RELATIVE HUMIDITY IN GREENHOUSES
We have already talked about the importance of temperature inside a greenhouse, but today we would like to talk about another equally important factor that is closely related to temperature: relative humidity.
Relative humidity is the amount of water contained in the air, and is related to the absolute humidity (amount of water vapour per m3 of air) and the saturation point (maximum amount of water vapour that the air can hold at a given temperature).
Relative humidity in a greenhouse is a climatic factor that is related to transpiration, and is essential for gas exchange and photosynthesis to take place. Depending on the species, the most favourable humidity levels for crop development are between 50%-75% (see table), and deviating from these ideal humidity conditions can have adverse effects on crop yields.
Adverse effects on the plant if there is high relative humidity:
- Excess moisture reduces transpiration, which reduces nutrient uptake and affects plant growth.
- It also hinders pollination and favours the spread of pests and diseases.
Adverse effects on the plant if relative humidity is low:
- The lack of moisture means that the plant has to make an extra effort to keep the stomata open, and this can cause it to wilt due to dehydration.
- It also affects production, as there is a loss of fruit quality and reduced fruit set.