Why are my plant’s leaves turning yellow?
Yellowing is the single most common houseplant complaint and it has about six causes that look superficially alike. Sorting them takes three observations: which leaves (lowest or newest), how they feel (soft or crisp), and how long since you watered.
The causes, and how to tell them apart
Overwatering / early root rot most common
How to tell: Lowest leaves first, uniformly yellow, soft and limp rather than crisp. Soil is still damp several days after watering, and often smells sour. Sometimes fungus gnats.
What to do: Stop watering and let the pot dry well down. Check that the drainage hole is not blocked and that the pot is not sitting in a saucer of water. If the soil stays wet more than a week, unpot and look at the roots — healthy roots are firm and pale, rotting roots are brown, soft and pull apart easily. Cut those off and repot into fresh, faster-draining mix.
Underwatering common
How to tell: Yellowing arrives WITH crisping — dry, papery edges — and the whole plant looks slightly deflated. Soil has pulled away from the sides of the pot and water runs straight down the gap.
What to do: Soak thoroughly until water runs from the drainage hole, ideally by bottom-watering for 20 minutes so the rootball rehydrates evenly rather than the water tunnelling down the sides.
Natural leaf aging common and harmless
How to tell: One or two of the OLDEST, lowest leaves, while everything above looks healthy and new growth continues.
What to do: Nothing. Plants retire their oldest leaves to fund new ones. Remove it once it is fully yellow.
Too little light common
How to tell: Gradual, widespread pale-yellowing rather than dramatic drop, plus long bare stretches of stem between leaves and any variegation fading toward plain green.
What to do: Move it closer to a window. "Bright indirect" in practice means somewhere you could comfortably read a book all day without a lamp.
Nutrient deficiency less common indoors
How to tell: Yellowing between the veins while the veins themselves stay green, usually on NEWER growth. Typically on a plant that has been in the same soil for years.
What to do: Feed at half strength during the growing season, and repot into fresh mix if it has been more than two or three years. Never fertilize a bone-dry pot — water first.
Cold damage or a draft seasonal
How to tell: Sudden yellowing after a cold snap, often with greasy grey-green patches, and worst on the side facing a window or a door.
What to do: Move it away from the cold glass or the draft. Damaged leaves will not recover; watch the new growth instead.
By species
The likely answer differs by plant — these species have their own version of this problem:
Plantroster compares this month's photo against last month's, so slow changes — the ones you stop noticing because you see the plant every day — get flagged.
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General horticultural guidance, not a diagnosis of your specific plant.