Greenhouses are humid environments where the air contains a high concentration of water vapor. This humidity is necessary, as plants require a moist atmosphere to facilitate metabolic processes like photosynthesis and nutrient uptake. However, maintaining the correct balance is a continuous challenge for growers; excessive humidity can encourage fungal diseases, while air that is too dry can stunt plant growth. Successful operation depends on the precise monitoring and active control of the air’s water content.
The Processes That Drive Internal Humidity
The high moisture content within a greenhouse results from biological processes and the structure’s physical design. The primary source is plant transpiration, where water moves through the plant and evaporates from aerial parts like leaves, stems, and flowers. This moisture release from the dense plant canopy significantly raises the concentration of water in the surrounding air.
Water also enters the air through evaporation directly from surfaces like the growing medium, exposed soil, and any standing water on the floors or benches. Watering practices, especially those that leave foliage wet or create puddles, contribute to this evaporation. These sources continuously pump moisture into the air volume of the structure.
The greenhouse structure itself, typically made of glass or polycarbonate, acts as a containment barrier that traps this moisture-laden air. The structure prevents the natural dissipation of water vapor, allowing humidity levels to build up rapidly. This containment, combined with the warming effect of solar radiation, creates an atmosphere where water vapor accumulates.
Quantifying the Greenhouse Environment
To manage the air state, growers use specific metrics, the most common of which is Relative Humidity (RH). RH is expressed as a percentage, indicating the ratio of the actual amount of water vapor in the air compared to the maximum amount the air can hold at that temperature. While a high RH value signals a moist environment, it has a significant limitation: the air’s moisture capacity changes drastically with temperature.
Warm air holds far more water vapor than cool air. For instance, 70% RH at a cool temperature holds much less actual water than 70% RH at a warm temperature. This temperature dependency makes RH a less accurate indicator of a plant’s immediate water stress.
A more accurate metric for plant-air moisture relationships is the Vapor Pressure Deficit (VPD), which measures the difference between the moisture pressure inside a saturated leaf and the moisture pressure in the surrounding air. VPD is measured in pressure units, such as kilopascals (kPa), and is a direct predictor of the rate at which a plant will transpire. A high VPD indicates dry air that pulls moisture rapidly from the leaves, potentially stressing the plant, while a low VPD means the air is near saturation, effectively shutting down transpiration. Growers use calibrated electronic sensors or psychrometers to measure the temperature and moisture levels required to calculate these metrics.
Strategies for Humidity Management
Growers use a combination of active and cultural techniques to manage the air’s moisture content and maintain the optimal VPD range. The most fundamental technique is ventilation, exchanging moist internal air with drier external air. This can be achieved passively by opening roof and side vents or actively through the use of exhaust fans that pull air through the structure.
A common approach involves simultaneously heating and venting the greenhouse, especially during cooler periods. Heating the air immediately increases its capacity to hold water vapor, thereby lowering the RH percentage. When this warmer, lower-RH air is then vented, it efficiently removes a greater mass of water vapor from the structure.
Internal air circulation fans are also employed to prevent the formation of localized pockets of saturated air, particularly within the dense plant canopy where transpiration is highest. Moving the air constantly ensures a uniform humidity level throughout the structure, preventing condensation on leaf surfaces which can lead to disease. In highly controlled or closed-loop facilities, mechanical dehumidifiers are used to condense water vapor out of the air, offering a precise method of moisture removal when outside venting is not an option.
Cultural practices related to watering are an effective management tool. Watering plants earlier in the day allows the moisture added through irrigation and subsequent evaporation to be removed by daytime ventilation. Using drip irrigation systems, which deliver water directly to the root zone, reduces the amount of exposed, wet surface area, limiting the volume of water vapor added to the air.