Relative humidity measures the water vapor present in the air compared to the maximum amount the air can hold at a specific temperature. This factor directly influences a plant’s ability to perform transpiration, the process of moving water and nutrients. When the air inside a greenhouse becomes too dry, transpiration accelerates significantly, leading to water stress and causing the plant to close its stomata. Insufficient humidity can quickly result in symptoms like stunted growth, wilting, and leaf burn, where foliage edges dry out. Controlling the moisture content of the air is fundamental to maintaining a stable and productive greenhouse environment.
Active Mechanical Systems for Increasing Humidity
The most precise method for increasing greenhouse humidity involves specialized mechanical equipment that atomizes water into microscopic droplets. High-pressure fogging systems are the most effective solution for larger commercial operations, utilizing pumps that pressurize water to between 750 and 1,500 pounds per square inch (psi). This extreme pressure forces water through fine nozzles, creating a fog of micro-droplets, typically measuring between 5 and 20 microns. These droplets flash-evaporate almost instantly upon release, increasing humidity without settling on plant surfaces and providing evaporative cooling.
The chief advantage of high-pressure systems is their ability to deliver uniform humidity across a large area while preventing the leaf wetness that promotes fungal disease. However, the initial installation cost is high, and the fine nozzles are prone to clogging, requiring a high-quality water filtration system.
Ultrasonic Humidifiers
Ultrasonic humidifiers offer an alternative, suited for smaller, enclosed spaces or propagation areas. These units use high-frequency sound waves to vibrate a plate, atomizing water into an ultra-fine mist of approximately 5-micron droplets. They are generally quieter and more energy-efficient than pump-driven systems. However, they can produce a fine “white dust” residue if the water source is not purified.
Low-Pressure Misting Systems
A third option is the low-pressure misting system, which operates at standard tap pressure, typically between 15 and 70 psi, making it inexpensive and simple to install. The lower pressure produces larger water droplets, often in the 50 to 100-micron range. These heavier droplets do not evaporate quickly and are more likely to fall out of the air, leading to wet plants and soil. This persistent moisture can create an environment favorable for pathogens like Botrytis cinerea (gray mold), making low-pressure misting less desirable for mature plants.
Structural and Passive Environmental Techniques
Simple, non-mechanical methods focus on increasing the surface area for water evaporation or reducing the air exchange rate with the drier outside environment. The practice known as “damping down” involves wetting the greenhouse floor, paths, and staging benches. Evaporation of this standing water passively raises the moisture content of the air. This technique is most effective when performed frequently during hot, dry weather, often once in the morning and again around midday.
An adaptation involves creating specialized water reservoirs beneath the plants, known as a “gravel bench.” This is a fixed bench with a tray filled with a layer of gravel or pebbles, kept constantly wet. The water level must remain below the base of the pots. The large surface area of the wet gravel provides a consistent source of slow evaporation, contributing to a stable, localized increase in relative humidity.
Grouping plants closely together is another effective passive strategy because the collective transpiration from the foliage creates a humid microclimate around the plants. This concentrated moisture helps to slow down individual plant water loss and provides a temporary buffer against dry air conditions.
Managing the greenhouse’s air exchange rate can also help retain moisture generated by the plants. During periods of low humidity, slightly reducing the opening of roof vents and side walls limits the rate at which dry external air replaces the moister internal air, effectively trapping the humidity inside the structure.
Measuring and Maintaining Target Humidity Levels
Effective humidity management requires continuous monitoring to prevent the environment from becoming too dry or excessively saturated, which promotes fungal growth. The primary tool for this is the thermo-hygrometer, a device that measures both air temperature and relative humidity, providing real-time data. For automated control, a humidistat acts as a thermostat for moisture, automatically activating mechanical systems when the relative humidity drops below a pre-set threshold.
The target relative humidity must align with specific crop requirements, as ideal levels vary significantly based on plant type and growth stage. Delicate seedlings and tropical plants, such as orchids, thrive when humidity is maintained between 70 and 90 percent. Conversely, crops like tomatoes and vegetative-stage plants are best kept in the 50 to 70 percent range, while succulents and cacti require a drier environment, often below 50 percent. Regular monitoring and automated controls ensure the environment remains within the optimal range, balancing moisture retention with preventing condensation and disease.