Yellow Cannabis Leaves: 12 Causes & Exact Fixes (With Photos)

Yellow leaves could mean nitrogen deficiency, pH lockout, overwatering, or 9 other problems — and the wrong fix makes it worse. Our visual guide diagnoses the exact cause from leaf position and color.

Rule #1: Mobile vs. Immobile Nutrients

Before you add nutrients, look at WHERE the yellowing is.

Mobile Nutrients (N, P, K, Mg): The plant can steal these from its bottom leaves to feed the top. If bottom leaves are yellowing, it's a mobile deficiency.
Immobile Nutrients (Ca, Fe, S): The plant cannot move these. If NEW growth is yellowing, it's an immobile deficiency.

The pH Lockout Trap

90% of 'deficiencies' are actually pH problems. If your soil pH is 7.5, the plant *cannot* absorb Iron, even if the soil is full of it. This is called 'Lockout.'

Always test your runoff with a calibrated pH probe. If your runoff is outside the 6.0-6.8 range, flush with pH 6.0 water until the meter stabilizes. Never 'eye-ball' your nutrients.

Overwatering vs. Root Rot

If leaves are yellow AND drooping (the 'heavy' look), you are likely overwatering. This suffocates the roots. If they stay yellow and the stems turn purple, check for Root Rot (Pythium). Lift your plant out of its pot—healthy roots are white and smell like clean earth. Rotting roots are brown, slimy, and smell like a swamp. Switch to fabric pots and add more perlite to your grow medium to prevent this.

Light Burn: The Silent Bleacher

High-intensity modern LEDs can actually 'bleach' the chlorophyll out of leaf tips. The leaves will turn yellow or even white at the very top of the canopy, while the rest of the plant remains dark green. Raise your light height or dim it to 75% intensity.

Frequently Asked Questions

Why are my cannabis leaves turning yellow?

Yellow cannabis leaves have many causes. The location tells you the cause: bottom leaves yellowing = Nitrogen deficiency (mobile nutrient); top/new growth yellowing = Iron or Calcium deficiency (immobile nutrient); yellowing between veins = Magnesium deficiency; all-over yellow with crispy tips = nutrient burn; yellow + drooping = overwatering. Check pH first—90% of deficiencies are actually pH lockout.

What pH causes yellow leaves in cannabis?

pH lockout is the #1 cause of yellowing. Soil ideal pH is 6.2–6.8. Below 6.0, Iron and Manganese lockout causes interveinal yellowing on new growth. Above 7.0, Calcium and Magnesium become unavailable, causing widespread yellowing. Always test runoff pH with a calibrated probe, not test strips.

Is it normal for lower leaves to turn yellow during flowering?

Yes. Some lower leaf yellowing in mid-to-late flower is normal and expected. The plant 'cannibalizes' nitrogen from its fan leaves to fuel bud development. This is a sign of a healthy finish. Only worry if yellowing progresses rapidly up the plant, affects more than 30% of leaves before week 6 of flower, or if leaves are also drooping.

How do I fix a Magnesium deficiency in cannabis?

For soil: add Cal-Mag supplement at 5 ml/gallon or foliar spray with 1 tsp Epsom salt per liter. First, flush your medium to reset pH to 6.2–6.5 (Mg is locked out below 5.8 and above 7.0). Recovery is visible within 5–7 days on new growth. Affected leaves will not fully recover, but healthy new growth confirms the fix worked.

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