Short answer
Raise grow-tent humidity by holding on to the water your plants transpire: turn the exhaust down first, then add a humidifier on a humidistat. Aim for 65–75 % RH for seedlings, 55–70 % in veg and 50–60 % in early flower. Grower practice, built on VPD physics.
Why
Relative humidity sets how hard the air pulls water from the leaf, the vapour pressure deficit [1]. Dry air at normal temperatures means a high VPD: stomata partly close, CO₂ uptake drops and leaf edges crisp.
Seedlings and fresh clones have few roots. In dry air they lose water faster than they can replace it, which is why young plants get the highest RH.
Under LEDs, leaves often sit 1–3 °C (2–5 °F) below air temperature [2], so the leaf feels less dry than an air reading suggests. Read leaf temperature before chasing a number.
Don't overshoot. Damp air and wet surfaces in flower invite Botrytis and powdery mildew [3].
The numbers
| Stage | RH target | Air temperature |
|---|---|---|
| Unrooted clones (dome) | 75–90 % | 23–26 °C (73–79 °F) |
| Seedlings | 65–75 % | 23–26 °C (73–79 °F) |
| 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) |
What to do
- Check the hygrometer is accurate and sits at canopy height.
- Slow the exhaust fan, keeping temperature in range.
- Cool the room a degree or two if it runs hot; warm air reads drier.
- For seedlings, use a vented dome or group the pots.
- Add a humidifier with a humidistat, low in the tent, away from lights and plugs.
- Use RO or distilled water in it if your tap water is hard.
Common mistakes
- Misting plants instead of humidifying the air.
- Running the exhaust flat out in a dry winter room.
- A humidifier with no controller, soaking the tent overnight.
- Raising RH in late flower, when 40–50 % is the goal.
Related
- Full article: Humidity by stage
- How do I lower the humidity in my grow tent?
- Should I mist my plants?
- VPD explained
- Terms: humidifier · leaf VPD
