Plants are affected by humidity as a result of the concentration of water vapour in the air and the moisture content of the soil. The concentration of atmospheric humidity is usually expressed as 'relative' humidity (RH): the ratio of the amount of water in the air to the amount of water in the air at saturation point. This is expressed as a percentage.

The saturation point of air varies according to temperature; the warmer it is, the more water vapour it can hold. This means that for the same relative humidity, the air would contain more moisture at 80°C than it would at 60°C. Likewise, if air contains a certain amount of moisture, its relative humidity will drop as the temperature rises.

The majority of plants grow well at a relative humidity of between 65% and 80%. Growth is negatively affected below this value, whilst levels over 80% can encourage fungal diseases such as grey mould (botrytis). Transpiration is restricted at values above 90%; as this affects the movement of nutrients, this may lead to symptoms of deficiency.

However, there are a number of plants that thrive outside this range; xerophytes (plants such as cacti that thrive in dry conditions) and hygrophytes (plants such as mosses and ferns that enjoy moist conditions).

Humidity can be reduced in the greenhouse by ventilation, increasing the temperature or by using dehumidifiers. Humidity can be increased by damping down, overhead irrigation or misting and fogging systems.

Damping down is carried out to increase the humidity and lower the temperature in very hot, dry weather and can also help to discourage red spider mite. This may be done by simply wetting the greenhouse floors and staging.