Relative humidity (RH) is the ratio of the actual water vapour pressure in the air to the saturation vapour pressure at the same temperature, as a percentage. Because warm air can hold far more water than cold air, RH depends on temperature: warm a room without adding water and RH falls; cool it and RH rises, reaching 100% at the dew point.
In practice
Common ranges by stage:
| Stage |
RH |
| Clones |
70–85% |
| Seedlings and early veg |
60–70% |
| Late veg |
50–65% |
| Flower |
40–55% |
| Late flower |
40–50% |
High RH in flower raises the risk of bud rot and powdery mildew; Corredor-Perilla and colleagues (2025) reviewed the effects of high humidity in cannabis. Low RH with high temperature drives water loss faster than roots can supply.
RH alone is an incomplete target. 60% RH at 20 °C and 60% RH at 30 °C produce very different VPD, so experienced growers set RH from a VPD target and the current temperature. Cheap hygrometers can be off by 5% or more; check them against a calibrated reference or a two-salt test.
RH (%) = actual vapour pressure ÷ saturation vapour pressure × 100.
Don't confuse with
- Absolute humidity: grams of water per cubic metre of air.
- VPD: the drying power, in kPa.
- Jar RH in curing: a proxy for water activity, covered under harvest and cure.
Dew point is the temperature at which air becomes saturated and water starts condensing; when leaves or buds cool to it, free water forms and mould risk jumps.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.Botrytis cinerea, the fungus behind bud rot or grey mould, which rots dense colas from the inside in cool, humid conditions late in flower.A fungal disease, on cannabis caused mainly by Golovinomyces ambrosiae, that grows as white powdery patches on leaves and buds and thrives in moderate humidity.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.Absolute humidity is the actual mass of water vapour in a volume of air, in grams per cubic metre, independent of temperature.A 0-1 measure of how freely available the water in a material is; for dried cannabis flower the accepted range is about 0.55-0.65 aw.A dehumidifier removes water vapour from grow-room air, usually by condensing it on a cold coil; it is the main tool for controlling humidity in flower.A humidifier adds water vapour to the air to raise relative humidity, used mostly for clones, seedlings and veg, or dry climates.
