J. Raymond Kessler, Jr.
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 cannot 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 the problem?
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.
Current water concentration in air
RH = _______________________________________ × 100, at constant temperature
Maximum water concentration the air could hold
The discussion above assumes a specific temperature, but temperature within a greenhouse change 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 an 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. The RH increases as long as water does not condense out of the air.
Figure 1 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 70°F during the day and the RH is 40% (small dotted line). If the temperature at night drops to 60°F, the RH increases to 50%. Conversely, a 10-degree increase in temperature will decrease the RH. In reality, the amount of change in RH is about 2% for each 1°F change in temperature.
Now suppose the temperature in a greenhouse is 75°F during the day and the RH is 50%. If the temperature at night drops too just below 45°F, the RH will increase to 100%. 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 point.
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 a 95% 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 inside RH.
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 many 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 the leaves. 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, 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 that it will turn on about 8:00 or 9:00 PM in the evening. 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. The cool outside air should activate the heating system and bring the greenhouse air temperature up to the set point. The moist, cooler air from outside will warn-up and have a lower RH.
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!