Greenhouse Ventilation and Cooling
Greenhouse Humidity Control
Humidity control is most difficult during the fall and spring seasons, when the outside temperature and humidity is similar to those inside the greenhouse. igh humidities are not likely to occur during freezing weather, since the relative humidity of the outside air is very low. Humidity in greenhouses is controlled to minimize spread of fungal pathogen such as Botrytis and powdery mildew and to regulate transpiration. Moisture buildup can be a major problem in greenhouses used for plant production. At high levels of relative humidity the risk for condensation on leaves is high (especially at night) and thus the risk of Botrytis and other fungal diseases to develop increases. There are few fungi that thrive under low relative humidity. The amount of moisture in the air (aka “humidity”) affects the transpiration rate of plants, which is responsible for moving water and nutrients from the root zone to other parts of the plant. When the humidity levels are too high or too low, transpiration will slow, inhibiting plant health, growth, and development. As a general guide, it is often recommended that greenhouse relative humidity be maintained between 65 to 75 percent during the night and 80 to 90 percent during the day for healthy plant growth.
Relationship between Humidity and Temperature
The amount of moisture in the air is generally expressed as relative humidity (RH), which is the ratio between the weight of moisture actually present in the air and the total moisture-holding capacity of a unit volume of air at a specific temperature and pressure. This term can sometimes be misleading, because it is temperature-dependent. Warm air has a higher moisture-holding capacity than cooler air; therefore, as the temperature of air increases, the relative humidity decreases even though the amount of water remains constant.
Dew Point Temperature
Dewpoint temperature indicates the temperature at which water will begin to condense out of moist air. At any given temperature and pressure, there is a maximum amount of water that can be held in the air. This is known as the saturation point. At any time when the air is nearly saturated with water vapor, all it takes is a slight drop in temperature to reach the dewpoint. The higher the moisture content of the air, the higher the dewpoint temperature.
Humidity-Measuring Instruments
Sling Psychrometer
An aspirated psychrometer, a sling psychrometer, or an electronic humidity meter can be used to determine the relative humidity. The difference between a sling psychrometer and an aspirated psychrometer is the way the airstream is provided. A sling psychrometer is mounted on a swiveled handle and whirled rapidly, while an aspirated psychrometer uses a small fan to provide air movement. The sling psychrometer consists of two thermometers exposed to the same airstream (See Figure 5.13).
Hand Held Humidity Meters
Hand held electronic humidity meters that display relative humidity are more convenient than wet bulb psychrometers (See Figure 5.14). However, wet bulb temperature is a more useful measurement for evaporative cooling systems because it directly determines the temperature to which air can be cooled by evaporative cooling alone. Some electronic humidity meters do have an option to display wet bulb temperatures.
Wireless Temperature and Humidity Sensors
Wireless temperature and humidty sensors are another option and are more suited to larger greenhouse operations. The wireless greenhouse temperature monitoring system is composed of wireless temperature and humidity sensors, data collector, and a managing server. The sensors installed inside a greenhouse for collecting environmental data.
Reducing Greenhouse Humidity
Greenhouse growers usually try to avoid humidity levels near the dew point since free water condensing onto plant surfaces can promote the growth of disease organisms. Excess humidity is usually more problematic in the spring and fall seasons when the weather is cool and moist. High humidity is not likely to occur during freezing weather since the relative humidity of the outside air is very low. The common strategy used to reduce greenhouse humidity involves the following methods.
Cultural Practices
Proper planting dates, adequate spacing, and morning watering (so that foliage can dry prior to lower night temperatures) are good cultural practices for managing relative humidity and controlling plant diseases. Closely-spaced plants and overlapping canopies can create microclimates different from the rest of the structure.
Ventilation and Heating
A common dehumidification practice is simply to open the windows, allowing moist greenhouse air to be replaced by relatively dry outside air. Venting for humidity control is most effective when outside air is significantly cooler and drier than that inside the greenhouse. As cool, dry air heats up in the greenhouse, it absorbs moisture and lowers the humidity. Humidity reduction by bringing in outside air can be somewhat effective even if the outside air is very humid, as long as it is significantly cooler than the inside air. In practical terms, however, outside air should be significantly cooler and drier to justify the cost of ventilation.
Air Circulation
Air circulation is another way of important consideration in managing humidity in the greenhouse. Air that is moving is continually mixed resulting in very small temperature differences. Horizontal air flow fans and polytubes can be used to move air in the greenhouse. When the greenhouse is heated with hot air furnaces, continuous air movement can be obtained by running the fans continuously.
Bottom Heat
Bottom heat will improve air circulation inside plant canopies and will help to prevent condensation on leaf surfaces. The warm air that rises creates air movement around the plants. Bottom heat also keeps the plant surfaces warm, preventing condensation on the plants.
Greenhouse Design
A sloped roof (rise to run of 1:2) will encourage moisture to move toward the gutter and collect without dripping, compared with a roof with a shallow slope. A double-layer glazing will have a warmer interior-layer surface temperature because of the air-gap insulation between the layers, and thus less condensation.
Anti-Drip Plastic
The use of a wetting agent either sprayed on the interior surface or as part of the formulation of the glazing on poly covered greenhouses can also help to reduce the humidity level.
Vapor Pressure Deficit
Instead of relative humidity, the more accurate way to express the driving force of water loss from a leaf is vapor pressure deficit (VPD). Its value is independent of temperature. Vapor pressure deficit is the difference between the amount of moisture in the air and how much moisture the air could potentially hold when it’s saturated. It’s often measured in pounds per square inch (psi) or kilopascal (kPa). A high VPD (> 1.0 kPa) means that the air can still hold a large amount of water. Therefore, there’s a large gradient between plants (nearly saturated with water) and the air, enabling the plants to transpire and, over time, dry out. A low VPD indicates the air is near saturation.
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