Vapor pressure deficit (VPD) is the primary driver of evapotranspiration inside the indoor plant environment. VPD is a measure of the amount of water in the air versus the maximum about of water the air can hold at a given temperature (the saturated condition). VPD is measured in units of pressure, typically in kilopascals (kPa), but may also be reported as bars, millibars, inches of Hg, or other pressure units.
In general, a low VPD means the air is "humid" and a high VPD indicates the air is "dry." VPD is directly responsible for the stomatal opening of plant leaves. If VPD is too high ("dry" air), stomata will close to conserve water. If the VPD is too low ("humid" air), stomata may be fully open, but evapotranspiration will be slow since the difference in water vapor in the air versus at the leaf is very small. Both conditions can cause leaf wilting, leaf tip burn, and other crop maladies, including susceptibility to pests and molds. On the other hand, when VPD is managed correctly, plants will transpire freely, moving nutrients readily to cells and maximizing CO2 uptake and photosynthesis.
The correct VPD level depends on the type of crop, as well as the stage of crop for fruiting and flowering plants.
Use our VPD calculator on this website to see what VPD levels are right for your crop.
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