Vapour pressure deficit (VPD) is the difference between the saturation vapour pressure of air at a given temperature and the actual vapour pressure, in kilopascals (kPa). It measures the drying power of the air. High VPD pulls water from leaves quickly; low VPD slows transpiration, nutrient flow and drying of surfaces. It combines temperature and humidity into one number that tracks plant water use far better than RH alone.
In practice
Common targets (leaf VPD):
| Stage | kPa |
|---|---|
| Clones | 0.4–0.8 |
| Veg | 0.8–1.2 |
| Flower | 1.0–1.5 |
| Late flower | 1.2–1.6 |
Too low (below about 0.4 kPa in flower) invites mould and soft growth. Too high (above about 1.6–1.8 kPa) can cause stomata to close, stalling photosynthesis, and leaves to curl.
Units and formula
Saturation vapour pressure (Tetens, kPa): es = 0.6108 × exp(17.27 × T ÷ (T + 237.3)), T in °C.
- Air VPD = es(air) × (1 − RH ÷ 100)
- Leaf VPD = es(leaf) − es(air) × RH ÷ 100
Example: 26 °C air at 60% RH gives an air VPD of about 1.35 kPa. If the leaf is 24 °C, leaf VPD is about 1.0 kPa.
Don't confuse with
- Leaf VPD: uses leaf temperature; what the plant actually experiences.
- Relative humidity: a percentage that means different things at different temperatures.
