Cannabis light burn turns leaves yellow or pale at the top of the canopy, closest to the lamp, while lower growth stays green. The reason is simple: the upper leaves absorb more light than they can use in photosynthesis, so chlorophyll breaks down and the tissue bleaches. If your yellow leaves appear at the bottom first and move upward, you are looking at a nutrient or root problem instead of light burn.
What Light Burn Looks Like on Cannabis
Light burn tends to follow the shape of your light spread. Plants directly under the fixture show symptoms first, and the tallest tops suffer most. Symptoms typically progress in this order:
cannabis ph imbalance yellow leaves
- Top leaves fade from deep green to light green, then yellow
- Leaf edges and tips on upper leaves crisp or curl upward
- Bleached white or silver patches on the highest buds and sugar leaves
- Leaves that feel dry and papery but sit in properly watered soil
- A clear line between damaged upper growth and healthy lower growth
Buds can also stop stacking and produce foxtails when light intensity stays too high through flower.
Cannabis pH Imbalance Yellow Leaves: Causes and Fixes
Light Burn vs. Nutrient Deficiency
Location is your fastest diagnostic tool. Mobile nutrients like nitrogen and magnesium move from old leaves to new growth, so deficiencies show up low on the plant. Light damage hits the tissue that receives the most photons, which is the top.
- Yellow starts at the top and spreads down: light or heat stress
- Yellow starts at the bottom and spreads up: nitrogen, magnesium, or pH issues
- Yellow between the veins, lower leaves first: magnesium deficiency
- Uniform pale green across the whole plant: often pH lockout or overwatering
Check the medium, runoff pH, and feed schedule before you dim anything. Fixing the wrong problem can cost you a harvest.
Why Too Much Light Turns Leaves Yellow
Leaves have a ceiling on how much light they can convert. Push past it and the plant produces reactive molecules that damage the chlorophyll in the leaf, a process researchers call photoinhibition. The leaf responds by reducing its light-absorbing pigments, and that reduction is what you see as yellowing.
Two numbers describe the load on your canopy. PPFD measures how much light arrives each second, and DLI measures the total light delivered over a full day. A fixture that measured fine in veg can burn a flowering canopy when you raise the intensity, extend the photoperiod, or let plants stretch toward the lamp.
How to Fix Light Burn
- Raise the fixture 6 to 12 inches and watch new growth for three to five days.
- Dim the driver by 10 to 20 percent if you cannot move the light higher.
- Measure PPFD at the tallest top with a meter or a phone app, then compare it to the recommended range for that growth stage.
- Lower canopy temperature and keep humidity inside your target VPD range so leaves can cool themselves.
- Remove leaves that are fully crisp or bleached. Partially yellow leaves can still photosynthesize, so leave them on the plant.
New growth should come back green within a week or two. Leaves that already bleached will not turn green again, so judge recovery by the tips and fresh nodes, not by the damaged tissue.
When Yellow Leaves Are Not Light Burn
Several problems mimic the same symptom. Overwatering and poor drainage starve roots of oxygen, which shows as drooping yellow leaves anywhere on the plant. Root rot, cold medium, and a pH that locks out nutrients all produce similar signs. Heat stress without excess light usually curls leaf edges upward and keeps color, while light burn bleaches. If your tops are yellow but the lamp is far away and dim, look at the root zone first.
Preventing Light Burn on the Next Run
- Increase intensity in small steps instead of jumping to full power.
- Keep a fixed distance between the lamp and the tallest cola, then adjust as plants stretch.
- Train plants with LST or a trellis so no single top sits far above the rest.
- Track PPFD and DLI at the canopy, not at the wall.
- Recheck your setup after switching fixtures, since LEDs deliver more usable light per watt than older lamps.
Read the plant before you read the bottle. Yellow at the top means the light is too strong. Yellow at the bottom means the medium is short on something. That single habit solves most cannabis light burn cases before they spread.