Light burn: bleached leaves closest to the lamp

The giveaway is position: the damage is worst on the leaves nearest the light, while lower leaves in the same plant look fine.

What it looks like

Growers commonly describe these signs, listed in our light height guide:

• Leaves bleached or pale at the very top
• Leaf edges curling up toward the light
• Crisping on the leaves closest to the light

These are grower reports. The two controlled trials we read don’t describe them, as explained below.

Why it happens

Leaves can only use so much light at once. In one trial, the amount a leaf could use topped out well below the highest light level tested, and a measure of photosynthetic health (Fv/Fm) fell steadily as the light on a leaf increased (Rodriguez-Morrison et al. 2021). So high light can put measurable strain on a leaf.

Light and heat also work together. Photosynthesis in one study peaked at 30°C (86°F) and 1,500 µmol/m²/s, and at that temperature it fell at higher light levels. The authors concluded that high temperature combined with high light had an adverse effect (Chandra et al. 2008). A home tent may get warmer under a strong lamp than a research room does.

How to confirm it

1. Check where it is. Is it mainly at the top and directly under the lamp? If it’s on lower leaves or all over, look elsewhere.
2. Check your light level. Use a PAR meter at the canopy or the manufacturer’s PPFD chart for your height and dimmer setting.
3. Measure the temperature at the top of the canopy, not just across the room.
4. Watch what happens after you change one thing. New growth is the place to look, since damaged tissue won’t turn green again.

What else it could be: heat stress can look similar. Pale new growth can also be a nutrient or pH problem. Paleness alone doesn’t prove anything about the light.

What to do

1. Dim the light or raise it a little. Stay within the manufacturer’s clearance and hanging limits.
2. Bring the top of the canopy down to the same level, so a few tall tips aren’t taking the brunt. See our training guides for how.
3. Improve cooling and airflow at the top of the plant.
4. Raise intensity gradually as plants get bigger, rather than all at once.
5. Don’t change the feed for this, unless you find a separate reason to.

Don’t dim too far without a reason. More light means more flower: yield rose steadily up to 1,800 µmol/m²/s in one trial, and in proportion to light from 600 to 1,000 in another (Rodriguez-Morrison et al. 2021, Llewellyn et al. 2022). Don’t treat that as a target for a home tent, but it does mean dimming has a real cost.

How much to trust this

“Light burn” is less well established than you’d think. The 2021 trial grew plants up to 1,800 µmol/m²/s for 81 days and yield kept rising. Neither that paper nor the 2022 trial describes bleaching or visible burn.

That doesn’t mean it never happens. Those trials ran in controlled rooms, about 25°C (77°F) in the 2021 trial, using specific cultivars, and yield doesn’t tell you how each leaf looked. Your plants have different genetics, and likely different temperature and airflow. So we can’t give you a safe number, or tell you how much of what growers call light burn is light rather than heat. The Chandra measurements are of photosynthesis in leaves, not of damage or yield, and we read that abstract only.