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Light Burn and Bleaching: Too Close, Too Bright, or Too Hot?
By the Cultivar grow team · published 2026-08-04 · updated 2026-08-04 · how we source these numbers
Light burn is a modern problem. Older lamps ran out of heat tolerance long before they ran out of intensity, so growers hit a temperature ceiling first. Efficient LEDs changed that: it is now entirely possible to deliver more photons than a plant can process while the tent sits at a perfectly comfortable temperature.
The plant's response is bleaching — the tissue closest to the lamp loses pigment and fades toward pale yellow or white.
Identifying it
Light burn has a strong spatial signature. It follows the lamp, not the plant.
- Only the top canopy is affected. Leaves 30cm lower are fine.
- Damage maps to the fixture's footprint — worst directly beneath, tapering toward the edges.
- Leaves go pale, yellow or bleached white, rather than brown and crispy.
- Affected leaves often stay firm and upright, sometimes with the veins still green while the tissue between fades.
- In flower, the top bud tips bleach to a distinctive near-white.
Compare against the two things it is most often confused with:
| Symptom | Light burn | Heat stress | Nutrient burn |
|---|---|---|---|
| Where | Whole leaf surfaces nearest the lamp | Top canopy, hottest zone | Leaf tips, anywhere well-fed |
| Colour | Pale, yellow, bleached white | Yellowing with edges curling up | Brown, dry, crispy |
| Leaf shape | Stays fairly flat and firm | Edges taco upward | Tips crisp and curl |
| Test | Raise light — new growth clean | Measure canopy temp | Measure runoff EC |
The single most useful diagnostic: is the damage at a consistent height, or on the best-fed leaves regardless of height? Height means light. Vigour means feeding.
What intensity is actually right
Hang height is a poor proxy because fixtures differ so much. Measure PPFD at the canopy if you can. Consistent with our light schedules guide:
| Stage | PPFD (µmol/m²/s) |
|---|---|
| Seedling | 200–400 |
| Vegetative | 400–600 |
| Flower | 600–1000 |
If you have no meter, the manufacturer's recommended hang height for your model and stage is a better starting point than any generic rule, and a phone lux app is a rough but usable relative guide for finding hot spots within your own tent.
Fixing it
Raise the light, or dim it. Most modern fixtures have a dimmer, and dimming is often the better tool because it preserves your coverage footprint. Raising the lamp reduces intensity but also spreads it wider, which can help edge plants and hurt centre penetration.
Make one change at a time, and give it several days before judging.
Check the corners. A fixture centred over a tent produces markedly higher intensity in the middle. If only your centre plants are bleaching, the answer may be rearranging plants rather than changing the light.
Consider the stage. A seedling under flower-strength light will bleach quickly. Intensity should climb with the plant, not start at maximum.
Rule out heat while you are there. Put a thermometer at canopy height, directly under the lamp, not clipped to a tent pole in the shade. If canopy temperature is high, you may be dealing with both problems at once, and see heat stress.
About bleached buds
Bleached bud tips are the version of this that costs you real money, because the damage lands on the harvest itself. Bleached tissue is generally lower quality — commonly reported as weaker and worse-tasting than shaded material from the same plant.
If you find bleaching mid-flower:
- Raise or dim immediately. Damage already done will not reverse, but you can stop it advancing.
- Do not remove bleached bud during flower; it will not recover but it is still part of the plant's structure.
- At harvest, consider separating bleached tips from the rest rather than letting them drag down the jar.
Confirming recovery
Bleached leaves do not re-green. As with most leaf damage, you judge the fix on new growth:
- New leaves emerging at the top come in a normal green.
- The bleached zone stops expanding downward or outward.
Expect roughly a week to read clearly. If new growth is still bleaching after you have raised or dimmed, either the change was too small or you are looking at heat rather than light.
The trade-off worth understanding
More light is not free. Photosynthesis saturates: past a point, additional photons stop producing additional growth and start producing damage, and the plant's ability to use high intensity depends on everything else being right — temperature, humidity, CO₂, and nutrition. A plant that bleaches at an intensity another grower runs happily is usually telling you that something else in the environment is the actual limit.
Running slightly under your fixture's maximum costs very little yield and removes an entire category of problem.
- PAR / PPFD meter — the only way to know what your canopy is actually receiving rather than guessing from hang height
- Thermometer + hygrometer — separates light burn from heat stress, which look similar at the top of the plant
- Adjustable rope ratchet hangers — makes raising the fixture a thirty-second job instead of a project
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FAQ
What does light burn look like on cannabis?
Leaves closest to the lamp go pale, yellow, or bleached white while the rest of the plant stays green. On flowering plants the top bud tips bleach almost white. The affected leaves often stay firm and stand upright rather than drooping, and the veins may remain green while the tissue between them fades.
How far should my LED be from the canopy?
Follow the manufacturer's figure for your specific model and stage, because output varies enormously between fixtures. The reliable check is PPFD at the canopy: roughly 200-400 for seedlings, 400-600 in veg, and 600-1000 in flower.
Is bleached bud still usable?
Yes, but it is generally lower quality. Bleaching indicates the tissue was damaged by excess light, and bleached bud tips commonly test lower in potency and taste worse than the shaded material lower on the same plant.
How do I tell light burn from nutrient burn?
Position and pattern. Light burn affects whole leaf SURFACES nearest the lamp and bleaches them pale or white. Nutrient burn starts at leaf TIPS, is brown and crispy rather than pale, and appears anywhere on the plant that is well fed.
Can a plant get light burn if the temperature is fine?
Yes. Photobleaching is caused by light intensity beyond what photosynthesis can use, independently of heat. That said, lamps produce heat, so the two frequently occur together and it is worth measuring canopy temperature before assuming which one you have.
Keep reading
- Nutrient Burn: Why Cannabis Leaf Tips Turn Brown and Crispy
- Heat Stress in Cannabis: Taco Leaves, Foxtails and What to Fix First
- Yellow Leaves on Cannabis: a 6-Question Decision Tree
- Brown or Rust Spots on Cannabis Leaves: Calcium, pH or Pests?
- Bud Rot (Botrytis): Catching It Before It Takes the Harvest
- Drooping Cannabis Leaves: Overwatered or Underwatered?
- Fungus Gnats: Small Flies in Your Grow and the Larvae That Matter
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