Heat Stress
Definition: Heat stress happens when cannabis plants get hotter than they can cool themselves through transpiration, typically when canopy temperatures stay above about 85°F (29°C). Signs include leaf edges curling upward, leaves folding like tacos, drooping, and in flower, loose, foxtailing buds that can lose aroma as volatile terpenes evaporate.
How Heat Stress works
Plants cool themselves by releasing water vapor through tiny pores in their leaves. When the air is too hot or too dry, they close those pores to avoid dehydration, which also shuts down gas exchange and slows growth. Leaves then curl or fold to reduce the surface facing the light.
Comfortable ranges
- Vegetative, lights on: 70 to 85°F (21 to 29°C)
- Flowering, lights on: 68 to 80°F (20 to 27°C)
- Lights off: about 5 to 10°F cooler than lights on
- Root zone: under about 75°F (24°C)
Where it comes from
Indoors, the usual suspects are a light hung too close, a tent in a hot attic or garage, or an undersized exhaust fan. Outdoors, heat waves above 90°F (32°C) and black pots baking in full sun do the damage. Warm root zones are a hidden problem, especially in hydro, where warm water holds less oxygen and invites root rot.
Why flowering plants suffer most
Late-flower heat can cause buds to grow upward in thin spires, a pattern called foxtailing, and can drive off some of the aromatic compounds that give a strain its smell.
Cooling fixes
- Size the exhaust fan to replace the tent’s air every 1 to 3 minutes.
- Run lights at night when ambient temperatures are lower.
- Raise or dim the fixture, and keep oscillating fans moving air across the canopy.
- Outdoors, use light-colored or fabric pots, water early in the morning, and consider 30% shade cloth during extreme spells.
Watching VPD alongside temperature helps you catch dry, hot conditions before they cause visible damage.