Relative humidity (RH) is the ratio of the amount of water vapor in the air compared to the maximum amount the air can hold at that specific temperature. In a greenhouse, the enclosed structure traps moisture released from plant transpiration and watering the soil, creating a highly dynamic environment. Humidity levels fluctuate widely and quickly, moving from plant-friendly to pathogen-friendly conditions in hours. Active humidity management is integral to successful cultivation because of this need for precise environmental balance.
Understanding Humidity’s Impact on Greenhouse Ecosystems
Maintaining the correct moisture balance is fundamental because humidity directly affects a plant’s ability to respire and move nutrients. When air moisture levels are too high, the rate of transpiration slows down significantly, since the air is already close to saturation. This reduction in water movement within the plant can inhibit the uptake of necessary minerals, leading to nutritional deficiencies. Excess moisture also promotes the growth of fungal diseases like Botrytis (gray mold) and powdery mildew, which thrive when water condenses on leaf surfaces.
Conversely, when the humidity drops too low, plants transpire too rapidly, causing excessive water loss and stress. To conserve moisture, the plant’s stomata—small pores on the leaves—may close, which slows growth and can lead to wilting or desiccation. Growers monitor the Vapor Pressure Deficit (VPD) for a more precise measure of plant comfort. VPD is the difference between the actual water vapor pressure and the saturated water vapor pressure, serving as a better indicator of the plant’s water-stress level than relative humidity alone.
Techniques for Reducing Internal Moisture Levels
The most common challenge in a greenhouse is removing excess moisture, and the primary method for this is ventilation. This involves exchanging humid indoor air with cooler, drier air from outside. Natural ventilation relies on passive air movement through roof vents and side louvers, using the buoyancy of warm air to draw it out while cooler air enters below. Mechanical ventilation uses exhaust fans to forcibly pull the moist air out, creating a vacuum that draws fresh air in through intake shutters on the opposite side.
Air circulation fans, often called Horizontal Air Flow (HAF) fans, complement ventilation. These fans break up stagnant air pockets, especially within the dense plant canopy where humidity can be 20% to 40% higher than the surrounding air. Moving air helps keep the leaf surface dry, reducing the risk of condensation and fungal disease development. They also destratify the air, mixing warm air near the ceiling with cooler air near the floor to promote a uniform climate.
A popular strategy for active dehumidification, particularly during cooler periods, is the “heat and vent” method. This involves briefly turning on a heat source to raise the air temperature, which instantly lowers the relative humidity because warmer air can hold significantly more moisture. Once the air’s moisture-holding capacity is increased, the humid air is vented out and replaced with cooler, drier outside air. This technique is often cycled multiple times an hour during the night or early morning when temperatures are lowest and condensation risk is highest.
Cultural practices also manage moisture inputs. Watering plants only in the morning allows excess water evaporating from the soil to be vented out during the day when temperatures are high. Avoiding plant overcrowding is also helpful, as closely packed foliage traps moisture and creates microclimates ideal for disease. Refrigeration-based dehumidifiers can be used in highly controlled environments to condense water, but they are energy-intensive and most effective in closed-loop systems.
Methods for Raising Humidity When Levels Are Too Low
In arid climates or during winter when heating dries the air, a grower must actively add moisture back into the environment. Simple overhead misting systems use lower water pressure, typically producing larger water droplets around 100 to 200 microns in size. These larger droplets do not evaporate quickly and are more likely to settle on plant leaves, potentially causing surface wetness that can lead to disease. For this reason, simple misting is often reserved for propagating cuttings where leaf wetness is desired.
A more refined approach is high-pressure fogging, which uses pumps operating at very high pressure to force water through tiny nozzles, creating micro-fine droplets less than 10 microns in size. These ultra-fine particles are small enough to remain suspended in the air, creating an artificial fog that evaporates almost instantly before it can wet the leaf surface. This rapid evaporation process also absorbs heat from the air, providing a cooling effect while simultaneously increasing the ambient humidity level.
Evaporative cooling, frequently used in hot, dry regions, is a dual-purpose system that both cools and humidifies the air. This system uses exhaust fans on one end of the greenhouse to draw outside air across thick, water-saturated cellulose pads installed on the opposite wall. As the air passes through the wet pads, the water evaporates, cooling the air by 10°F to 20°F while adding moisture. This method is effective only when the outside air is dry, as it increases the humidity substantially.
Passive methods can also achieve a modest increase in moisture levels. “Damping down” involves manually wetting concrete floors or walkways, allowing the water to slowly evaporate into the air. Placing shallow trays filled with water and gravel beneath plant benches increases the surface area for evaporation, raising localized moisture. Grouping plants closely together also helps create a higher-humidity microclimate from their combined transpiration.