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.
Humidity
Humidity is the amount of water vapor in
the air. Water
vapor is the gaseous state of water and is invisible. Humidity indicates the
likelihood of precipitation, dew, or fog. Higher humidity reduces the
effectiveness of sweating in cooling the body by reducing the rate of evaporation of
moisture from the skin. This effect is calculated in a heat index table
or humidex, used
during summer weather.
There
are three main measurements of humidity: absolute, relative and specific. Absolute humidity is the water content
of air. Relative humidity, expressed
as a percent, measures the current absolute humidity relative to the maximum for that temperature. Specific humidity is a ratio of the water vapor content of the
mixture to the total air content on a mass basis.
Humidity is a measure of the
amount of water vapor in the air, not the total amount of vapor and liquid. For
clouds to form, and rain to start, the air does have to reach 100% relative
humidity, but only where the clouds are forming or where the rain is coming
from. This normally happens when the air rises and cools. Often, rain will be
falling from clouds where the humidity is 100% into air with a lower humidity.
Some water from the rain evaporates into the air it's falling through,
increasing the humidity, but usually not enough to bring the humidity up to
100%. Humidity can be less than 100% when it's raining.
Humidity is important in
climate change. Water vapor in the air, the humidity, plays an important part
in global climate. Like carbon dioxide, water vapor is a greenhouse gas.
Climate scientists have found that carbon dioxide human activities is adding to
the air is causing the Earth's average climate to warm.
Relative Humidity
The
amount of water vapor actually in the air divided by the amount of water vapor
the air can hold. Relative humidity is expressed as a percentage and can be
computed in a variety of ways. One way is to divide the actual vapor pressure
by the saturation vapor pressure and then multiply by 100 to convert to a
percent.
Actual vapor pressure:
The partial pressure exerted by the water vapor present in a parcel. Water in a
gaseous state (i.e. water vapor) exerts a pressure just like the atmospheric
air. Vapor pressure is also measured in millibars.
Saturation vapor
pressure: The maximum partial pressure that water vapor
molecules would exert if the air were saturated with vapor at a given
temperature. Saturation vapor pressure varies with atmospheric pressure. When a
given atmospheric pressure is steady, then the saturated vapor pressure is
directly proportional to the temperature.
Measurement
There
are various devices used to measure and regulate humidity. A device used to
measure humidity is called a psychrometer or hygrometer. A humidistat is a
humidity-triggered switch, often used to control a dehumidifier.Humidity
is also measured on a global scale using remotely placed satellites.
These satellites are able to detect the concentration of water in the troposphere at
altitudes between 4 and 12 kilometers. Satellites that can measure water vapor
have sensors that are sensitive to infrared radiation. Water vapor specifically absorbs and re-radiates radiation in this
spectral band. Satellite water vapor imagery plays an important role in
monitoring climate conditions (like the formation of thunderstorms) and in the
development of future weather forecasts.
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 the 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 samespace, 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 thecontainer
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 sameamount of thermal
energy that is used when water evaporates is released when water 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.
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.
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. Under operational conditions, increasing temperature is more of a controlling
factor than humidity in determining evapotranspirationaldemand. 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).
Container tree seedlings develop significant boundary layers 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. 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
transpirationalgradient from the leaf to the air, even at high humidities.
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
oftenpropagated 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 orconservation purposes, such as poplars and willows, are propagated
vegetatively.Maintenance of the proper humidity is of particular concern
invegetative propagation. The transpiration rate of cuttings must be kept low
for several weeks or even months so that they canmaintain 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 humiditiesManaging 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. 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.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
andpenetrate 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 ofDouglas- 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.
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.
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 theirtarget RH values decreased during the
growing season, ranging from 60 to 80% in the establishment phase to 45 to 50%
during the hardening phase. Several nurseries stated that they did not really
have targets for humidity because it is so difficult tocontrol, especially in
growing structures that are not fully enclosed,such as shadehouses. Most agreed,
however, that high humiditieswere definitely important during seed germination
and emergence. Establishment phase. Managing the humidity is most
criticalduring 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" . Maintaining high relative
humidities of 60 to 90% eliminates the
need for frequent irrigation, which would keep the growing medium too wet and
promote damping-off.
