What VPD measures
The vapor pressure deficit (VPD) is the difference between the amount of water vapor the air could hold when saturated and the amount it actually holds. Unlike relative humidity, it tells you directly how strongly the air pulls water out of leaves. Growers use it to manage transpiration, nutrient uptake and disease risk in greenhouses and grow rooms.
Formula
The saturation vapor pressure, in kilopascals, is estimated with the Tetens equation:
SVP = 0.6108 × e^(17.27 × T ÷ (T + 237.3)), with T in °C
- Actual vapor pressure: AVP = SVP(air) × RH ÷ 100.
- Air VPD = SVP(air) − AVP.
- Leaf VPD = SVP(leaf) − AVP, using the leaf temperature.
Leaves are often slightly cooler than the air because transpiration cools them, so the leaf VPD is usually a little lower than the air VPD. Under high-intensity lamps, leaves can instead be warmer than the air.
Example
At 77 °F (25 °C) and 60% relative humidity, SVP ≈ 3.17 kPa and AVP ≈ 1.90 kPa, so the air VPD is about 1.27 kPa. With leaves 4 °F cooler, the leaf VPD is about 0.87 kPa.
Target ranges
| Leaf VPD | Situation |
|---|---|
| under 0.4 kPa | too humid: mold, mildew and weak roots |
| 0.4 to 0.8 kPa | seedlings, cuttings and clones |
| 0.8 to 1.2 kPa | vegetative growth |
| 1.2 to 1.6 kPa | flowering and fruiting |
| over 1.6 kPa | dry air: stomata close, growth slows and plants may wilt |
These are general ranges; tropical foliage plants prefer the lower end and many fruiting crops tolerate higher values.
How to adjust VPD
- VPD too high: raise humidity with a humidifier or misting, lower the temperature, or reduce ventilation with dry outside air.
- VPD too low: run a dehumidifier, raise the temperature slightly, improve air circulation and avoid watering late in the day.
Measuring tips
Place the thermometer and hygrometer at canopy height, shaded from lamps. An infrared thermometer pointed at the leaves gives the leaf temperature. Check VPD at both the start and the end of the light period, when humidity often changes the most.