Diagnose by position first
Cannabis yellowing, or chlorosis, is a symptom and not a diagnosis. The fastest way to narrow it down is to check where the yellow starts. Yellow on the oldest, lowest leaves that spreads upward points to a mobile nutrient shortage, and nitrogen is the usual suspect. Yellow that starts on new growth at the top points to an immobile nutrient problem, light stress, or root trouble. Yellow between the veins while the veins stay green points to magnesium or iron depending on leaf age. Match the pattern before you add anything to the reservoir.
Cannabis Yellow Leaf Treatment: Causes and Fixes
What to look for
Leaf position and pattern
- Lower leaves, uniform pale yellow, soft texture: nitrogen draw during stretch or plain underfeeding.
- Lower leaves with yellow margins and brown spots: potassium shortage, common in mid-flower.
- Interveinal yellow on middle leaves: magnesium. On new leaves: iron, often a pH lockout rather than a true shortage.
- Yellow tips that move inward with curled edges: nutrient burn or heat, not deficiency.
Root zone and watering
Overwatering and poor drainage produce yellowing that looks identical to a nitrogen shortage, because soggy roots cannot take up nutrients. Lift the pot and check its weight, then look at the drainage holes. In containers, water when the top inch or two is dry. In coco, do not let the medium go bone dry between feeds.
pH and EC
Nutrient lockout from an off pH is the most common false deficiency. Measure the input and the runoff. Compare them: if runoff EC is far higher than input, salt has built up and a flush is the fix. If runoff pH has drifted, correct it before adding more feed.
diagnosing cannabis yellow leaves
Light, temperature, pests
Light burn shows as bleached or yellow tops closest to the fixture. Cold root zones slow uptake and cause yellowing with purple stems. Check the undersides of leaves with a loupe for mites and thrips before you blame nutrients.
Cannabis Yellow Leaf Disease: Guide to Prevention and Treatment
Parameter bands to check against
- Soil pH: 6.0 to 7.0. Coco and hydroponics: 5.8 to 6.2.
- Runoff EC within roughly 0.3 to 0.5 mS/cm of input in coco and hydro.
- Daytime canopy temperature: 22 to 28 C (72 to 82 F). Root zone below 18 C (65 F) slows uptake.
- VPD in veg: 0.8 to 1.0 kPa. In flower: 1.0 to 1.5 kPa.
- Reservoir water temperature: 18 to 22 C (65 to 72 F).
Pitfalls
- Adding nitrogen to every yellow leaf. Late flower yellowing on fan leaves is the plant mobilizing nutrients, not a fault to correct.
- Feeding on top of a pH problem. You will raise EC and make lockout worse.
- Judging by a single leaf. Track the canopy and photograph it every few days.
- Ignoring the schedule. Yellowing that appears right after a feed change points to that change.
- Running a pH pen you cannot calibrate. A drifting meter is worse than drops.
What to keep in your diagnostic kit
Buy a pH pen with a replaceable probe and two calibration buffers, 4.0 and 7.0, plus storage solution so the bulb does not dry out. Add a conductivity meter for EC, a 30x to 60x loupe for pests, and a moisture probe or a pot light enough to lift. A notebook or phone log matters more than any single tool, because the pattern over a week tells you what a snapshot cannot.
FAQ
Should I cut yellow leaves off?
Leave them until they pull away with a light tug. A yellowing leaf is still exporting sugars and mobile nutrients to new growth. Remove only dead, crispy tissue that can harbor mold.
Is yellowing normal in flower?
Some fade in the last two to three weeks is normal as the plant redirects nitrogen to buds. Yellowing that starts at week three or four of flower and climbs fast usually means the feed is too light or the pH has drifted.
Can too much light cause yellow leaves?
Yes. Tops nearest the fixture bleach to pale yellow or white while lower leaves stay green. Raise the fixture or dim it, then watch new growth.
How fast should a fix show?
New growth should look better within five to seven days. Leaves that already turned yellow will not turn green again, so judge the correction on new leaves.