Cannabis vegetative stage humidity level
Hold relative humidity at 60% to 70% at canopy height during vegetative growth. Air temperature: 75 F to 80 F. That pair puts vapor pressure deficit (VPD) near 0.9 to 1.1 kPa. In that band stomata stay open and the plant moves water and nutrients.
Three pairs that land in range: 75 F at 70% RH (0.89 kPa), 78 F at 65% RH (1.15 kPa), 80 F at 70% RH (1.05 kPa). Drop RH below 50% in veg and transpiration outruns root supply. Leaves curl at the edges, growth slows, and EC in the root zone climbs.
cannabis vegetative stage vs flowering
RH by week
- Week 1 to 2 after transplant: 65% to 75%
- Week 3 to 4: 60% to 70%
- Week 5 to flip: 55% to 65%
Late veg runs drier. Bud sites start to form before the light flip, and dense leaf stacks trap moisture. A 5 point drop in the last week cuts botrytis pressure when flowers appear.
ideal temperature for cannabis vegetative stage
VPD targets
- 0.8 kPa: rooted clones, first leaves
- 1.0 kPa: mid veg
- 1.2 kPa: late veg, plants over 18 in tall
VPD uses leaf temperature, not air temperature. Measure leaves with an infrared thermometer. Under LED at 1.0 kPa, leaf temperature runs 2 F to 5 F below air temperature. That offset shifts VPD by about 0.2 kPa.
Night hours
Lights off drops air temperature 5 F to 10 F. RH rises 5 to 10 points as the air cools. Keep night RH under 75% and run exhaust on a humidity controller. A 4x4 ft tent needs 30 to 50 CFM of exhaust at night.
Equipment and placement
- Hygrometer at canopy top, shaded, away from walls and wet media
- One oscillating fan per 4x4 ft, running 24 hours, aimed above the canopy
- Dehumidifier: 30 pint unit for 100 sq ft of canopy at 70% RH target
- Exhaust fan on a controller set to trip at 55% RH in late veg
Wall-mounted sensors read low. Media-level sensors read high. Both give errors of 5 to 10 points against a canopy reading.
Limits
Above 70% RH at 80 F, powdery mildew and botrytis risk climbs. Keep leaf wetness near zero and never foliar feed in late veg. Published cultivar-specific humidity optima for vegetative cannabis are thin. The 60% to 70% figure comes from general greenhouse and VPD practice, not from per-strain trials.