Vapor Pressure & VPD for Agriculture

VPD combines temperature and humidity into a single measure of how strongly the air pulls moisture out of a leaf. The same relative humidity is far more drying at 30 °C than at 18 °C; VPD captures that, so it predicts transpiration, stomatal behavior and nutrient uptake better than humidity alone. That is why greenhouse and indoor growers tune to a VPD target rather than an RH number.

As a rule of thumb: roughly 0.4–0.8 kPa for propagation and young plants, 0.8–1.2 kPa during vegetative growth, and 1.2–1.6 kPa during flowering or fruiting. Too low and transpiration stalls and disease pressure rises; too high and plants close their stomata and growth slows. Optimal values vary by crop, so treat these as starting points and the calculator above as the way to hit them.

When a leaf or fruit surface cools to the dew point — overnight, or under cold glass — water condenses on it. Sustained leaf wetness is what many fungal and bacterial diseases need to establish, so the dew point is a practical disease-risk indicator. Below freezing the relevant threshold is the frost point, computed over ice.

Transpiration cools leaves a couple of degrees below the surrounding air, and VPD should be evaluated at the leaf surface where evaporation happens. The calculator includes a leaf-temperature offset (a default of −2 °C is common) so the VPD reflects what the plant actually experiences; set it to 0 for the plain air VPD.