VPD (Vapor Pressure Deficit)
Definition: Vapor pressure deficit (VPD) is the difference between how much moisture the air could hold at its current temperature and how much it actually holds, measured in kilopascals (kPa). It combines temperature and humidity into one number that predicts how fast cannabis leaves transpire water and pull nutrients up from the roots.
How VPD (Vapor Pressure Deficit) works
Humidity alone doesn’t tell the whole story. Air at 60% relative humidity feels very different to a plant at 68°F than at 85°F, because warm air can hold far more water. VPD captures that by measuring the drying power of the air.
Why growers watch it
- VPD too low (humid, cool): leaves transpire slowly, nutrient uptake drags, and moisture lingers in dense buds, raising the risk of bud rot.
- VPD too high (hot, dry): plants close their stomata to save water, growth slows, and leaf edges can curl or crisp.
Typical targets
| Stage | Target VPD | Example air conditions |
|---|---|---|
| Seedlings and clones | 0.4 to 0.8 kPa | 75°F (24°C) at 75% RH, about 0.75 kPa |
| Vegetative | 0.8 to 1.2 kPa | 77°F (25°C) at 65% RH, about 1.1 kPa |
| Early flower | 1.0 to 1.4 kPa | 77°F (25°C) at 60% RH, about 1.3 kPa |
| Late flower | 1.2 to 1.6 kPa | 72°F (22°C) at 45% RH, about 1.5 kPa |
These are air VPD values. Leaves under LED lights often run 2 to 4°F cooler than the air, which lowers the true leaf VPD a little, so an infrared thermometer pointed at the canopy gives a more accurate picture. You don’t need to hit an exact number; staying inside the range for each stage matters more than chasing decimals.