Heat Stress

Heat stress is one of the most common preventable causes of cannabis crop damage, particularly in indoor grow rooms with inadequate cooling capacity and outdoor grows in hot summer climates. Cannabis evolved across a wide temperature range but performs optimally in the 68-78°F (20-26°C) zone, with absolute upper limits around 85°F (29°C) before serious physiological stress begins. Above this threshold, plants exhibit: upward leaf curling (the diagnostic "taco curl" or "praying" leaves) as the plant attempts to reduce sun exposure on leaf surfaces; closed stomata reducing CO2 absorption and halting photosynthesis; transpiration spike that can deplete root zone moisture rapidly; nutrient burn appearance from concentrated salts as water evaporates faster than it can be replaced; reduced terpene production as volatile terpenes evaporate from heat-stressed flowers; and during flowering, heat stress triggers airy "foxtail" bud formations and dramatically increases hermaphroditism risk (intersex bananas appearing in late flower). Indoor cooling solutions: AC mini-splits sized 1500 BTU per 100W of LED draw, sealed-room CO2 enrichment (allows higher temperatures up to 88°F since photosynthesis stays active), exhaust fans with thermostat control, and dehumidifiers to manage humidity rise from heat. Outdoor mitigation: shade cloth at 30-50% during peak afternoon hours, deep mulching to maintain soil temperature, careful watering schedules avoiding noon, and selecting heat-tolerant strains (most landraces from desert origins like Hindu Kush and Afghani). Recovery: heat-stressed plants can recover fully if temperatures normalize within 24-48 hours, but repeated heat events progressively compound damage and reduce final yield by 10-30%.