in 

more detail in volume one of this series.)

The other instrument that is commonly used to monitor humidity in 

container tree nurseries is the hygrothermograph (fig. 3.1.31C), 

which measures both air temperature and relative humidity (fig. 

3.2.12). Because proteins in hair (keratin) change length with 

changes in humidity, human hair is often used in 

hygrothermographs. Hygrothermographs are precise within 3% 

RH, but their accuracy decreases at extreme humidities and they 

are slow to respond to changes (ASHRAE 1989). They have the 

advantage of continually recording RH values to show diurnal and 

daily trends. A good approach is to install a shaded 

hygrothermograph to provide a permanent record of RH, and then 

occasionally check the instrument with a sling psychrometer.

Electrical RH sensors offer improved accuracy and are durable 

and compact. They also feature digital readout capability, which 

makes them easy to use. The two most widely used electrical RH 

sensors, the Dunmore element and the Pope cell, both use wire 

grids in a substrate containing a hygroscopic salt. The electrical 

resistance of the substrates declines as the humidity of the 

surrounding air rises. Although they are accurate and quick to 

respond, both of these RH sensors are highly sensitive to 

contamination, which reduces their useful lifetime in a container 

nursery application (Gaffney 1978).

A new environmental control system can measure VPD around 

plant foliage. The computer uses sensors to measure leaf 

temperature and the temperature and RH of the air. Because the 

air within the stomata is always near saturation under normal 

nursery conditions, the VPD of the leaf can be determined from its 

temperature. The computer system calculates VPD every few 

seconds and uses an accumulated value to estimate plant water 

use and schedule irrigation (Barrett 1990).
68

3.2.5.2 Fog and mist

Three basic types of mist controllers are available: time clocks, 

mechanical sensors such as the "artificial leaf," and 

computer-assisted control equipment that monitors humidity or 

radiant energy. With time controllers, the grower sets the hours of 

operation and the duration of the mists on a mechanical clock (fig. 

3.2.17A/B). These relatively simple and inexpensive controls are 

often wired in series to provide intermittent mist during certain 

hours. The artificial leaf is another inexpensive control system 

consisting of a square of wire gauze on one end of a balance arm 

(fig. 3.2.17C). When the mist that has settled on the leaf becomes 

heavy enough, the balance arm tips, triggering a mercury switch to 

shut off the mist system. After the water evaporates from the leaf, 

the balance arm rises again to the "on" position. Thus, the misting 

cycle is repeated at intervals that are determined by the 

evaporation rate in the growing area.

Fogging requires more sophisticated electronic controls. New 

environmental computers monitor humidity and other factors, such 

as solar radiation, and integrate this information to activate the fog 

system.
69
3.2.6 Conclusions and Recommendations

Atmospheric humidity in container tree nurseries is biologically 

important because it affects seedling water relations and the 

growth of fungal pathogens. The principal influence of humidity is 

on evapotranspiration rates. When the air is still, the rate of 

evaporation from a wet surface is a function of relative humidity 

and temperature and is proportional to the vapor pressure deficit. 

At a constant temperature, the higher the relative humidity, the 

lower the vapor pressure deficit, and the less the plant moisture 

stress. When the vapor pressure deficit increases, the seedlings 

come under greater stress, which results in stomatal closure and a 

reduction in net photosynthesis. For maximum growth, the stomata 

must remain open as long as possible. Growers can promote 

growth by maintaining relatively high humidities in the growing 

areas.

It is difficult to maintain optimum humidity levels for container tree 

nurseries because relative humidity varies so much with 

temperature. Ideal humidity levels also change during the growing 

season depending upon the seedlings' stage of development. High 

relative humidities are most important during seed germination and 

seedling emergence. Target relative humidity during the 

establishment phase is 80%, with a range of 60 to 90%, and during 

the rapid growth phase the target RH drops to 60%, with a range of 

50 to 80%. During the hardening phase, outside ambient relative 

humidities are usually acceptable.

Except for vegetative propagation, very high relative humidities 

(90 to 100%) are not desirable because growth of pathogenic 

fungi, moss, algae, and liverworts is stimulated under these 

conditions and can result in a lowering of seedling quality or 

even in mortality.

