Growing guide
VPD in plain words: why humidity alone is not enough
Vapour pressure deficit combines temperature and humidity into one number that tells you how hard your plants are drinking. What it is, how to calculate it and which ranges to aim for.
Two rooms can both read 60% relative humidity and still behave very differently. At 20 °C the air is close to saturated for the plant; at 30 °C the same 60% pulls water out of the leaves much faster. VPD (vapour pressure deficit) captures that difference in one number.
What VPD measures
Warm air can hold more water vapour than cool air. VPD is the gap between how much moisture the air could hold at a given temperature and how much it holds now, measured in kilopascals (kPa). A bigger gap means the air pulls water from the leaves faster, so the plant transpires more.
- Too low (air nearly saturated): plants transpire slowly, take up fewer nutrients, and leaves stay wet — good conditions for mould and mildew.
- Too high (air very dry): plants close their stomata to save water, which also slows photosynthesis; tips can burn and growth stalls.
How to calculate it
Strictly, the leaf matters more than the air. Leaves are often 1–3 °C cooler than the air under LED lighting, which lowers the real VPD at the leaf. If you have an infrared thermometer, use leaf temperature for SVP at the leaf and air temperature for the air side.
Ranges to aim for
| Stage | Target VPD (kPa) |
|---|---|
| Propagation and clones | 0.4–0.8 |
| Early vegetative | 0.8–1.0 |
| Late vegetative and early flower | 1.0–1.2 |
| Mid to late flower | 1.2–1.5 |
Young plants with few roots need gentle conditions; mature plants can handle and use drier air. Late in flowering many growers run towards the top of the range to keep dense flowers free of mould.
Controlling VPD in practice
- Change one thing at a time: temperature moves VPD faster than humidity does.
- Keep air moving through the canopy. Still air around the leaves stays humid even if the room sensor reads fine.
- Watch the night. When lights go off, temperature drops and humidity jumps — that is when condensation and mould start.
- Place sensors at canopy height, not on a wall near the door.
G_Control calculates VPD continuously from its temperature and humidity sensor, records the history and raises an alarm when a room drifts out of range. In the Big G plus app you see the VPD of every room on one screen and get a push notification when one needs attention.
Planning a room?
Send us the room size and the crop. Our engineers will suggest a lighting and control layout that fits.
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