Heat Stress

Heat stress is the plant losing water through its leaves faster than the roots can replace it. Cannabis wants 70 to 82 F with the lights on. Past about 85 F it starts protecting itself instead of growing.

What you are seeing#

The upper canopy first, because heat rises and the lamp is up there, but unlike light burn it affects shaded leaves too.

Leaf edges curl upward along the length of the blade, so the leaf folds into a taco shape. This is the plant reducing its exposed surface area. Tips then brown and crisp, working back from the point.

The whole plant droops in the last few hours of the light period and perks back up an hour or two after lights off. That daily rhythm is diagnostic on its own.

Leaves may yellow between the curls. In flower you get foxtailing, which is new spiky growth pushing out of an existing bud, plus loose airy buds and a harsh smell.

Why it happens#

At high leaf temperature the plant closes its stomata to conserve water, which stops carbon dioxide intake and therefore stops photosynthesis. Sustained heat also destroys terpenes in developing flowers, which is why late heat is so expensive.

Common causes:

• Light hung too close, so the lamp is both a light source and a heater.
• Exhaust fan undersized or not running, so hot air sits in the tent.
• Tent in an attic, garage or unconditioned room in summer.
• No air exchange, only a circulating fan. Moving hot air around does not cool it.

How to fix it#

1. Put a thermometer at canopy height, not floor level. Read it in the last hour of the light period, the daily peak.
2. If it reads above 85 F, raise the light 4 to 6 inches today and dim the fixture to 70 percent until it is back under 82 F.
3. Increase exhaust. A 4 inch inline fan should exchange the tent's full air volume every one to three minutes. If it is already maxed, that fan is too small.
4. Flip the light schedule so lights run overnight and off during the hottest hours. This is free and often solves a summer tent by itself.
5. Over the next week, aim for 75 to 82 F lights on in veg and 70 to 78 F in flower. The lower end in late flower preserves terpenes.
6. Do not respond to drooping by watering more. Heat drooping is not thirst, and extra water adds root problems on top of the heat.

How to tell it apart#

Versus light burn. Same neighborhood, different footprint. Light burn is confined to leaves directly under the lamp and bleaches them pale or white with no daily droop cycle. Heat stress affects the whole upper canopy including shaded leaves, curls edges upward, and produces the afternoon droop and overnight recovery. A thermometer at canopy height settles it in thirty seconds.

Versus underwatering. Both wilt. The discriminator is the pot, so lift it. An underwatered pot is very light and the wilt does not recover until you water. A heat stressed plant sits in a normally heavy pot and recovers on its own after lights off.

Versus overwatering. Overwatered leaves droop limp and downward with no edge curl, the pot stays heavy for days, and the symptom ignores the light cycle. Heat curls edges upward and follows the clock.

Versus wind burn. Wind burn also curls and crisps leaves, but only the ones sitting directly in a fan's airflow. If the damaged leaves face your clip fan, aim the fan at the tent wall instead.

How long until it looks better#

Leaves already tacoed and crisped mostly stay that way. Badly curled leaves may partially relax within a day or two of the temperature dropping, which is unusual and worth knowing, but browned tips are dead tissue.

Get the temperature right and the afternoon droop should disappear the same day, with new growth coming in flat within four to seven days. Damage to flowering buds, foxtailing and lost smell, is permanent, so your response speed matters more here than on any nutrient page.