Short answer. If the damage is only at the top, nearest the light, and lower leaves look fine, suspect light or heat before nutrients. Measure canopy PPFD and leaf temperature first. Fix by dimming or raising the light 10–20 %, bringing leaf temperature under ~30 °C (86 °F) [1], and making sure the plant can transpire (airflow, root zone, VPD).
How to recognise it
| Symptom | Where | Most likely cause |
|---|---|---|
| Leaf edges curl up, leaves "taco" or "canoe" | Top canopy, mid-day | Heat or high VPD; the plant is cutting transpiration |
| Leaves point straight up and stay that way all day | Top | Too much light (Grower practice sign) |
| Bleached white or pale yellow top leaves/bracts, veins green at first | Top colas closest to lamp | Light bleaching (photo-oxidation); worse with heat |
| Crispy brown spots on top leaves only | Top | Light/heat burn |
| New calyxes stacking on top of finished buds ("foxtailing") | Top colas | Heat/light stress in late flower, or genetics |
| Droop at lights-on, recovers later | Whole plant | Root zone (over/under-watering), not light |
Lower canopy healthy + top damaged = light/heat. Old leaves damaged first = check nutrients and pH instead.
What's happening
At high leaf temperature cannabis photosynthesis and water-use efficiency fall; in one gas-exchange study high temperature combined with high light reduced net photosynthesis [1]. When transpiration can't keep the leaf cool (dry air, still air, sick roots), leaf temperature can climb 8–12 °C (14–22 °F) above air [2]. Excess absorbed light then damages chlorophyll (bleaching). UV at high doses also reduced photosystem efficiency (Fv/Fm) in cannabis [4].
Note that the whole-plant yield response to light did not saturate at 1,800 µmol/m²/s in a well-run trial [3], so "too much light" usually means too much for the conditions: heat, low CO₂, root problems, or a cultivar that dislikes it.
Differential diagnosis
- Nitrogen toxicity: dark, glossy, clawed leaves throughout, not just at the top.
- Calcium/boron issues: spotting or distorted new growth across the plant, often with pH or low VPD.
- Windburn: damage on the fan side.
- Root zone: wilting that tracks irrigation, not light.
Numbers that matter
| Parameter | Target | Alarm |
|---|---|---|
| Leaf temperature (top) | 24–29 °C (75–84 °F) | > 31 °C (88 °F) |
| Canopy PPFD, ambient CO₂ | ≤ 1,000–1,200 µmol/m²/s | Bleaching at any level = too much for your conditions |
| Leaf VPD | 0.8–1.5 kPa | > 1.7 kPa |
| Light distance | Follow the fixture's PPFD map, not a rule of thumb |
How to fix it
- Dim 10–20 % or raise the light; re-measure PPFD at the tallest tops.
- Lower air temperature; move drivers out of the tent if possible.
- Add airflow across the top canopy (not directly onto colas).
- Check irrigation and roots; a plant that can't drink can't cool.
- Supercrop or tie down the tallest colas to even the canopy (HST).
- Bleached tissue won't recover. Judge by new growth.
How to confirm it worked
New leaves at the top are flat and green within 3–5 days, leaf temperature at the tallest tops stays within ~2 °C of air, and no new bleached bracts appear.
Common mistakes
- Flushing or changing nutrients for what is a light problem.
- One tall cola dictating the light height.
- Turning lights up in late flower "for density" just when foxtailing risk is highest.
Evidence
Temperature × light effects on cannabis gas exchange are Supported [1]. Leaf temperature physics is Supported [2]. Visual diagnostic patterns and foxtailing causes are Grower practice; foxtailing has no controlled study we know of.
