Yellow leaves are a symptom, not a disease. Work through this order of likelihood and you will almost always find the cause within minutes.

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Educational comparison photo showing healthy green leaf next to yellowing leaf to help diagnose the cause of yellow leaves
Use visual patterns: uniform yellow = often water or age. Interveinal yellow = nutrients or light stress.

Step 1: Check Soil Moisture (Most Common Cause)

Stick finger 2 inches down. Soggy = overwatering (yellow + soft/mushy stems or mold). Bone dry all the way through + crispy edges = underwatering. Fix: adjust frequency, improve drainage, never let pots sit in saucers of water.

Step 2: Evaluate Light

Too little light causes uniform pale yellowing and leggy growth. Too much direct sun (especially through glass) scorches and yellows. Move plant and wait 2-3 weeks for new growth to show improvement.

Step 3: Age and Normal Shedding

Older lower leaves naturally yellow and drop on many plants (especially palms, dracaena, ficus). If only 1-2 bottom leaves and the rest look perfect, this is normal. Remove cleanly with sharp scissors.

Step 4: Nutrient Issues

Interveinal yellowing (yellow tissue, green veins) on newer leaves often = iron or magnesium. Overall pale yellow older leaves = nitrogen. See the nutrient guide for exact fixes.

Step 5: Pests

Spider mites, thrips, or scale can cause stippled yellowing. Flip leaves and check stems with a magnifying glass or phone macro. Treat with insecticidal soap or neem oil weekly for 3 weeks. Isolate the plant.

Step 6: Temperature Shock or Chemicals

Cold drafts, sudden moves, fluoride/chlorine in tap water, or leaf shine products cause sudden yellow patches. Use room-temperature filtered or rainwater when possible.

Quick Decision Rule

Yellow + wet soil = stop watering, improve air flow, check roots for rot.
Yellow + dry soil = water deeply, then establish a finger-test schedule.
Yellow + perfect soil and light = check for pests or nutrients.

Photograph the whole plant and the yellow leaves with the analyzer tool above. It combines visual analysis with care database recommendations to narrow the cause faster.

See also: If the leaves are curling as well as yellowing, read Leaves Curling Up vs. Down: What Each Direction Means.