There are many terms used to describe the moisture content of the air, including relative humidity (RH), absolute humidity (AH), and dewpoint temperature. In horticulture, vapor pressure deficit (VPD) is most commonly used, as it describes the relationship between water in the air and water at the surface of the leaf. VPD is the difference in water vapor pressure at a given air temperature and RH and the vapor pressure at the same air temperature when it is saturated with water (100% RH). This difference in vapor pressure is literally what “forces” water through the plant.
The majority of horticultural crops are C3 plants, which have evolved to thrive in temperate climates with a good source of water (as opposed to CAM plants, such as cactus, which evolved to thrive in hot and dry climates). Much research has been performed demonstrating that, in general, horticultural crops enjoy a VPD within the range of 0.65 to 1.25 kPa when well-watered. For vegetative plants, such as leafy greens, they tend to do better in the lower half of the range. Whereas fruiting and flowering plants, such as tomatoes and cannabis, tend to like the VPD in the higher range, especially when they are mature crops. When the VPD is lower than 0.65 kPa, stomata begin to close, restricting transpiration and CO2 assimilation. When the VPD is higher than 1.25 kPa, the plant will begin to stress and, in an effort to conserve water, will also close their stomata.
Many growers are afraid of “high” humidity levels, for fear of harboring molds and bacteria, and they will try to drive the RH as low as possible to avoid these pests. Unfortunately, this action can be counterproductive. Just like over-worked humans who have suppressed immune systems, plants that are water-stressed will be more susceptible to microscopic invaders and diseases. And just like your mom telling you to “drink lots of water” and “eat chicken soup,” the same prescription is good for plants. By properly managing the moist air environment and providing an adequate supply of water at the root zone, plants can freely absorb water and nutrients, build up their defense systems, and put more resources into carbon fixation, photosynthesis, and reproduction.
When it comes to managing humidity, plants are like Goldilocks: they like it “just right”; not too high and not too low.