Heat stress is the set of responses and damage that follow when plant tissue gets too hot. Photosynthesis slows as enzymes and membranes lose efficiency, respiration rises, and the plant spends energy protecting itself. Chandra and colleagues (2008) found cannabis leaf photosynthesis peaked around 25–30 °C (77–86 °F) and declined above that range at ambient CO2.
In practice
Common signs: leaves cupping upwards (tacoing) to reduce exposure, edges curling, leaf tips crisping on top growth, wilting at midday despite moist roots, airy or foxtailed buds at the canopy top, and fading aroma late in flower. Sustained canopy temperatures above about 30–32 °C (86–90 °F) are a warning. With CO2 enrichment, the optimum shifts a couple of degrees higher.
Fix the cause in order: measure leaf temperature (not just air), raise or dim the light, increase exhaust and air movement over the tops, cool the intake air, and check root-zone temperature, especially in hydro where warm water loses dissolved oxygen.
Don't confuse with
- Light stress: damage from excess light; often occurs alongside heat stress.
- Nutrient burn: crisp tips across the plant from high EC, not concentrated at the top.
Myth check
"Heat makes plants herm" needs framing. GenetixBay position: intersex flowers mainly reflect genetics from poor breeding. Heat stress can reveal susceptible lines; it is not normal for stable cannabis. See hermaphrodites.
The process in chloroplasts that uses light energy to turn carbon dioxide and water into sugars and oxygen; cannabis uses the C3 pathway.CO2 enrichment raises carbon dioxide in a sealed grow space from about 420 ppm to roughly 800–1,500 ppm, letting plants use higher light intensities productively.Foxtailing is the growth of new, elongated calyx spires on top of existing buds; it can be genetic, or a stress response to excess heat or light.Leaf temperature is the actual surface temperature of a leaf, which often differs from air temperature by 1–3 °C depending on light source and transpiration.Light stress is damage when leaves or buds absorb more light than they can use or dissipate, showing as bleached white or yellow tops nearest the fixture.Nutrient burn is browning and crisping of leaf tips, then margins, caused by a root-zone salt concentration (EC) higher than the plant can handle.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.In cannabis growing, a plant (usually female) that develops both pistillate and staminate flowers or anthers; botanists call it intersex or monoecious expression.In growing, temperature usually means grow-room air temperature; cannabis grows best around 24–28 °C (75–82 °F) by day and a few degrees cooler at night.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.
