Section 3: Humidity
Because of evaporation of water from floors, root substrates and other surfaces as well as transpiration (loss of water from plant leaves), the relative humidity in a greenhouse is often high, especially when vents are closed. High humidity promotes the development of certain diseases (i.e. black spot, powdery mildew) as well as various physiological abnormalities (i.e. leaf edge burn in poinsettia and blossom end rot of tomatoes caused by a calcium deficiency induced by high relative humidity) in some greenhouse crops. Additionally, high humidity can increase condensation on the inside of the glazing, thus reducing light levels and causing water to drip onto plants.
During the summer, vents are usually open and the ambient relative humidity out-of-doors is the humidity at which the greenhouse will be maintained (although the relative humidity in the greenhouse may still be somewhat higher than that outside due to evapotranspiration). However, during cool months when vents are closed, very high relative humidities can occur inside of the greenhouse. To control the relative humidity, greenhouse managers may periodically vent the warm saturated air out of the greenhouse. Additionally, horizontal airflow fans may be used to circulate air within the greenhouse which helps to reduce the effective relative humidity experienced by the plant by reducing the boundary layer around the plant surfaces.
In some cases, such as propagation houses and seed germination chambers, it may be desirable to increase the relative humidity level. For propagation, mist, irrigation booms or fog systems (see the "Irrigation and Water Quality" learning unit for more information) are most often used to increase the relative humidity and reduce water loss from cuttings by transpiration. If seed are being germinated, fog or a fine mist may be used. Large water droplets should be avoided as they result in splashing and may displace seed. In germination chambers, fog systems are usually used to maintain a 100% relative humidity level without actually applying additional water to the root substrate in the plug trays. This allows for a more even moisture level to be maintained and reduces the amount of free water that accumulates of the floor.
