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.
Cannabis Vegetative Stage Yellowing Leaves: Causes and Fixes
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 humidity level
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.
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.
cannabis vegetative stage leaves curling
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.