Most container nurseries are not designed with specific 

equipment for controlling humidity, but utilize existing heating, 

ventilation, and irrigation equipment to maintain humidities 

within the desired ranges.

70
3.2.7 Literature Cited

Aldrich, R.A.; Bartok, J.W. Jr. 1989. Greenhouse engineering. 

Ithaca, NY: Cornell University, Northeast Regional 

Agricultural Engineering Service. 203 p.

ASHRAE, 1989. ASHRAE Handbook: fundamentals. Atlanta, GA: 

American Society of Heating, Refrigerating, and Air Conditioning 

Engineers, Inc. 792 p.

Barrett, J. 1990. Plants and water 101: understanding VPD. 

Grower Talks, September 1990: 48-52.

Clawson, K.L.; Jackson, R.D.; Pinter, P.J. Jr. 1989. Evaluating 

plant moisture stress with canopy temperature differences. 

Agronomy Journal 81:858-863.

Gaffney, J.J. 1978. Humidity: basic principles and measurement 

techniques. HortScience 13(5):551-555.

Garzoli, K. 1988. Greenhouses. Canberra, Australia: Australian 

Government Publishing Service. 185 p.

Gordon, I. 1989. Fogging systems for propagation. International 

Plant Propagators' Society Combined Proceedings 38:84-86.

Hallett, R.D. 1982. Contrasting approaches to containerized 

seedling production. 3. The Maritime Provinces. In: Scarratt, 

J.B.; Glerum, C.; Plexman, C.A., eds. Proceedings, Canadian 

Containerized Tree Seedling Symposium. COJFRC 

Symposium Proceedings O-P-10. 1981, Sept. 14-16. Toronto, 

ON. Sault Ste. Marie, ON: Canadian Forestry Service, Great 

Lakes Forest Research Centre: 129-138.

Hanan, J.J.; Holley, W.D.; Goldsberry, K.L. 1978. Greenhouse 

management. New York: SpringerVerlag. 530 p.

Hartmann, H.T.; Kester, D.E. 1983. Plant propagation: principles 

and practices Englewood Cliffs, NJ: Prentice-Hall. 727 p.

Jarvis, P.G. 1980. Stomatal response to water stress in conifers. 

In: Turner, N.; Kramer, P., eds. Adaption of plants to water and 

high temperature stress. Oxford: Blackwell: 105-122.

Kramer, P.J. 1983. Water relations of plants. New York: Academic 

Press. 489 p.

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

New York: Academic Press. ft 1 1 n

Krizek, D.T.; Bailey, W.A.; Klueter, H.H. 1971. Effects of relative 

humidity and type of container on the growth of F, hybrid 

annuals in controlled environments. American Journal of Botany 

58:544-551.

Mastalerz, J.W. 1977. The greenhouse environment. New York: 

John Wiley and Sons. 629 p.

Peterson, M.J.; Sutherland, J.R.; Tuller, S.E. 1988. Greenhouse 

environment and epidemiology of grey mould of 

container-grown Douglas-fir seedlings. Canadian Journal of 

Forestry Research 18(8):974-980.

Peterson, M.J.; Sutherland, J.R. 1989. Grey mould control by 

seedling canopy humidity reduction through under-bench 

ventilation and styroblock aeration. FRDA Rep. 077. Victoria, 

BC: Canadian Forestry Service and BC Ministry of Forests. 16

p.

Schroeder, M.J.; Buck, C.C. 1970. Fire weather: a guide for 

application of meteorological information to forest fire control 

operations. Agric. Handbk. 360. Washington, DC: USDA Forest 

Service. 229 p.

Seiler, J.R.; Johnson, J.D. 1984. Abnormal needle morphology in 

loblolly pine induced by low humidity. HortScience 

19(4):521-522.

Van de Sanden, P.A.C.M. 1985. Effect of air humidity on growth 

and water exchange of cucumber seedlings: preliminary report. 

Acta Horticulturae 174:259-267.

Weed, J. 1989. A look at fog cooling systems. Greenhouse 

Manager 8(6):110-113.

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