The Short Answer
Yellow leaves on a cannabis plant point to one of six problems. Nitrogen shortage is the most common cause in vegetative growth. Natural fade is the most common cause in the last 2 to 3 weeks of flower. Overwatering, pH lockout, magnesium shortage, and root damage cover most of the rest.
Read the Position of the Yellow Leaf
Match the yellow leaf to its spot on the plant. Nitrogen, magnesium, potassium, and phosphorus move through plant tissue. When supply drops, the plant pulls these nutrients from old leaves to feed new growth. Iron, calcium, sulfur, and zinc do not move. A shortage of these shows up on new leaves first.
Mobile Nutrient Shortages
Nitrogen
Older leaves turn yellow from the tip toward the stem. The leaf fades to pale green, then yellow, then drops. This shows in weeks 3 to 6 of veg when the plant doubles in size and the feed stays the same. Fix: raise nitrogen to 100 to 150 ppm in the reservoir.
cannabis nitrogen deficiency yellow leaves
Magnesium
Older leaves show yellow between the veins. Veins stay green. Damage starts mid-leaf. This is common in coco coir and soft water. Fix: add 1 to 2 ml per gallon of cal-mag.
Potassium and Phosphorus
Potassium shortage yellows and burns leaf edges on older leaves. Phosphorus shortage turns leaves dark green, with purple stems and slow growth.
Immobile Nutrient Shortages
Iron shortage yellows new leaves while veins stay green. It shows when pH climbs above 6.5 in coco or hydro. Zinc shortage shortens new growth and narrows leaflets.
pH Lockout
pH controls which nutrients roots can take up. Soil range: 6.0 to 7.0. Coco and hydro range: 5.5 to 6.5. A pH of 7.2 in coco blocks iron and manganese. The plant starves with a full reservoir.
Overwatering and Root Problems
Wet roots cannot take up oxygen. Lower leaves yellow, droop, and feel soft. Soil that stays wet for 3 days or more kills root hairs. Root rot adds a sour smell and brown roots.
Light Stress
Too much light bleaches top leaves to pale yellow or white. Too little light yellows lower leaves in shade. Veg plants need 400 to 600 PPFD. Flower plants need 600 to 1000 PPFD.
Late-Flower Fade
In weeks 7 to 9 plants pull nitrogen from fan leaves. Leaves yellow from the bottom up. This is senescence, not a deficiency. Feed more nitrogen at this stage and buds hold green leaf tissue at harvest.
Order of Checks
- Test pH of runoff or reservoir.
- Check leaf position, old growth or new growth.
- Lift the pot to judge water weight.
- Compare feed strength to the week of growth.
- Check leaf surface temp at the canopy.
What Yellowing Is Not
Yellow leaves rarely mean bud rot, mold, or a virus. Check for pests with a 30x loupe before you change the feed.