Quick answer
Yellow leaves on a cannabis plant are light burn when the damage starts at the top of the canopy and follows the shape of the light. The leaves closest to the fixture turn pale, yellow, or crisp first, while lower leaves stay green. Raise the light, check the distance to the tallest cola, and cool the space between the lamp and the plant. Do not add nutrients to fix a light problem.
Before you start
- Tape measure
- Infrared thermometer or a hand test at canopy level
- Adjustable light hanger or yo-yo hangers
- Timer set for the correct photoperiod
Confirm light burn
Light burn is a location problem, not a feeding problem. Check the plant from the top down.
Cannabis Yellow Leaves During Flowering: Normal or a Problem?
- Damage starts at the highest leaves under the lamp.
- Upper leaves look bleached, yellow, or curled up at the edges.
- Lower and middle leaves keep a normal green color.
- Symptoms show 1 to 3 days after a light move, a fixture swap, or a growth spurt.
- The leaf surface feels hot, or an infrared thermometer reads above 82°F.
Fix light burn
- Dim the fixture to 50 percent for the rest of the light cycle.
- Measure the distance from the lamp to the tallest cola.
- Raise the fixture 6 to 12 inches for LED and 12 to 18 inches for HID.
- Point a small fan at the gap between the lamp and the canopy.
- Check leaf surface temperature and keep it below 82°F.
- Remove only leaves that are crisp and brown.
- Wait 3 to 5 days before you change the nutrient mix.
New growth should look green and flat within one week. Old bleached tissue will not turn green again. Use the new leaves at the top as your progress check.
Cannabis pH Imbalance and Yellow Leaves: Buying Guide for Fast Fixes
Prevent repeat burn
- Set hang height for the current stage before you flip the light schedule.
- Measure canopy height every 2 days during the stretch phase.
- Raise the light when a cola comes within the manufacturer's minimum hang distance.
- Keep room temperature at 75 to 80°F with lights on.
- Clean the fixture glass and check that the driver or ballast runs within spec.
What it is not
Nutrient burn starts at leaf tips and spreads along the margins. Nitrogen toxicity makes leaves dark green and clawed. A pH problem shows mixed symptoms across the plant, not a clean line under the lamp. If only the top leaves turn yellow, treat the light first.