How far the air sits below saturation
VPD is the gap between the moisture the air holds and the most it could hold at that temperature. You get it from air temperature and relative humidity: compute saturation pressure, take the fraction the air is missing.
SVP is the saturation vapor pressure at the leaf or air temperature, from the Tetens formula. Warmer air has a higher SVP, so at the same humidity a warm room runs a higher VPD than a cool one.
Target range by growth stage
| Stage | VPD (kPa) | Typical RH |
|---|---|---|
| Clones / seedlings | 0.4 – 0.8 | 70 – 85% |
| Vegetative | 0.8 – 1.2 | 60 – 70% |
| Early flower | 1.0 – 1.4 | 50 – 60% |
| Late flower | 1.2 – 1.6 | 40 – 55% |
Young plants have few roots and little water uptake, so they want low VPD and humid air. The deficit rises as the plant matures.
Too low and too high both cost you
Below about 0.4 kPa transpiration stalls and gray mold and mildew take hold; above about 1.6 kPa the stomata shut and growth halts. The productive band sits in the middle.
- Measure at the leaf. Leaves in direct light run a few degrees warmer than the air, which raises their real VPD above the room reading.
- Sensor error matters. A hygrometer off by 10 percent RH shifts VPD by about 0.15 kPa, enough to move you out of the target band.
Common questions
What is VPD in a grow room?
Vapor pressure deficit is how much drier the air is than fully saturated air, in kilopascals. It drives how fast plants transpire. Low VPD means humid, stagnant air that risks mold; high VPD means dry air that closes the stomata and stalls growth.
What VPD should I aim for?
Roughly 0.4 to 0.8 kPa for seedlings and clones, 0.8 to 1.2 kPa in vegetative growth, and 1.2 to 1.6 kPa in late flower. Higher VPD later dries the canopy and lowers gray-mold risk on dense flowers.
How do I raise VPD without cooling the room?
Dry the air. Run a dehumidifier, add airflow, or vent more, which lowers relative humidity and raises VPD. Warming the room also raises VPD because warmer air holds more moisture, so its saturation pressure climbs.
