Why have my container plants gone yellow and stopped growing?

This is the most predictable failure in container growing and the one people diagnose as a disease. A pot is a closed system: every nutrient the plant gets, you put there, and there is nothing underneath for the roots to reach into. Around six weeks after planting, the supply runs out.

What you'll need

• Balanced liquid feed
• Slow-release fertiliser granules (optional)
• The next pot size up (optional)

Step by step

1. Check the timing against when you planted.Yellowing and stalled growth four to eight weeks after potting into fresh compost is almost always the built-in feed running out. It is that predictable.
2. Look at which leaves went yellow first.Oldest and lowest leaves first, working upwards, with the plant otherwise sound: nitrogen shortage. Newest leaves yellow with green veins: usually the wrong compost or hard water on a lime-hating plant. All leaves at once with wilting: a root problem.
3. Feed and watch for a response.A balanced liquid feed at the label rate, weekly. If it is hunger, colour improves within one to two weeks and that confirms the diagnosis retrospectively.
4. Use the right feed for what you want.A balanced or high-nitrogen feed for leafy growth and green colour, a high-potash feed for flowers and fruit. Feeding a flowering plant nothing but nitrogen gives you a large green plant with few flowers.
5. Allow for peat-free behaviour.Wood-fibre based composts use nitrogen as they decompose, so they can run hungry earlier than peat-based mixes did. Starting feed at four to six weeks rather than waiting is sensible with peat-free.
6. Rule out waterlogging.Lift the pot: heavy and cold, with a sour smell and green algae on the surface, means saturated compost. Check the drainage hole is clear, take away any saucer, and raise the pot on feet. Roots in waterlogged compost cannot take up nutrients even when they are present, so feeding makes it worse.
7. Slide the plant out and check the roots.A solid circling mass means pot-bound: the reservoir is gone and so is the nutrition. Brown, soft, sour-smelling roots mean rot. Fat white C-shaped grubs mean vine weevil, and the plant will collapse suddenly if it is not dealt with.
8. Check the water you are using.Hard tap water raises the pH at the root zone over a season, which locks up iron and manganese and yellows the newest leaves on camellias, blueberries, rhododendrons and pieris. Rainwater from a butt is the fix for those plants.
9. Check the compost is right for the plant.An acid-lover in ordinary compost yellows between the veins of the new leaves. It is not common, but it is instantly recognisable and easy to correct by repotting.
10. Pot on if the plant has outgrown the pot.Feeding a root-bound plant helps for a fortnight. One pot size up with fresh compost is the real fix, and slow-release granules mixed in save the weekly job.
11. Set up next season so it does not happen.Mix slow-release fertiliser into the compost at planting, and diary a weekly liquid feed from six weeks. Between them, most container plants keep going all season.

Tips

• Six weeks after planting is when the compost is empty. Put it in the diary rather than waiting for the plant to tell you.
• Do not feed a plant that is wilting from drought or sitting in waterlogged compost. Fix the water first — fertiliser on damaged roots makes things worse.
• A yellowing plant that greens up within a fortnight of feeding was hungry, and that is worth knowing for next year.

Common mistakes

• Feeding at double strength to catch up — Excess salts scorch roots and make the plant look worse. The label rate weekly is far more effective than a large occasional dose.
• Feeding a plant that is stressed from drought, waterlogging or root damage — Damaged roots cannot use the feed and concentrated fertiliser around them causes further injury. Correct the water first, then feed once it is growing again.
• Assuming it is a disease — Uniform yellowing without spots, lesions or wilting is a nutrition or root problem, not a pathogen.
• Watering a lime-hating plant with hard tap water all season — Alkalinity accumulates in the pot and locks up iron. Rainwater is the answer, and a butt on the shed is the practical way to get it.

If it doesn't work

Questions people ask