Light Burn

Light burn is too much light intensity for what the plant can process. The leaf receives more photons than its chemistry can use, the excess energy damages the chlorophyll, and the tissue bleaches.

What you are seeing#

Only the top. Specifically, only the leaves closest to the lamp, and usually only the ones directly under the center of the fixture where intensity is highest.

Those leaves lose color from the tips and outer edges inward, going pale green, then yellow, then in a bad case chalky white. The veins often stay green longer than the surrounding tissue. Edges curl upward into a taco shape as the leaf reduces its exposed surface.

Tips can crisp brown after the bleaching. In flower, the top colas bleach white and lose smell and potency where they bleached.

The key observation: a leaf six inches lower or a few inches out of the beam is completely normal. Draw an imaginary circle under your lamp and the damage sits inside it.

Why it happens#

LED lights got very powerful very quickly, and beginners hang them at seedling distance and never move them, or run them at 100 percent from day one.

Common causes:

• The light hung too close as plants stretch up into it during weeks one to three of flower.
• Running a new LED at full power for a seedling or a young plant.
• The center of the tent getting far more intensity than the edges, so only the middle plants burn.
• Low humidity, which reduces the plant's ability to handle intensity.

How to fix it#

1. Raise the light today. Measure from the lamp face to the canopy. A safe starting point is 24 to 30 inches for a 150 watt LED, 18 to 24 inches for a seedling under a dimmed light.
2. If the fixture has a dimmer, drop it to 50 or 60 percent for a week, then step back up 10 percent every few days while watching the tops.
3. Verify it is light and not heat. Put a thermometer at canopy height under the lamp. Under 82 F with leaves still bleaching means intensity, not temperature.
4. Over the next week, even the canopy. Tie the tallest tops down so no branch sits four inches closer to the lamp than the rest.
5. Keep humidity at 50 to 60 percent in veg and 45 to 55 in flower. A plant transpiring properly tolerates more light.

How to tell it apart#

Versus heat stress. These travel together and the distinction is real. Heat stress affects the whole upper canopy including leaves in the shade, curls edges upward, and the plant droops noticeably in the last hours of the lights-on period then recovers overnight. Light burn is confined to the lit footprint and does not droop. The test is a thermometer at canopy height: above 85 F means heat is at least part of it. If temperature is fine and only the lit leaves are pale, it is light.

Versus nutrient burn. Nutrient burn browns the tips of all fast growing leaves regardless of where the lamp is pointing, and the rest of each leaf stays dark green. Light burn bleaches the whole outer portion of leaves inside the lamp footprint and leaves shaded growth untouched. Shade position is the discriminator.

Versus wind burn. Wind burn produces clawing and crisped, tattered edges only on the leaves directly in front of a fan, at any height. Trace your fan's airflow before blaming the lamp. If the damage follows the breeze rather than the beam, move the fan.

How long until it looks better#

Bleached tissue is permanently destroyed. Those leaves stay pale or white for good, and bleached bud tops do not recover their smell or potency. This is one of the few problems that costs you finished product rather than just leaves.

Once the light is raised, damage should stop spreading within two to three days. New growth should come in a normal green within five to seven. In flower, expect the bleached top to stay bleached through harvest, so raise the light early rather than waiting to be sure.