Recognizing and fixing heat stress in cannabis

Heat stress refers to damage to metabolism and tissue when leaf temperature rises persistently above the physiological optimal range.

What matters is leaf temperature, not air temperature: under strong LED/HPS radiation, the leaf can be several degrees warmer than the room air.

Above the temperature optimum, net photosynthesis declines because photorespiration increases and enzymes (including Rubisco activase) lose efficiency.

If the transpiration demand (high VPD) exceeds the water supply, the stomata close — evaporative cooling collapses and the leaf continues to heat up.

The leaf curls its edges upward (tacoing) to reduce the irradiated surface area, and stands upright ('praying') — both are active protective responses.

Under the lamp, the topmost leaves closest to the light show symptoms first; with sustained load, marginal, pale to necrotic burns follow.

Stage 1 (mild): Slight upright posture of the leaves, beginning tacoing at the tips closest to the lamp, recovery in the evening.

Stage 2 (moderate): Persistent tacoing, pale edges, bleached spots directly under the lamp, inhibited stretch growth.

Stage 3 (severe): Marginal necrosis, bleached 'foxtails' during flowering, loose buds, and loss of aroma/yield.

1. Lamp too close / PPFD too high: radiant load exceeds the leaf's cooling capacity.

2. Air temperature too high: weak exhaust/intake air, hot ambient room, summer operation.

3. VPD too high: dry, hot air forces stomatal closure despite moist substrate.

4. Weak air movement: heat pockets above the canopy without air circulation.

Step 1: Do the edges curl upward (tacoing) and do the leaves stand upright? Yes → check heat/VPD instead of nutrients.

Step 2: Are only the leaves closest to the lamp/topmost leaves affected? Yes → radiant/temperature load likely.

Step 3: Measure air temperature and VPD at the canopy. Temp > 30 °C or VPD > 1.6 kPa → heat stress confirmed.

Step 4: Rule out root problems (moist substrate but wilting) — drought stress can look similar.

1. Reduce radiant load: hang the lamp higher or dim it until the tacoing subsides.

2. Control temperature: increase exhaust air, use cooler intake air, shift the light phase into the cooler hours.

3. Bring VPD into the window: if VPD is too high, raise the relative humidity so the stomata reopen.

4. Move air: set air circulation so the canopy is evenly surrounded by airflow without causing windburn.

Define a climate corridor (temp 24–28 °C, VPD 1.0–1.5 kPa) and monitor it at the canopy, not at the tent wall.

Plan for summer heat waves: night-time light phase, stronger exhaust, air conditioning if needed.

Increase PPFD gradually and observe leaf behavior instead of going straight to maximum.

Heat plus high VPD intensifies water demand and can trigger a secondary calcium deficiency (transpiration-dependent).

Persistently high temperatures also favor spider mites, which love warm, dry conditions.

Confusing tacoing (upward) with the 'claw' of a nitrogen (N) excess (downward) and reacting incorrectly.

Only lowering the temperature but overlooking the high VPD — the stomata stay closed.

Fertilizing against the presumed 'burn' when it is actually radiant/heat stress.

An infrared thermometer on the leaf shows the true leaf temperature and exposes heat pockets the room sensor doesn't see.

In CO2-supplemented rooms, the temperature optimum is somewhat higher — heat and CO2 strategy must be planned together.