Why did feeding my plant make it worse?

Feeding is what people reach for when a plant looks unwell, and it is the wrong reflex more often than not. Fertiliser helps a healthy plant that has run out of nutrients. It harms a plant whose problem is water, roots or light, and in a container it can do that damage quickly.

Safety

What you'll need

• Plain water
• Fresh compost (optional)
• A larger pot (optional)

Step by step

1. Recognise fertiliser scorch.Brown, dry, crisp leaf edges and tips, worst on the oldest leaves; wilting even though the compost is wet; growth stopped; and often a white or yellowish crust on the compost surface or around the pot rim.
2. Understand why it happens.Fertiliser dissolves into a salt solution. If that solution is more concentrated than the sap inside the root cells, water moves out of the root rather than in. The plant then behaves exactly as though it were in a drought, in wet compost.
3. Flush a container heavily.Water thoroughly from the top with plain water and let it run out of the base, repeated three or four times over an hour. That leaches the excess salts out. Do it outdoors or in a sink, and let the pot drain completely afterwards.
4. Scrape off any surface crust.Remove the top 10 to 20mm of compost, including the crust, and replace it with fresh. That is where accumulated salts concentrate.
5. Repot if it was a heavy overdose.Slide the plant out, shake off as much compost as comes away easily, rinse the roots gently and repot into fresh compost. Do not feed for at least a month afterwards.
6. Work out why you fed in the first place.If the symptom was wilting, yellowing or poor growth, check water, drainage, roots, light and pot size before feeding again. Feeding is the last thing to try, not the first.
7. Never feed a dry plant.Concentrated feed on dry compost is the classic route to scorch. Water first, wait, then feed — or feed with the watering at label strength.
8. Never feed a waterlogged plant.Roots in airless compost are already damaged and cannot take up anything. Feed added on top is pure injury. Sort the drainage and wait until it is growing.
9. Never feed a dormant plant.In winter most plants are not taking anything up. The fertiliser accumulates as salts, and any growth it does force is soft and frost-tender.
10. Do not feed newly potted plants.Fresh compost carries four to six weeks of feed already, and new roots are the most easily scorched. Wait.
11. Recognise the opposite problem too.Too much nitrogen produces plenty of lush dark green leaf and very few flowers, and makes growth soft and attractive to aphids. That is not scorch; it is the wrong analysis, and the fix is a high-potash feed instead.
12. In the ground, water it in generously.Granular fertiliser scattered on dry soil and left there scorches roots and foliage where it touches. Water it in thoroughly, keep it off leaves and stems, and never exceed the label rate.

Tips

• A white crust on the surface of a pot is the most reliable visible sign of over-feeding.
• Half strength twice as often is safer than full strength occasionally, and usually works better as well.
• If in doubt, water. Almost nothing that looks unwell in a garden is short of fertiliser as its first problem.

Common mistakes

• Feeding a struggling plant as a first response — The commonest causes of a struggling plant are water, roots, light and pot size. Fertiliser addresses none of them and can add a second injury.
• Using more than the label rate — The rate is set by what the roots can tolerate, not by what the manufacturer can sell. More is straightforwardly harmful.
• Feeding dry compost — The undiluted solution sits against dry roots. Water first.
• Feeding a seedling in seed compost — Seed compost is deliberately low in nutrients because germinating roots scorch easily. Wait for the first true leaves and then feed at quarter strength, or pot on.

If it doesn't work

Questions people ask