Short answer
Lower grow-tent humidity by removing the water your plants breathe out: exhaust more air if the room is drier, and if it isn't, run a dehumidifier sized to roughly the litres you irrigate per day. Aim for 40–50 % RH in late flower and keep it at or below 55 % at lights-off. Supported for the disease risk; sizing is Grower practice.
Why
Almost all irrigation water leaves the plant as vapour. Four big plants in a 1.2 × 1.2 m (4 × 4 ft) tent can add 6–8 L a day. That water has to go somewhere.
High humidity in flower is how bud rot and powdery mildew start [2][3]. In a 2025 cannabis study, roughly 78–98 % RH delayed flowering and lowered biomass and cannabinoids compared with 37–58 % RH [1].
When the lights go off, air cools and relative humidity rises even though no water was added. Surfaces near the dew point get wet [4].
The numbers
| Stage | RH target | Air temperature |
|---|---|---|
| Veg | 55–70 % | 24–28 °C (75–82 °F) |
| Flower weeks 1–4 | 50–60 % | 23–27 °C (73–81 °F) |
| Flower week 5 to harvest | 40–50 % | 20–25 °C (68–77 °F) |
| Lights-off in flower | ≤ 55 %, surfaces ≥ 3 °C (5 °F) above dew point | |
| Dehumidifier capacity | ≈ litres irrigated per day, at real conditions |
What to do
- Check the hygrometer first (salt test) and read it at canopy height.
- Raise exhaust speed if the room air is drier than the tent.
- Empty saucers and runoff trays; lid any reservoir.
- Thin dense interior leaves so air reaches the middle of the canopy ("tuck, don't pluck" first).
- Add a dehumidifier on a humidistat, inside or beside the tent, and keep it on through lights-off.
- Ramp temperature down over 30–60 minutes at lights-off.
Common mistakes
- Buying a dehumidifier on its 30 °C / 80 % RH rating; it removes far less at 24 °C / 50 %.
- Watching the daytime average and missing the lights-off spike.
- Turning fans off at night.
- Pulling "dry" air from a room that is just as humid as the tent.
Related
- Full article: Humidity by stage
- How do I raise the humidity in my grow tent?
- How do I check if my hygrometer is accurate?
- VPD explained
- Terms: dehumidifier · dew point
