Cannabis Leaf Yellowing Diagnosis Chart: Read the Plant

Cannabis Leaf Yellowing Diagnosis Chart

Start with one question before you touch a bottle of nutrients: is the yellow on old leaves or new leaves? That single detail splits almost every cause in half. Mobile nutrients move out of old growth to feed new growth, so a shortage shows up low on the plant first. Immobile nutrients cannot move, so the newest leaves suffer while the old ones look fine. Get that answer and the rest of the chart falls into place.

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Yellow on older, lower leaves: mobile nutrients

• Nitrogen: the whole leaf fades from green to pale yellow, bottom up. Growth slows and lower leaves may droop or drop.
• Magnesium: yellow between the veins while the veins stay green. Starts on lower and middle leaves and moves upward.
• Potassium: yellow and brown at the leaf edges and tips. Margins look burned and crisp.
• Phosphorus: leaves go dark green with purple or bronze tones, and older leaves can yellow in late flower.

Yellow on new, upper leaves: immobile nutrients

• Iron: bright yellow new growth with sharp green veins. Common in high pH soil and soilless mixes.
• Calcium: yellow or brown spots on new leaves, crinkled edges, and tip burn on buds.
• Zinc: small new leaves with yellow between the veins and tight spacing between nodes.
• Sulfur: the entire new leaf turns pale, all the way to the veins. Rare in most setups.

When the color points to the environment, not the menu

Overwatering is the most misread cause of yellow leaves. Soggy roots cannot take up oxygen, so lower leaves yellow and droop while the soil stays wet. Underwatering gives you dry, crisp edges that crumble. Light stress bleaches the top canopy and leaves the lower plant alone. A sudden cold night can turn leaves purple or yellow within a day.

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Check pH before you add anything

Nutrient lockout mimics a deficiency almost perfectly. In soil, aim for 6.0 to 7.0. In coco or other soilless mixes, aim for 5.5 to 6.5. If pH is off, pouring in more nutrients makes the burn worse, not better. I check runoff pH and the color of the runoff before I change any feed schedule.

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Pests and disease that look like a deficiency

Spider mites leave fine stippling that reads as yellow speckling. Thrips scrape the surface and leave silvery patches. Root aphids stunt the plant and yellow it from the bottom. Fusarium wilt yellows one side of a plant before the stem collapses. Mosaic viruses cause patchy light and dark green mottling that no bottle will fix.

How I read a yellow leaf

Four checks, in order: location on the plant, pattern across the leaf, age of the affected leaf, and whether it is one plant or the whole row. One plant points to pests, root damage, or a hot spot. Every plant points to feed, water, pH, or temperature. Water it properly, wait three to five days, and look again before adding anything.