Is humidity important? How can/should I try to control it?
Proper greenhouse humidity is important both in preventing plant diseases and promoting healthy plant growth. High humidity can promote the Botrytis and other fungal diseases. High humidity also restricts plant transpiration, which in turn limits evaporative leaf cooling and can lead to overheating of plant foliage. If high humidity persists for a long time, the restriction of transpiration can limit the "transpiration stream" of nutrients and can lead to nutrient deficiencies.
Low humidity is best avoided because it may increase foliar transpiration to the extent that the root system cannot keep up.
Humidity is most accurately measured in terms of the "vapor pressure deficit" or VPD. This can be thought of as the drying power of the greenhouse air. However, most people measure humidity as relative humidity. Charts and programs are available to convert relative humidity to VPD.
Humidity is perhaps the most difficult of the greenhouse conditions to control. Most growers simply aim to avoid the extremes of humidity. Over most temperature ranges, a GH humidity of 50-85% is generally safe.
Low humidity can be managed with misters and foggers. It is also useful to shade plants under conditions of low humidity to reduce the rate of transpiration.
Transpiring plants add water vapor to the greenhouse air, increasing the humidity inside the greenhouse. Therefore, managing high humidity starts with ventilation...replacing warmer, humid greenhouse air with cooler, drier outside air. Ventilation also involves significant energy losses, and therefore ventilation often must be accompanied by heating. Therefore, lowering greenhouse humidity with a combination of ventilation and heating can cost a lot of energy.
Information provided by: Stressbaby