Have you every walked into a greenhouse early in the morning
on a cool, crisp spring or fall day, slammed the door behind you and found
yourself soaking wet? You look up to try to find the source of this indoor
precipitation and see rows of big, fat drops of water coating the inside of the
glazing. With a sigh, you walk among the benches of plants and observe a film
of water coating the leaves and maybe a spot or two of gray, fuzzy mold growing
on a leaf or flower petal. Other than your discomfort, should this drippy
deluge be a concern for the health of your crops? Certainly it should! High
relative humidity at night and condensation can not only lead to disease
problems, but will also reduce light intensity. So what conditions lead to this
problem, and what steps can be taken to prevent it?
A unit for measuring the amount of water dissolved in air
that is familiar to most people is relative humidity (RH). Relative humidity is
a ratio between the amount of water dissolved in the air to the maximum amount
of water the air can hold at a specific temperature and atmospheric pressure.
From a practical viewpoint in a greenhouse, atmospheric pressure changes will
be so small that a constant pressure can be assumed. Therefore, greenhouse
relative humidity is the current concentration of water in the air divided by
the maximum amount of water the air could hold at a specific temperature,
expressed as a percent.
The discussion above assumes a specific temperature, but
temperature within a greenhouse changes under various conditions, e.g. day to
night. Suppose that a container of water is placed within an enclosed space
(similar to a greenhouse), water molecules evaporate into the air of the
enclosed space until equilibrium is reached (similar to plants, floors, etc.,
evaporating water in a greenhouse). At equilibrium, the air is saturated with
as much water as it can hold. When the temperature of the enclosed volume of
air at a certain RH increases, the air expands and can hold more water. If no
additional water is added, the RH decreases. Conversely, when the temperature
of an enclosed space at a certain RH decreases, the air contracts and can hold
less water. As long as water does not condense out of the air, the RH
Figure 1, right, shows the relationship between RH and
temperature and is called a psychometric chart. For an example of this
relationship, suppose the temperature in a greenhouse is 70º F during the
day and the RH is 40 percent (small dotted line). If the temperature at night
drops to 60º F, the RH increases to 50 percent. Conversely, a 10-degree
increase in temperature will decrease the RH. In reality, the amount of change
in RH is about 2 percent for each 1-degree change in temperature.
Now suppose the temperature in a greenhouse is 75º F
during the day and the RH is 50 percent. If the temperature at night drops to
just below 45º F, the RH will increase to 100 percent. Any further
temperature reduction and the concentration of water in the air will rise above
the saturation point and condense as water droplets. This is called the dew
High RH in a greenhouse is important in relation to the
incidence of several foliar diseases, especially Botrytis and powdery mildew.
Powdery mildew spores germinate best at a 95 percent RH or higher. However, low
RH and high temperature are needed for fungal maturation and spore release.
Diseases related to high RH are usually more common in the spring and fall when
environmental conditions are highly variable. Diseases due to high RH are
usually not a problem during the heating season because raising the air
temperature using heat lowers the RH. Air vented into the greenhouse during the
winter may be very moist but is also very cool. Heating this moist, cool air
reduces its RH. In the summer, the RH of outside air is usually lower than the
air in the greenhouse, so ventilation is the most practical means of lowering
The spring and fall often have warm, bright days and cool
nights, and the moisture content of the outside air is high. In the early
evening as temperature drops, ventilation does little to decrease the inside
RH. As the temperature drops at night and ventilation stops, the RH can be very
high even though the greenhouse may not be cool enough to require heat.
Leaf temperatures during the day are usually a few degrees
warmer than the air temperature because they absorb sunlight. After sunset,
however, the leaves may radiate heat through the glazing to the cooler air
outside (radiation cooling), resulting in leaf temperatures below the
greenhouse air temperature. At high RH in the greenhouse, the dew point is
reached in a thin boundary layer of air around the leaves, and a film of water
forms on them. This free water is an invitation to diseases.
The best way to prevent water from condensing on leaves
during the spring and fall is to provide constant internal air circulation and,
if necessary, to heat the greenhouse for a short period of time in the evening
with a ventilation fan running. Internal air circulation from horizontal
airflow fans or similar methods prevents formation of a saturated boundary
layer around the leaves and thus prevents condensation. If high RH is a serious
problem, place one ventilation fan on a time clock so it will turn on at
approximately 8:00 or 9:00 p.m. The fan should run just long enough to complete
one air exchange of the greenhouse. The warm, moist air in the greenhouse will
be removed and replaced by cooler, moister air from the outside, which should
activate the heating system and bring the greenhouse air temperature up to the
set point. The moist, cooler air from the outside will warm up and have a lower
During the winter, the temperature difference between the
greenhouse air and the outside air can be large. The glazing material loses
heat rapidly and becomes cold. Warm, moist air circulating over the inside of
cold glazing can reach the dew point, and water condenses on the inside of the
glazing material. With glass, condensation dripping is usually not a big
problem because of the surface tension properties of glass. Water tends to
spread out into a thin film on glass. However, the surface tension properties
of plastics are such that condensed water very quickly forms large drops that
can rain down on crops in the greenhouse. This problem is especially serious in
houses covered in polyethylene.
Covering the greenhouse with two layers of polyethylene and
creating an air space between the layers by inflation insulates the inside
layer and reduces the temperature difference between the outside and inside.
Internal air circulation and heating the greenhouse with a fan running in the
evening also helps. New polyethylene products have been developed with a
special coating on the inside to reduce the surface tension and thus reduce
dripping. Be sure to install this kind of polyethylene with the coated side
toward the inside of the greenhouse.
This article was reprinted with permission from Southeastern
Learn what causes this problem and how to prevent it.