Modern LED grow lights are powerful — a 480W LED at 12 inches from the canopy delivers more photon energy than the plant can use, causing cellular damage that looks nothing like a nutrient problem. Light burn is one of the most misdiagnosed issues in cannabis growing because the symptoms (bleaching, yellowing, curling) overlap with several nutrient deficiencies.
The key diagnostic: light burn symptoms appear only on the leaves closest to the light, while the rest of the plant looks healthy. No nutrient deficiency is that selective about location.
Light burn vs. heat stress
These two problems often occur together (the light produces both photons and heat), but they have different causes and different fixes.
| Feature | Light burn (photon damage) | Heat stress |
|---|---|---|
| Cause | Too many photons hitting the leaf | Air temperature too high at the canopy |
| Primary symptom | Bleaching — tops turn pale yellow or white | Taco curling — leaf edges curl upward |
| Location | Only the top 2–4 inches of the canopy | Can affect the whole plant, worst at top |
| Leaf feel | Crispy, dry, sometimes papery | Dry but may not bleach |
| LED-specific | Yes — LEDs can light-burn without excessive heat | LEDs produce less heat than HPS at equal wattage |
| HPS-specific | Less common (HPS tends to heat-burn before light-burn) | Very common — HPS produces significant radiant heat |
Light burn symptoms
- Bleaching: the top leaves (closest to the light) turn pale yellow, then white. The bleaching starts at the leaf tips and progresses inward. Unlike nitrogen deficiency (which yellows from the bottom up), light burn yellows from the top down — only the leaves nearest the light
- No green recovery: bleached tissue is permanently damaged. The chloroplasts are destroyed. Unlike a nutrient deficiency that improves when corrected, bleached leaves don't green up
- Foxtailing: in flower, light burn causes buds to grow abnormal tower-like structures (foxtails) — new calyxes stacking on top of each other in thin columns instead of swelling outward. Some foxtailing is genetic, but widespread foxtailing on the top colas with none on the lower buds is light stress
Heat stress symptoms
- Taco curling: leaf edges curl upward, making the leaf look like a taco or canoe. This is the plant's attempt to reduce the leaf surface area exposed to heat
- Drooping in the heat: leaves droop during the hottest part of the light cycle and recover when temperatures drop
- Increased water consumption: the plant transpires faster to cool itself
- Fox-tailing and airy buds in flower
Optimal light distance
LED grow lights
LED hanging height varies by manufacturer and wattage. Use these as starting points, then adjust based on plant response:
| Light wattage | Seedling | Veg | Flower |
|---|---|---|---|
| 100W | 24–30" | 18–24" | 12–18" |
| 200–300W | 30–36" | 20–28" | 14–20" |
| 400–480W | 36–42" | 24–30" | 16–24" |
| 600W+ | 42–48" | 30–36" | 20–30" |
Use the dimmer. Most quality LEDs have a dimmer knob. Instead of adjusting height alone:
- Seedlings: 50% intensity, higher mounting
- Early veg: 60–75% intensity
- Late veg: 75–90% intensity
- Flower: 90–100% intensity
Measuring with a PAR meter
If you have a PAR/PPFD meter (or a phone app like Photone):
| Stage | Target PPFD (µmol/m²/s) |
|---|---|
| Seedling | 200–400 |
| Veg | 400–600 |
| Early flower | 600–800 |
| Peak flower | 800–1000 |
| Maximum (with CO₂) | 1000–1500 |
Above 1000 PPFD without supplemental CO₂, the plant can't photosynthesize faster — extra photons become damage, not growth. CO₂ supplementation raises this ceiling to ~1500 PPFD, but only if CO₂ levels are maintained at 1200–1500 PPM.
How to fix light burn
Step 1: Increase distance or reduce intensity
- Raise the light 4–6 inches above its current position
- Dim the light by 10–20%
- Do both if symptoms are severe
Step 2: Wait and assess
- Watch the plant for 3–5 days after adjustment
- Bleached leaves won't recover — judge improvement by NEW growth. New leaves should emerge green and healthy
- If new growth is still showing symptoms, raise or dim further
Step 3: Remove severely damaged tissue
- Leaves that are fully bleached (white) are no longer photosynthesizing and serve no purpose
- Remove them to redirect the plant's energy to healthy growth
- Don't remove green leaves that are merely at the top — only remove those that are permanently damaged
Special situations
Seedlings and young plants
Seedlings are extremely sensitive to light intensity. A light that's perfect for a mature plant will bleach a seedling in hours. Start seedlings at 50% dimmer and 30+ inches from the light. Gradually reduce distance and increase intensity over the first 2 weeks.
Post-transplant
Freshly transplanted plants are stressed. Reduce light intensity by 20% for 2–3 days after transplanting, then return to normal.
Autoflowers
Autoflowers get 18–20 hours of light per day, meaning the daily light integral (DLI) is higher than a 12/12 photoperiod plant at the same PPFD. Reduce intensity slightly compared to a 12/12 flower plant to compensate for the extra hours.
The center vs. edges
Most grow lights have a hot spot in the center of their footprint and drop off toward the edges. The center of the canopy directly under the light receives 20–40% more photons than the edges. If the center plants show stress while edge plants look healthy, raise the light or rotate plants regularly.
Common mistakes
- Assuming more light is always better — beyond the plant's photosynthetic capacity, additional light is pure damage. There is a ceiling, and exceeding it reduces quality and yield
- Blaming bleached tops on nitrogen deficiency — nitrogen deficiency yellows from the BOTTOM up. Light burn yellows from the TOP down. This distinction is diagnostic
- Not using the dimmer — many growers run their LED at 100% from day one because "I paid for all those watts." Dimming is not waste — it's precision. Use 100% only during peak flower when the plant can handle it
- Ignoring foxtailing — foxtails look exotic and some growers think they're a good sign. Light-stress foxtails produce airy, loose buds with reduced potency. If only the top colas foxtail, the light is too close
- Lowering the light to "fix" a leggy plant — stretchy growth is usually caused by insufficient light during veg. Suddenly lowering the light to compensate burns the canopy. Instead, gradually reduce distance over several days
For light selection and setup, see our LED lighting guide. For environmental management during flower, see our VPD guide. For DLI calculation, visit our grow tools.