If the leaves closest to the fixture are turning pale, yellow, or white — sometimes crisping or curling — while the lower canopy looks fine, it's usually too much light, not a nutrient problem. Past the saturation point the plant can't use the extra photons, and they start damaging the photosynthetic machinery; intense light also runs the leaf several degrees hotter than the air, compounding it. The fix is to dial back intensity or raise the fixture and check leaf temperature — not to add nutrients. (If the bleaching is interveinal on lower leaves instead of the tops, that's likely a nutrient or pH problem — see the rule-outs.)
This is one of the most misread problems in the room, because pale, yellowing leaves look like a deficiency, and the instinct — feed the plant — points the wrong way. The tell is location: light burn strikes the tissue nearest the fixture, where the photon load is highest, and leaves the shaded lower canopy alone. A deficiency doesn't care how close a leaf is to the light.
What to do right now
- Measure the intensity at the canopy. Use a quantum PAR meter at the height of the affected leaves, not the fixture rating. If PPFD is high for your crop and CO₂ level, that's your answer.
- Raise the fixture or dim it. Increase the distance, lower the output, or both, until the top of the canopy sits inside the crop's usable range. Move in steps and watch the new growth.
- Check leaf temperature. Point an infrared thermometer at a bleached leaf. Intense light runs the surface 2–5 °C above the air, so what reads as "light burn" is often light and heat together — if the leaf is hot, airflow and a cooler setpoint are part of the fix.
- Look at your CO₂ and airflow. Without enriched CO₂ the saturation point is lower, so you bleach at a PPFD an enriched room would handle easily; good airflow also cools the leaf. Raising CO₂ can make a high intensity usable rather than damaging.
- Don't feed a bleaching top. Adding nutrients to light-stressed tissue raises the salt load on a struggling leaf and treats a problem the plant doesn't have.
How to be sure it's this
Light burn has a signature distinct from the deficiencies it imitates:
- It's on the tops, nearest the light. The uppermost leaves and the colas closest to the fixture bleach first; the lower, shaded canopy stays healthy. A nutrient problem follows the plant's chemistry, not its geometry.
- It's worst directly under fixtures. Map it against your light map — the pale patches sit under the hot spots, the healthy tissue in the cooler, dimmer zones.
- The color is bleached. Pale, yellow-white, sometimes with crisping edges or leaves cupping/tacoing away from the light — a washed-out look, not the distinct interveinal pattern of an iron or magnesium issue.
- It followed an intensity change. It tends to appear after the lights were raised in output, lowered toward the canopy, or after the canopy grew up into the high-intensity zone.
Why it happens
Past the saturation point, the light reactions generate electrons faster than the Calvin cycle can consume them. The excess excitation energy has nowhere productive to go, and beyond the plant's ability to dissipate it safely — the carotenoid pigments that normally quench surplus energy get overwhelmed — it begins to damage the photosynthetic apparatus itself. This is photoinhibition, and at the extreme, photobleaching: chlorophyll is degraded faster than it can be rebuilt, and the tissue loses its color. Compounding it, 85–90% of the light a leaf absorbs becomes heat rather than sugar, so the most intensely lit leaves are also the hottest, pushing 2–5 °C above the air and toward the thermal stress zone. And the saturation point is not fixed: without CO₂ to let the Calvin cycle run faster, it sits lower, so an un-enriched room bleaches at an intensity a CO₂-enriched room would turn into yield. The damage isn't that the plant got "too much energy" in the abstract — it's that it got more than its current carbon machinery, at its current CO₂ and temperature, could use.
The trap: feeding a light problem, or chasing watts
The first trap is reading bleached, yellowing tops as hunger and feeding harder — which raises EC on already-stressed tissue and never touches the cause, because the leaf isn't short of nutrients, it's overexposed. The second is the "more watts, more yield" belief that drove the intensity too high in the first place: past saturation, extra photons don't add growth, they add bleaching and heat, and the electricity buys damage. The move that works is the opposite of adding — back the light off to what the plant can use, and if you want to use more light, raise the ceiling first by enriching CO₂ rather than just turning the fixture up.
Telling it apart from its look-alikes
- If the pattern is interveinal (green veins, yellow between) and on older or lower leaves, that's a nutrient story — iron and pH lockout up top, mobile nutrients like magnesium lower down — not light burn. Location and pattern separate them.
- If the leaf is hot but the intensity is moderate, lead with the heat: it's the leaf-air temperature gap, and the lever is airflow and setpoint, not the dimmer. → the science of air temperature. In practice light burn and heat stress travel together; check both, because the leaf is hotter than your air sensor reads.
- If new growth is twisted, clawed, or generally distorted across the plant, look at nutrient excess or other stressors rather than intensity.
- If the whole canopy is pale, not just the tops, it's more likely a room-level spectrum, nutrient, or pH issue than localized light burn.
Preventing it from coming back
The durable fix is to set intensity to what the plant can actually use and let it grow into the light rather than the reverse. Establish the crop's usable PPFD range for your CO₂ level, map the canopy so no zone sits in a hot spot, and re-check the distance as the canopy rises toward the fixtures. If you want to run high intensity, enrich CO₂ to raise the saturation point so the photons become yield instead of damage, keep airflow moving to cool the leaf, and watch leaf temperature alongside PPFD. The science of light page covers the light-response curve and the CO₂ interaction; the matrix gives the intensity and spectrum targets crop by crop.
When the cause is elsewhere
- If the bleaching is uniform across the whole crop, it's a room-level cause — a spectrum problem, a nutrient or pH issue — not intensity at the canopy top.
- If PPFD reads moderate for your crop and CO₂, and the leaves are still pale, look at nutrition, pH, or root-zone health instead.
- If it tracks a recent feed change or pH swing rather than a lighting change, follow that thread first.