Humidity is very important for plant health. Plants contain a large percentage of water. Water is also involved in photosynthesis, most of it is released from the plant during transpiration. Water enters the plant through the roots, and evaporates through the leaves – this process cools the plant. The level of relative humidity affects the rate of water flow through the plant: the higher it is, the slower evaporation occurs. Low relative humidity causes a stressful state of the plant, forcing it to spend too much energy transferring water through its tissues into the air. And too high humidity will not allow it to cool normally, because the evaporation process will be slowed down (and at 100% humidity it will be stopped altogether).
Different plants have different humidity control standards, today we’re talking about the general vegetable case: At the stage of seedling, the humidity level should be quite high - about 80% or more； at the vegetative stage of growth, when the plant has 2-3 pairs of leaves and the root system has already taken shape, the favorable level of relative humidity is 60-70%.
If environmental changes rapidly, it can cause damage to the plant. An increase or decrease in relative humidity by 20% within a few minutes can cause damage to the plant, during this time it does not have time to adapt. A rapid decrease in relative humidity can be caused, for example, by the sudden entry of dry air into a greenhouse opened for cooling purposes. A rapid increase in humidity can cause in the event of a sharp drop in temperature at night. If the plant before that quickly changes absorbed water, it will continue to absorb water after changing, because plant reaction to the environment is always slow. The moisture received by the plant after the relative humidity has already risen cannot be released into the air as freely through the leaves as under normal conditions, and instead will be pumped into the foliage and fruits, causing cell damage again.
Humidity in the greenhouse can be increased by changing of irrigation strategy, using the high-pressure system, wet pad system and so on. It can be decreased by using roof and sidewall window ventilation, heating system, or fans.