Why are my plant leaves turning yellow?

Read the pattern first

Penn State’s diagnostic key for indoor plants lists yellowing of the old leaves and yellowing of the young leaves separately, because they have different causes. Noting which leaves went first splits the causes in two before you touch the pot.

Lower, older leavesAgeing, drought or a hungry plant

Newest leaves paleLow light, or iron locked up by hard water

All overWet compost, too little light, cold

Old and new togetherOverwatering: check the roots

Lower, older leaves

A single lower leaf yellowing now and then is nothing to worry about: the RHS says it is normal for the odd lower leaf on a mature plant to yellow and fall, and Iowa State says not to worry if the rest looks healthy. Several at once is different. The RHS notes that on a plant left too dry, the older leaves at the base yellow and fall first. Clemson adds that lower leaves yellowing, browning and dying is generally nitrogen or iron shortage, with pot-bound plants especially prone, and the RHS lists too little light as another cause.

The newest leaves

When yellowing starts at the top, Penn State’s key points to low light, iron or manganese deficiency, or too much fertiliser. In lime-hating plants such as gardenias and azaleas, the RHS explains that hard tap water or ordinary compost leaves iron and manganese unavailable, so upper leaves turn yellow but stay firm, often between green veins. The fix there is rainwater at room temperature and ericaceous compost, not more feed.

All over

General yellowing is where overwatering dominates. Clemson says most yellowing is a symptom of overwatering; it also lists too little light, too little fertiliser, insects and mites. The RHS adds a pot that is too big, too shady a spot, cold, a sudden drop in temperature and draughts. Too much light can do it as well: NC State notes that light above a plant’s optimum destroys chlorophyll.

Overwatering or underwatering?

Both make leaves yellow and droop, and Missouri Botanical Garden points out that the symptoms can look the same. The compost and the roots tell them apart.

For a drought-struck plant, University of Maryland Extension suggests standing the pot halfway deep in warm water for an hour or two so the shrunken root ball can soak up water again. For an overwatered one, stop watering, empty the saucer, and if it is wilting, check the roots before anything else.

Does it need feeding?

Sometimes, but rarely first. A plant in the same compost for years, never fed, with lower leaves paling evenly, may well be short of nitrogen, and Clemson recommends regular feeding and repotting in fresh soil. Feed while it is growing and cut back in winter, when most plants rest (Missouri Botanical Garden). Too much feed causes problems of its own: salt build-up damages roots (Clemson), and Penn State lists overfertilising among the causes of yellow young leaves.

A worked example: yellowing all over in a dim corner

The chart above starts with a plant yellowing all over, compost wet, watered little and often, well back from a window, in the cold months. Overwatering ranks first because every answer supports it. Root rot comes next and is tagged red: in wet compost it is worth ruling out quickly, especially if the plant starts to wilt. Too little light is third: winter light is weak, and the RHS suggests moving plants closer to a south- or west-facing window for the winter.

The order of work is the order of checks. Feel the compost and lift the pot today. If it is wet and the plant is not wilting, let it dry before watering again and move it nearer the light. If it is wilting while wet, tip it out and look at the roots straight away.

Common mistakes

• Feeding first. In winter plants need less feed as well as less water (NC State). Check the watering before anything else, since Clemson names overwatering as the usual cause.
• Moving it from spot to spot. Every move makes a plant adjust again; NC State says most plants drop a few leaves after a move.
• Repotting a yellowing plant into a bigger pot. The RHS warns that a much bigger pot leaves compost sitting wet, causing yellowing weeks or months later.
• Ignoring the saucer. Missouri Botanical Garden says to water until it drains, then tip away the excess.

General information from the extension and botanic-garden pages listed below, not professional advice.