Light Burn and Heat Stress on Cannabis

Light burn vs nutrient burn

Both show on leaf tips — but the pattern differs. Nutrient burn usually hits leaf fingers evenly from the tips inward with dark green, overly lush foliage. Light burn bleaches the tops closest to the fixture — white or yellow patches on bud and upper leaves while lower canopy stays green.

If only the tallest colas under the light center look fried, raise the fixture or dim before adding more calmag.

Heat stress signs

Cannabis prefers roughly 75–80°F (24–27°C) with lights on. Above ~85°F consistently, plants struggle — especially without matching humidity and CO₂.

• Leaves curl upward like a taco or canoe — heat escape behavior
• Dry, crispy edges on upper leaves first
• Foxtailing — airy, heat-stressed calyx stacks on top colas
• Slow resin production; terpenes fade in extreme heat
• Wilting during hottest part of light cycle even with adequate water

Light burn signs

Too much PPFD bleaches chlorophyll faster than the plant can adapt — common when new growers crank LED power or hang too low in late flower.

• White or pale yellow patches on bud tips directly under the light
• Upper leaves only — lower plant looks fine
• Bleaching worsens over 3–5 days without distance change
• Distinct from deficiency — whole plant would show if it were feed-related

Fixes that actually work

Raise the light 6–12 inches or dial back intensity if your driver is dimmable. In late flower, many strains want more distance than mid-veg — bud stretch puts tops closer to the bar.

Improve exhaust: hot tents above 85°F need more CFM or active intake. Move drivers outside the tent when the fixture allows — it drops in-tent heat noticeably.

Increase airflow across the canopy with clip fans — stagnant hot air pockets cause localized burn even when room average looks fine.

• Target: 75–80°F lights on, 40–50% RH mid-flower
• Raise or dim light at first sign of bleaching
• Run exhaust on thermostat if available
• Defoliate only for airflow — not as a heat band-aid on a 400W monster at 12 inches

Prevention on your next run

Log max tent temperature during a 48-hour burn-in before plants go in. Fix ventilation first — you can't train away a 90°F tent.

Track DLI: seedlings need less than flowering plants. Ramping intensity with plant age beats full power from day one.