Heat stress happens when the air gets too hot — generally past 85°F (30°C) — and your plant starts losing water through her leaves faster than the roots can keep up. The cellular machinery that drives runs sluggishly at those temps, and the leaves curl upward in self-defense, trying to shrink the surface area exposed to all that heat. The classic look is "tacoing" or "canoeing," depending on how far the curl progresses.
Heat stress comes from some combination of high ambient temperature, weak airflow, a struggling exhaust setup, or low humidity (which makes water loss through even faster). It often happens alongside because high-output lights radiate heat, but the two are diagnostically distinct: heat curls and crisps the upper canopy, while light burn bleaches it. If she's flowering, sustained heat can trigger , weaken potency, and produce loose airy bud structure — terpenes are heat-volatile and start degrading above 85°F.
Solutions
✓Reduce temperatures to 70-85°F (21-30°C)
✓Increase air circulation with oscillating fans
✓Add AC, swamp cooler, or improve exhaust ventilation
✓Run lights during cooler night hours
✓Water more frequently in hot conditions
✓Consider silica supplements for heat tolerance
Plant Overview
How it appears across the plant
Heat stress shows up across the whole upper rather than on one leaf or one side of the plant. The whole top section looks like it's curling upward uniformly. Because the upper canopy sits closest to both the heat and the lights, that's where the damage stacks up; the lower leaves stay flat and healthy until things get really severe.
Early. The upper leaves start tacoing — edges lifting upward — with the serrated tips curling first before the full taco shape sets in. The leaves angle up toward the light. She'll drink water visibly faster than normal; pots dry out quicker than expected. There may be some slight wilting during the hottest hours of the day that resolves overnight.
Mid. The taco shape intensifies. Upper leaves fold close to half rather than just curling at the edges. The tips and edges on the worst leaves start browning. Yellowing creeps inward from the tips. If she's flowering, the topmost buds start showing the first signs of foxtailing.
Late. Severe tacoing — leaves folded nearly in half. The edges are dry, crispy, and brown across the whole upper canopy. Yellowing has moved several centimeters inward from the tips. The buds turn loose and airy, foxtailing gets pronounced, and the plant may start shedding upper leaves outright. Yield and potency take a real hit.
Early — how it first appears on a leaf
On a single upper-canopy leaf in the early stage, what you see is structure, not color. The serrated tips lift first, curling upward, and then the leaf edges follow into a shallow taco shape. The leaf overall angles upward toward the light. Color stays normal green. The feels firm. There may be a faint droop during the hottest part of the day that disappears once the lights go off and temperatures drop. Lower leaves on the same plant lie flat and unaffected — the contrast tells the story.
Mid — when damage starts to set in
By the mid stage the leaf is folded close to half — not just edge-curled but a real pronounced taco, with the two halves of the lifted nearly vertical. Browning shows up at the tips and along the upper edges, and importantly that brown is dry, not wet. That's a key tell against nutrient deficiencies, which tend to produce wet-looking . Yellowing creeps inward from the tips. The surface feels dry and slightly leathery. The tissue closest to the and midrib still holds green and structural integrity.
Late — past the point of recovery
In the late stage the leaf is severely tacoed — folded almost in half and rigid — with crispy brown edges that crumble at the slightest contact. The yellowing has advanced from the tips and edges several centimeters inward across the , leaving only the center strip near the midrib still green. The leaf is fragile and dry, and it may shed entirely if brushed. A leaf this far gone doesn't come back — but get your temps back into a livable range and you'll keep the rest of the plant from following it.
Commonly Confused With
Light Burn
Often co-occur but diagnostically distinct. Heat stress causes curling and structural deformation. Light burn causes bleaching and color change. Curling = heat, bleaching = light.
Both cause wilting. Heat stress produces edge tacoing on the upper canopy. Underwatering produces whole-leaf droop throughout the plant that perks up quickly after watering.
Both cause leaf curling but in opposite directions. Heat stress curls leaves UP with normal or yellowing color. Nitrogen toxicity curls leaves DOWN with dark green waxy coloring.
