Cannabis yellow leaf treatment: the short answer
Yellow leaves on a cannabis plant point to one of three faults: a nutrient shortage, a pH problem, or water stress. Test the pH of your input water and the runoff before you add anything. If pH is in range and the plant is in veg, feed nitrogen. If the plant is in the last 2 to 3 weeks of flower, yellow lower leaves are normal and need no treatment.
Read the leaf pattern
Where the yellow starts tells you which nutrient is short. Mobile nutrients move from old growth to new growth, so the damage shows on lower leaves first.
diagnosing cannabis yellow leaves
- Nitrogen: lower leaves fade from the tip back to the stem. The whole leaf turns pale.
- Magnesium: yellow patches between the veins. Veins stay green. Middle and lower leaves.
- Potassium: yellow and brown edges on older leaves.
- Sulfur: new upper leaves turn yellow. Lower leaves stay green.
- Iron: new leaves yellow with green veins. Common when pH runs above 7.0.
Test pH and runoff
Soil: 6.0 to 7.0. Coco coir and hydro: 5.5 to 6.5. Use a meter that reads to 0.1 pH and calibrate it with 4.0 and 7.0 solution before each run. Collect runoff in a clean tray. A runoff pH below 5.5 or above 7.5 locks out nutrients even when the feed is correct.
Cannabis Yellow Leaf Disease: Guide to Prevention and Treatment
Flush at 2 times the pot volume with pH 6.0 water. Feed at half strength for the next 2 waterings.
Fix water and roots
Lift the pot after watering. A light pot means the medium is dry. Water when the top 1 inch (2 to 3 cm) of the medium is dry. Cannabis in soil at 68 to 82 F (20 to 28 C) dries a 5 gallon pot in 2 to 4 days. Yellow leaves plus wet soil plus droop point to root damage. Check the roots: white and firm is healthy, brown and soft is rot.
Feed nitrogen
Use a nitrogen source at 100 to 150 ppm N for plants in veg. Do not double the label rate. Make one change per 3 days, then check new growth. Old yellow leaves do not turn green again. New leaves are the signal.
Remove leaves
Cut a leaf when more than half of it is dead or when it touches the medium. Leave leaves that are half yellow on the plant. They still photosynthesize, and the plant pulls mobile nutrients out of them.
When yellow leaves are normal
Lower leaves under a dense canopy get little light. One or two yellow leaves per week on a plant in week 6 of flower is normal senescence. The plant moves nitrogen to the buds.
Prevention
- Keep a log of pH, EC, and water volume for each pot.
- Water to 10 to 20 percent runoff in coco and soil.
- Keep medium temperature above 65 F (18 C).
- Run a calcium and magnesium supplement in coco at 2 ml per gallon.
- Do not mix two nutrient lines in one reservoir.