Grow science · 06
Day vs. night VPD
Your plants don’t sleep, but their stomata do. The night needs its own numbers.
Why the night is different
In the dark there’s no photosynthesis, so most stomata close. Transpiration drops sharply – and with it the plant’s need for a high VPD. At the same time the air cools and humidity rises on its own (see dew point & mold).
Targets for lights-off
| Stage | Day VPD | Night VPD |
|---|---|---|
| Clones & seedlings | 0.4–0.8 | 0.4–0.6 |
| Veg | 0.8–1.2 | 0.6–1.0 |
| Early flower | 1.0–1.4 | 0.8–1.2 |
| Late flower | 1.2–1.6 | 1.0–1.4 |
kPa, typical guideline values. The night value can sit a little lower – but never so low that humidity reaches the dew point.
The real risk is too low, not too high
At night nobody complains about stress – the problem is moisture. A night VPD under ~0.6 kPa in flower means humidity well above 70 %, with almost no transpiration to move water through the plant. That’s when bud rot and powdery mildew get their chance.
Practical setup
- Program a smaller temperature drop at night (2–4 °C).
- Run the dehumidifier on a night schedule or with a humidity target of ~50–55 %.
- Keep exhaust running – just slower, so it doesn’t pull in cold, humid air.
- Check your numbers in the first hour after lights-off: dew point calculator.