Relative Humidity (RH)
Definition: Relative humidity (RH) is the amount of water vapor in the air expressed as a percentage of the maximum the air could hold at its current temperature. Because warm air holds more moisture than cool air, the same grow room can read 50% RH with lights on and well over 65% after lights turn off.
How Relative Humidity (RH) works
Cannabis wants different humidity at different ages. Young plants with small root systems take in some moisture through their leaves and lose water slowly in humid air. Flowering plants with dense buds need drier air so moisture doesn’t collect inside the colas.
Common RH targets
- Seedlings and clones: 65 to 75%
- Vegetative growth: 50 to 70%
- Flowering: 40 to 50%
- Final 2 weeks of flower: 35 to 45%
- Drying room: about 55 to 62% at around 60°F (16°C)
The lights-off spike
When lights shut off, the temperature drops and the air’s capacity to hold water shrinks, so RH jumps even though no water was added. Plants also keep releasing moisture. That nightly spike is when bud rot and powdery mildew get their start, so many growers set dehumidifiers to work hardest during the dark period.
Measuring and controlling it
Place a digital hygrometer at canopy height, not on the floor or near the exhaust. Raise RH with a humidifier or by adding wet media to the space; lower it with a dehumidifier rated by pints per day and good air exchange. Because temperature and RH work together, many growers track VPD instead of chasing humidity alone.
When you hang your harvest, RH becomes the key setting for drying. Too low and buds crisp in days; too high and mold can set in.