The difference between the moisture the air can hold when saturated and the amount it currently holds. A key parameter for optimising plant transpiration.
Vapour Pressure Deficit (VPD) is the difference between the saturated vapour pressure and the actual vapour pressure of air at a given temperature and humidity. It is expressed in kiloPascals (kPa) and is one of the most important environmental parameters in controlled growing environments.
VPD directly drives plant transpiration - when VPD is high, plants transpire more rapidly to cool their leaves, pulling water and dissolved nutrients up through the root system. When VPD is too low, transpiration slows, reducing nutrient uptake and creating conditions favourable for fungal diseases.
Ideal VPD ranges by growth stage:
VPD is determined by both air temperature and relative humidity. Most growers monitor and adjust these two variables together to maintain the target VPD for each growth stage.
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