Light Burn (Light Bleaching)
Definition: Light burn, also called light bleaching, is damage to cannabis leaves and buds caused by more light intensity than the plant can use. It appears on the tops closest to the lamp as yellowing or white, bleached tissue, while lower leaves on the same plant stay healthy and green.
How Light Burn (Light Bleaching) works
Plants can only process so much light. Past that point, the excess energy damages the pigments and cells in the upper canopy. Modern LEDs run cool enough that leaves may not feel hot, so growers sometimes hang them too close without realizing the light itself is the problem.
Symptoms
- Top leaves turning pale yellow, then white, often starting in the middle of the leaf.
- Leaves curling upward or folding along the center vein.
- Bleached, almost white bud tips on the top colas in late flower.
- Crispy texture on the most exposed leaves.
Burn or deficiency?
Location is the giveaway. Light burn is concentrated at the top of the plant, directly under the fixture. Mobile-nutrient problems like nitrogen deficiency usually start on lower, older leaves. If only the tallest colas look sick and the rest of the plant is fine, suspect the light first.
Common causes
Hanging the light too close, running a dimmable fixture at 100% in a small space, or letting a few colas stretch far above the rest. In flower without added CO2, sustained PPFD above roughly 1,000 µmol/m²/s at the canopy is a common trigger. Heat makes it worse, so burn and heat stress often show up together.
Fixing it
Raise the light or dim it by 20 to 30%, then measure again at the top colas. Bleached tissue won’t turn green again, but new growth should come in healthy within a week. Many LED makers recommend 12 to 24 inches of clearance in flower; follow your fixture’s chart.