Why Are My Plant's Leaves Turning Yellow? 9 Causes and Fixes

A yellow leaf is the plant's cheapest way of telling you something changed. Chlorophyll is expensive to maintain, so when a plant is short of water, light, root oxygen or a specific nutrient, it dismantles the chlorophyll in the least valuable leaves first and moves the useful parts somewhere else. That is why yellowing almost always follows a pattern.

The pattern is the diagnosis. Before you change anything, look at which leaves went yellow, not just how many.

Start by reading the pattern

Mobile nutrients such as nitrogen and magnesium can be pulled out of an old leaf and sent to a new one, so a deficiency of those shows up at the bottom of the plant. Immobile nutrients such as iron and calcium cannot travel, so a shortage shows up in the newest growth while the old leaves stay green. Water and light problems have their own signatures too.

1. Lowest leaves yellow and drop, soil stays wet for days, stems feel soft near the base.Overwatering or poor drainage
2. Lowest leaves yellow and crisp, soil pulls away from the pot, leaves droop before they turn.Underwatering
3. Whole plant pales evenly and growth has slowed, oldest leaves worst.Nitrogen shortage
4. New leaves yellow with green veins still showing, old leaves fine.Iron shortage, often from alkaline water
5. Old leaves yellow between the veins in a V from the leaf edge inward.Magnesium shortage
6. Only the side facing the room yellows and drops, the window side is fine.Not enough light
7. Bleached, papery yellow patches on the leaves nearest the glass.Too much direct sun
8. Fine stippling or speckled yellow dots, sometimes with webbing under the leaf.Spider mites
9. One or two old leaves a month, everything else thriving.Normal leaf turnover, do nothing

1. Overwatering, which is really a root oxygen problem

This is the most common cause by a wide margin, and the name is misleading. Roots do not drown in water, they suffocate: waterlogged soil has no air gaps, roots cannot respire, they die back, and a plant with fewer working roots cannot take up water. That is why an overwatered plant often looks exactly like a thirsty one.

The tell is the soil, not the leaf. If the top few centimetres are still damp several days after watering, and the yellow leaves feel limp rather than crisp, you are watering too often or the pot is not draining. Move the plant somewhere brighter and warmer so it uses water faster, stop watering until the top third of the soil is dry, and check that the drainage hole is not blocked. A pot sitting in a full saucer will never dry out.

2. Underwatering

Less common indoors but easy to confirm. The soil is dry deep down, not just at the surface, the pot feels light, and the leaves went limp before they went yellow. Water thoroughly until it runs from the drainage hole, and if the soil has shrunk away from the sides of the pot, stand it in a few centimetres of water for twenty minutes so the root ball can rewet properly.

3. Light, in both directions

A plant getting less light than it needs will shed the leaves it cannot afford to run, starting with the shaded interior and the side facing away from the window. A plant getting more direct sun than it is adapted to gets bleached, dry, yellow-white patches on the most exposed leaves, sometimes with a papery brown centre. Rotating the pot a quarter turn each week evens out the first problem. A sheer curtain fixes the second.

4. Nutrients

Potting mix is mostly bark, coir and perlite. It holds very little nutrition of its own, and whatever slow-release fertiliser was mixed in at the nursery is usually spent within a few months. If a plant has been in the same soil for more than a year with no feeding, an even yellowing of the older leaves is a nitrogen shortage and nothing more dramatic.

Feed at half the strength on the label during the growing season, and not at all when the plant is not putting out new growth. More fertiliser will not make a stalled plant grow, and salt buildup causes its own brown leaf tips.

5. Cold, draughts and sudden moves

Tropical foliage plants sitting against a cold windowpane in winter, or in the path of an air conditioner, will yellow and drop leaves without any other symptom. So will a plant that was just moved from a bright shop to a dim corner: it drops the leaves it grew under better light and then stabilises. Give a newly moved plant a month before you change anything else.

6. Pests

Spider mites, thrips and scale all cause yellowing, but the pattern is fine and speckled rather than a whole leaf turning at once. Turn a yellow leaf over and look at the underside in good light, along the midrib and where the leaf meets the stem. If you see fine webbing, tiny moving dots, or small brown bumps that do not brush off, treat for pests before you change your watering.

What to actually do this week

Remove leaves that are more than half yellow: they will not turn green again and the plant is still spending energy on them. Leave anything that is mostly green. Change exactly one thing at a time, watering being the usual candidate, and give it two weeks before you judge the result. Plants respond on a slower clock than we do, and stacking four changes at once means you never learn which one worked.

If you are not sure which pattern you are looking at, photograph the whole plant plus one close up of a bad leaf and run it through Identium's diagnosis. It reads the symptoms from the photo, gives you the likely cause and a treatment, and tells you whether the problem spreads to your other plants, which is the part people usually find out too late.