Cannabis Yellow Leaf Problem: Short Answer
Yellow leaves on cannabis come from five causes: nutrient deficiency, pH drift, root damage from overwatering, light stress, or end-of-flower senescence. Older lower leaves yellow first when nitrogen is low. New growth yellows first when iron or sulfur is low. Yellow between green veins points to magnesium. Fix the cause, not the color.
Check These Numbers First
- Soil pH: 6.0 to 7.0
- Coco or hydro pH: 5.5 to 6.5
- Runoff EC within 0.3 of input EC
- Root zone temperature: 65 to 72 F (18 to 22 C)
- Water when the top 2 to 3 cm of medium is dry
pH outside the range blocks uptake even when feed is correct. A plant in soil at pH 7.5 cannot pull iron or manganese, and the top leaves turn pale.
Nitrogen Deficiency
Nitrogen is mobile. The plant moves it from old tissue to new growth. Lower leaves turn yellow first, then fade to pale green, then drop. Leaves stay flat and soft. Feed a nitrogen source and expect 5 to 7 days before color returns.
Magnesium Deficiency
Magnesium shortage shows as yellow between the veins while veins stay green. It starts on middle and lower leaves. Add magnesium sulfate or a calcium-magnesium supplement. Correct pH at the same time.
Overwatering and Root Damage
Roots need oxygen. Saturated medium suffocates them and rot sets in. Leaves yellow, droop, and feel limp. Check drainage, cut water volume, and let the medium dry. Root rot smells sour and shows brown, mushy roots.
Late Flower Fade
In the last 2 to 3 weeks of flower, plants pull mobile nutrients from fan leaves. Yellowing from the bottom up is normal in this window. It is not a problem unless leaves dry and curl.
Common Mistakes
Adding nutrients when pH is wrong makes the problem worse. Flushing a hungry plant removes food it needs. Measure pH, EC, and water volume before you change the feed.