Relative humidity (RH) measures the amount of water vapor in the air as a percentage of the maximum amount the air can hold at that specific temperature. This ratio is dynamic; if the temperature changes, the RH percentage will also change, even if the absolute amount of water vapor remains the same. Managing this balance is a significant part of successful greenhouse cultivation, placing humidity control alongside temperature and light as a foundational environmental variable. Precise regulation is necessary because the moisture content directly influences how a plant functions.
Understanding Humidity and Plant Transpiration
Air moisture directly influences transpiration, the process where water moves from the roots, through the plant’s vascular system, and evaporates from leaf pores called stomata. This constant flow delivers dissolved nutrients from the soil to the leaves. It also provides an evaporative cooling mechanism for the plant itself.
The speed at which a plant transpires is determined by the difference in moisture levels between the inside of the leaf and the surrounding air. This difference is called Vapor Pressure Deficit (VPD), which represents the air’s actual “drying power.” VPD is calculated from both air temperature and relative humidity, providing a more accurate indicator of plant stress than humidity alone. When the VPD is high, it signifies dry air, forcing the plant to transpire rapidly, which can lead to stress, wilting, and dehydration if water uptake cannot keep pace.
Conversely, a low VPD indicates a nearly saturated, high-humidity environment. In these conditions, the evaporative pull on the plant is significantly reduced, slowing transpiration to a crawl. This reduced water flow can hinder the uptake and distribution of essential nutrients like calcium, and it also compromises the plant’s ability to cool itself effectively. Therefore, managing greenhouse humidity is essentially managing the VPD to encourage a steady, moderate rate of transpiration.
Establishing Optimal Greenhouse Humidity Levels
While VPD offers the most scientific measure for climate control, the general question of “what should the humidity be?” can be answered with a healthy range of relative humidity percentages. For most common greenhouse crops in their vegetative growth stage, a relative humidity between 60% and 80% is considered the optimal environment. Within this range, the air is moist enough to prevent excessive water loss but dry enough to allow for steady transpiration and nutrient delivery.
The specific target level must be adjusted according to the plant’s current life stage, as its water needs change dramatically over time. Young seedlings, clones, and plants in the rooting stage lack a fully developed root system, making them highly susceptible to desiccation. For these delicate stages, a higher relative humidity, often exceeding 80%, is beneficial to reduce moisture loss until roots are established.
As plants mature and transition into the flowering or fruiting stage, the humidity level should be gradually lowered to the 60% to 70% range. This reduced moisture level encourages stronger transpiration, supporting the higher metabolic demands of producing fruit or flowers. Monitoring these levels requires a simple hygrometer and a thermometer; advanced growers use both readings to calculate the Vapor Pressure Deficit.
Active Management Techniques for Humidity Control
Managing greenhouse humidity involves a continuous cycle of measurement and adjustment, utilizing techniques to either add or remove moisture from the air. High relative humidity is a common issue that must be addressed to prevent the proliferation of fungal pathogens like Botrytis (gray mold) and powdery mildew, which thrive on wet leaf surfaces. The most effective method for decreasing humidity involves a combination of heating and ventilation.
Briefly raising the air temperature increases the air’s capacity to hold water vapor, immediately lowering the relative humidity percentage. This warm, moist air is then exchanged for drier outside air using exhaust fans or by opening vents, a process called “heating-and-venting.” Growers also reduce humidity by ensuring plants are spaced adequately to promote air circulation. Dedicated dehumidifiers condense water vapor into liquid and remove it from the atmosphere.
Conversely, when the air is too dry, typically falling below 50% RH, the focus shifts to increasing the moisture content. Low humidity accelerates transpiration, leading to water stress and a reduction in fruit or flower quality. Misting and fogging systems are a common solution, using high-pressure nozzles to release an ultra-fine spray of water that quickly evaporates and raises the humidity.
Wetting the greenhouse floor and benches is a low-tech method that utilizes the principle of evaporation to humidify the local environment. Evaporative cooling systems, such as fan-and-pad setups, also add significant moisture to the air as they simultaneously cool the greenhouse. The timing of watering is also a factor, as watering in the early morning allows the excess moisture to transpire and evaporate throughout the day, avoiding the dangerous combination of high humidity and falling nighttime temperatures.