# Heat stress

> Heat stress is damage or slowed growth when leaf or canopy temperature rises above what the plant can handle, typically above about 30–32 °C (86–90 °F) in cannabis.

- Canonical: https://genetixbay.com/knowledge/glossary/heat-stress.html
- Also written: heat stressed, heat damage, high temperature stress, heat burn, too hot
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Heat stress is the set of responses and damage that follow when plant tissue gets too hot. [Photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) 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](https://genetixbay.com/knowledge/glossary/co2-enrichment.md).

## In practice

Common signs: leaves cupping upwards ([tacoing](https://genetixbay.com/knowledge/glossary/taco-leaf.md)) to reduce exposure, edges curling, leaf tips crisping on top growth, wilting at midday despite moist roots, airy or [foxtailed buds](https://genetixbay.com/knowledge/glossary/foxtailing.md) at the canopy top, and fading aroma late in flower. Sustained [canopy temperatures](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) 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](https://genetixbay.com/knowledge/glossary/ec.md), not concentrated at the top.

## Myth check

"Heat makes plants [herm](https://genetixbay.com/knowledge/glossary/hermaphrodite.md)" 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](https://genetixbay.com/knowledge/diagnosis/hermaphrodites.md).

## Related terms

[Temperature](https://genetixbay.com/knowledge/glossary/temperature.md), [Leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md), [Light stress](https://genetixbay.com/knowledge/glossary/light-stress.md), [Foxtailing](https://genetixbay.com/knowledge/glossary/foxtailing.md), [VPD](https://genetixbay.com/knowledge/glossary/vpd.md)

## Read more

- [Heat and light stress: taco leaves, bleaching and foxtails](https://genetixbay.com/knowledge/environment/heat-light-stress.md)
- [Temperature by stage: day, night and root-zone targets](https://genetixbay.com/knowledge/environment/temperature-by-stage.md)

## Sources

1. Chandra S, Lata H, Khan IA, ElSohly MA, 2008. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants 14(4):299–306. https://doi.org/10.1007/s12298-008-0027-x
2. Nelson JA, Bugbee B, 2015. Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS ONE 10(10): e0138930. https://pmc.ncbi.nlm.nih.gov/articles/PMC4598144/