Heat stress: when the tent is running too hot

Most of this comes down to a thermometer. Get a reading at canopy height during the hottest part of the lights-on period before you change anything else.

What it looks like

Good descriptions are scarce. We didn’t find a controlled study that catalogs what heat stress looks like on cannabis leaves at the temperatures a home tent reaches.

• Wilting: in one hemp experiment at an extreme 45 to 50°C (113 to 122°F), plants were completely wilted within 3 days, whatever their water supply (Park et al. 2022). Most tents won’t get that hot.
• Abnormal flower formation: a 2025 Wageningen thesis reports that elevated temperatures reduced cannabinoid concentrations because flowers formed abnormally. Its effect on yield varied by genotype.
• Curling and crisping near the lamp: growers report this often, but we can’t point to a tested description. “Foxtailing” of buds is often blamed on heat too, and we couldn’t verify that either.

Why it happens

Hot leaves lose water faster, and photosynthesis suffers past a certain point. In leaf measurements, photosynthesis peaked at 30°C (86°F), and temperatures above that had an adverse effect on the leaf’s pores (stomatal conductance). Water loss kept rising up to 40°C (104°F) (Chandra et al. 2008).

That point differs by variety. Among three drug-type varieties, the best temperature for photosynthesis ranged from 25°C to 30 to 35°C (77°F to 86 to 95°F) (Chandra et al. 2011). So there’s no single danger line.

A tent usually copies the room around it, and lamps add heat. Our tent temperature guide goes through common causes.

How to confirm it

1. Measure at canopy height, at the hottest part of the day. A max/min thermometer shows you what happened while you were away.
2. Check leaf temperature with an infrared thermometer. It can differ from the air temperature.
3. Rule out water. Lift the pot. If the plant wilts in a moist pot and recovers when it’s cooler, or at lights-off, heat becomes more likely.
4. Look for a pattern. Damage nearest the lamp could be heat or light, so change one thing at a time.

What else it could be: overwatering, underwatering, root rot and salt buildup can all cause drooping. Check those too, because they’re easy to rule out.

What to do

1. Cool the room, not just the tent. Vent hot exhaust out of the room rather than back into it.
2. Improve air exchange and airflow across the canopy.
3. Run lights during cooler hours, such as overnight.
4. Dim or raise the lamp if the top of the plant is the hot spot, staying within the manufacturer’s limits.
5. Watch watering. Hot plants use more water, so check pots more often.
6. Don’t feed or spray to fix it. Neither one lowers the temperature.

For temperature ranges, our temperature and humidity guide lists common starting points. Those are grower ranges, not tested limits, so treat them as a guide.

How much to trust this

This page has real gaps. The leaf measurements tell us about photosynthesis, not damage or yield. They suggest trouble beyond about 30°C for many varieties, but they don’t show what room temperature is best for flower, and some varieties peaked above it. The extreme heat experiment was far hotter than a typical tent, and it also caused water stress, so its authors couldn’t fully separate the two. We only read the abstracts of the Chandra papers and the thesis.

The tent advice is practical, not tested. It’s low-risk, and a thermometer will show you whether it’s working.