Why are my plant leaves turning yellow?

Work down the list

1. Too much water, and roots that have started to rot

   What you would see

   The oldest leaves at the base yellow first and drop, the plant may wilt even though it has just been watered, and the compost has stayed wet for a fortnight.

   The check

   Lift the plant out of its pot. Healthy roots are white or tan and firm. Rotted ones are brown to black and soft, and the outer layer slides off between finger and thumb.

   Root rot and overwatering

2. Too little water

   What you would see

   Leaf edges and tips go brown and crisp before the yellowing spreads, the lower leaves are shed first, and the whole plant looks limp on a warm afternoon.

   The check

   Lift the pot. A dry one is noticeably light, and the compost has pulled away from the sides so that water runs straight down the gap and out of the bottom.

   Drought stress and underwatering

3. The leaf has reached the end of its life

   What you would see

   One leaf at a time at the very bottom of the plant, yellowing evenly and then dropping, while everything above it is green and growing.

   The check

   Count how many. A monstera or a rubber plant shedding its lowest leaf every couple of months is doing what it is supposed to do. Three or four at once is a different question.

4. Not enough light

   What you would see

   The whole plant pale rather than any one leaf yellow, new leaves smaller than the old ones, long bare stretches of stem, and the plant leaning towards the window.

   The check

   Stand where the plant stands and look at the window. If you cannot see a good piece of open sky from that position, the plant cannot either.

5. A nutrient shortage

   What you would see

   Yellowing in a pattern rather than all over. Between the veins with the veins themselves staying green, or a whole plant uniformly pale and small.

   The check

   Note whether it started at the bottom of the plant or the top. Bottom means a mobile nutrient such as nitrogen or magnesium; top means an immobile one such as iron, which is usually a pH problem rather than a shortage.

   Nutrient deficiencies

6. Something is drinking the sap

   What you would see

   Yellowing with the leaf surface also dulled, stippled or sticky, and often worse on one part of the plant than another.

   The check

   Turn a leaf over and look at the underside with the light behind it. Fine pale speckling and webbing at the shoot tips is red spider mite; brown domes on the veins are scale; white fluff in the joints is mealybug.

   Red spider mite

7. Cold, a draught, or a move

   What you would see

   Yellowing that appeared within a week or two of the plant being moved, repotted or left near an open door in winter.

   The check

   Think back three weeks. Tropical houseplants drop leaves after a change in position, a cold delivery van or a night beside a draughty window, and they stop once they have settled.

8. A leaf disease

   What you would see

   Yellowing around defined marks rather than spreading evenly, and getting worse week by week rather than staying put.

   The check

   Turn the leaf over again. Raised pustules that rub off as coloured powder are rust. Flat dark blotches with a fringed edge on a rose are black spot.

   Rust

9. Something eating the roots

   What you would see

   A container plant that yellows and then collapses over a few days, usually in autumn or spring, with no obvious cause above ground.

   The check

   Tip the pot out. White C-shaped grubs with orange-brown heads are vine weevil, and there will be very little root left.

   Vine weevil