Light Burn

Light burn (sometimes called light bleaching) is increasingly common in modern indoor cannabis cultivation as high-output LED fixtures push light intensity beyond the plant's photosynthetic capacity. Cannabis evolved under outdoor solar conditions delivering 1500-2000 µmol/m²/s PPFD at peak summer noon, but only for a few hours per day with cloud cover modulation. Modern indoor LEDs can deliver 1500+ µmol/m²/s continuously across 12-18 hours, exceeding what cannabis can metabolize. When light intensity exceeds the plant's photosynthetic capacity, excess energy damages chlorophyll and other photo-sensitive pigments, causing visible bleaching: pure white-tipped colas (the "white widow" effect that some growers mistake for desirable trichome density), yellow burned leaves immediately below the light source while lower leaves remain healthy, leaves curling upward at the edges (taco-curl) as a stress response, and reduced cannabinoid and terpene production in bleached tissue. PPFD targets by lifecycle stage: Seedlings 100-300 µmol/m²/s; Vegetative 400-700 µmol/m²/s; Flower 700-1000 µmol/m²/s without CO2 supplementation, 1000-1500 µmol/m²/s with CO2 enrichment. Diagnosis: bleached tops with healthy lower foliage indicates light burn; bleached tops with overall plant burn indicates heat stress. Treatment: raise lights immediately to reduce PPFD by 20-30% (use a quantum sensor to measure), or dim the fixture if it has a controller. Bleached tissue does not recover but the plant continues developing healthy growth below. Prevention: invest in a quality PAR meter (Apogee MQ-500 or similar) and measure actual PPFD at canopy height rather than relying on manufacturer claims.