Light stress (light burn, bleaching) happens when the light a tissue absorbs outpaces its capacity to use the energy in photosynthesis or shed it safely as heat. Excess energy forms reactive oxygen species that destroy chlorophyll and other pigments. The classic sign is bleaching: top leaves and bud bracts closest to the light turn pale yellow, then white, while lower growth stays green.
In practice
Bleaching shows up most often under high-output LEDs hung too close, because LEDs emit less radiant heat and growers do not feel the warning on their hand the way they would under HPS. It is more likely when PPFD is high (above about 1,200 µmol/m²/s at the tops), CO2 is ambient, leaf temperature is wrong or VPD is very high, and late in flower when leaves are senescing.
Signs in order: upward cupping (tacoing) of top leaves, yellowing between veins at the top only, then bleached white bracts. Fix by raising or dimming the fixture to bring top PPFD back to 900–1,100 µmol/m²/s, and check leaf temperature and VPD. Bleached buds do not regreen; they are usually lower in potency and aroma.
Don't confuse with
- Nutrient deficiency: yellowing that follows mobility patterns (old vs new leaves), not distance from the light.
- Heat stress: driven by temperature; often occurs alongside light stress.
- Foxtailing: a growth form change, which can be a response to light or heat stress.
The process in chloroplasts that uses light energy to turn carbon dioxide and water into sugars and oxygen; cannabis uses the C3 pathway.In cannabis, the small leaf-like sheath (perigonal bract) that wraps each female flower's ovary and carries the densest capitate-stalked trichomes.An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.Leaf temperature is the actual surface temperature of a leaf, which often differs from air temperature by 1–3 °C depending on light source and transpiration.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.Heat stress is damage or slowed growth when leaf or canopy temperature rises above what the plant can handle, typically above about 30–32 °C (86–90 °F) in cannabis.The light saturation point is the PPFD above which photosynthesis stops rising; for cannabis it is unusually high and moves upwards with CO2 and temperature.Foxtailing is the growth of new, elongated calyx spires on top of existing buds; it can be genetic, or a stress response to excess heat or light.