Rapid growth phase
As soon as the
seedlings have established root systems, the relative humidity should be
reduced to 50 to 80% . This will keep evapotranspiration low, but the surfaceof
the growingmedium and the seedling foliage will remain dry. A VPD of
approximately 1.00 kPais a reasonable target for this phase.
When
temperatures in the growing area become excessive, many nurseries apply a fine
mist, often in combination with shade, to cool the seedlings. 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 highhumidity because the moving air creates
a vapor pressure gradient from the foliage to the atmosphere.
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 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. Shelterhouses
areparticularly beneficial during this period because their sides can beraised
to promote good cross ventilation.
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; 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.
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 humiditieswithin the desired range. The type of growing
structure has an overriding effect because some greenhouses hold humidity better
than others.
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. Wellinsulatedgrowing 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. These differences were culturally
important because humidity -related disease losses were 8 times higher in the fiberglass
house than in the polyethylene-covered structure.
Humidification
Humidification
is used operationally to retard evapotranspiration under the following
conditions:
·
During
the establishment phase, when seeds, germinants, cuttings, and new grafts
requireconditions that are "moist but not wet."
·
At
times during the growing season when the outside air is much colder than in the
greenhouse because cold air contains less moisture.
·
In
arid climates, where the outside air is oftenhot 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.
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 uninsulatedcoverings, 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. 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 pumpto raise the
pressure to 2,700 to 10,000 kPa (400 to 1,500 poundsper 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
waterinto fog.The choice of system will depend on the type of crop, climate, greenhouse
ventilation system, cultural objectives, and
water
quality. 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.
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
saturated 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. Some
growers have outfitted mobile booms with multiple spray heads, one of which is
a misting nozzle. The effectiveness of cooling with irrigation was found to be
short-lived, however, as the increase in RH lasted less than 1 hour. No matter
what type of irrigation system is used, the water should be filtered to remove
suspended solids that can cause problems.
Evaporative cooling
In
arid climates, an evaporative cooling system can be an effective means of
humidification during warm weather. 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 principalsource of
humidity, however, but rather as a beneficial effect of temperature control.
Dehumidification
Dehumidification
is necessary to reduce high atmospheric humidity and prevent problems such as
excessive condensation. High humidity most often occurs when
·
After
irrigation, especially when the growing area cannot be immediately ventilated.
·
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. 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. High ventilation rates with very dry air can result in
seedling water stress
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. 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. 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.The seedling canopy can also be
dehumidified with fans, which can be moved into place after irrigation or
directly mounted on the irrigation boom. One grower has successfully used a portable
leaf blower to dry the foliage after irrigation. Overhead radiant heaters
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.
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.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 thatmeasures 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 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. 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 lessaccurate at low
temperatures, and special psychrometric charts are necessary under subfreezing
conditions. 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. The other instrument that is commonly used to monitor
humidity in container tree nurseries is the hygrothermograph, which measures
both air temperature and relative humidity. Because proteins in hair 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. 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. 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 tocontamination, which reduces their useful lifetime in a
container nursery application. 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 wateruse and
schedule irrigationThe basic device for controlling humidity is the humidistat,
whichhas a relative humidity sensor connected to an electrical switch.
Humidistats can be wired to close when humidities rise, switching on a
dehumidification system, or when humidities fall, triggering a humidification
cycle.
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.
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. When the mist that has settled on the leaf becomesheavy 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.
Reference:
Aldrich, R.A.; Bartok,
J.W. Jr. 1989.Greenhouse engineering.Ithaca, NY: Cornell University, Northeast
Regional Agricultural Engineering Service. 203 p.
Gaffney, J.J.
1978. Humidity: basic principles and measurementtechniques. HortScience
13(5):551-555.
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
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