Relative humidity (RH) measures the amount of water vapor in the air compared to the maximum amount the air can hold at a specific temperature. For many specialized plants, such as tropical varieties and young seedlings, maintaining a high RH is necessary to mimic their native environment. This elevated moisture minimizes transpiration stress, the excessive loss of water vapor from the plant’s leaves, promoting strong, steady growth.
Low-Tech and Passive Methods
Achieving higher humidity can begin with simple, manual techniques utilizing evaporation. A common method is “damping down,” which involves manually wetting the greenhouse floor, walkways, and benches with a hose. The water evaporates as the air temperature rises, steadily adding moisture and increasing the relative humidity throughout the space.
Grouping plants closely together creates a localized microclimate. The collective transpiration from the leaves raises the humidity immediately surrounding the foliage, making the air less dry for neighboring plants. Utilizing trays filled with pebbles and water beneath potted plants serves a similar purpose without waterlogging the roots. As water evaporates from the pebble surface, it increases the ambient RH near the plant canopy, providing localized moisture.
Capillary mats are highly absorbent fabric pads placed on benches that act as a continuous, passive moisture source. When kept wet, these mats slowly release water vapor into the air and provide a steady water supply to the plant pots resting on them. Hanging wet materials, such as burlap or porous cloth, along the walls or under benches also offers a significant surface area for water evaporation. This allows for a passive, continuous release of moisture that can be refreshed as needed.
Automated and Mechanical Systems
Automated systems offer precise and consistent moisture control by introducing water vapor with minimal droplet size. High-pressure fogging systems are the most advanced, using pumps that force water through specialized nozzles at pressures typically between 500 and 1,500 PSI. This extreme pressure atomizes the water into micro-fine droplets (5 to 20 microns) that flash-evaporate almost instantly. Flash evaporation humidifies the air without wetting the plant foliage, which helps prevent water-borne diseases.
Standard misting nozzles operate at lower pressures, producing larger water droplets that do not evaporate as quickly. While misting raises humidity, the larger droplets can leave a film of water on leaves and surfaces, increasing the risk of fungal growth. Dedicated industrial or ultrasonic humidifiers provide an alternative approach, using high-frequency vibration to create a fine, cool mist. Ultrasonic units are energy-efficient and effective for smaller, enclosed spaces or propagation areas requiring high, stable humidity.
The trade-off for the superior control and efficiency of these mechanical systems is the initial cost and maintenance. High-pressure systems require regular cleaning and good water quality to prevent mineral buildup from clogging the fine nozzle openings. Selecting the right system depends on the greenhouse size, the target RH range, and the grower’s tolerance for maintenance and initial investment.
Strategic Management of Airflow and Temperature
Managing ambient temperature and airflow is important for maintaining desired humidity levels. The relationship between heat and moisture is inverse: warmer air has a greater capacity to hold water. Heating the air without adding moisture causes the relative humidity to drop, so regulating the temperature helps stabilize the RH.
To prevent the loss of newly introduced moisture, ventilation should be managed strategically, especially during cooler periods. Venting the greenhouse to reduce humidity is most effective when the outside air is significantly cooler and drier than the air inside. This allows the drier air to absorb moisture as it warms up within the greenhouse.
Incorporating thermal mass, such as large water barrels or masonry, can help temper rapid fluctuations in both temperature and humidity. These materials absorb heat during the day and release it slowly at night, stabilizing the internal environment. This thermal buffering prevents sudden drops in temperature, which would otherwise cause the relative humidity to spike and lead to condensation on plant surfaces.
Monitoring and Maintaining Optimal Levels
Successful humidity management requires accurate measurement using a calibrated hygrometer, often integrated into a thermo-hygrometer unit. This device provides a real-time percentage reading of the relative humidity, allowing the grower to know when adjustments are needed. For many common greenhouse crops, an optimal RH range is often cited around 80%, though this varies widely depending on the plant species and its growth stage.
Maintaining this balance is delicate because excessive humidity carries significant risks, primarily promoting fungal diseases. When the RH remains above 80% for extended periods, conditions favor pathogens like Botrytis (gray mold) and powdery mildew. Even in high-humidity environments, air circulation is necessary to prevent stagnant, saturated air pockets from forming around the plants. Oscillating fans ensure gentle, constant air movement, which keeps leaf surfaces dry and reduces the chance of disease.