Can You Over-Fertilise Plants? Signs of Too Much Fertiliser

More fertiliser does not mean more growth — it can mean dead plants. Over-fertilising is a real and common problem, especially for enthusiastic beginners who assume that if a little is good, more must be better. Recognising the signs of over-fertilising early can save a plant or at least prevent you from repeating the mistake.

What Fertiliser Burn Actually Is

Fertiliser burn is caused by excessive salt concentration in the soil solution. Most fertilisers are salts at the chemical level. When the concentration of salts around the roots is higher than inside the root cells, water is drawn out of the roots by osmosis rather than into them — the plant effectively becomes dehydrated even in moist soil. Roots die, the plant cannot take up water or nutrients, and leaves wilt and burn.

Symptoms to Look For

The symptoms of over-fertilising can look confusingly similar to drought or some diseases, but the context (you just fed the plant) is usually the clue:

• Brown, crispy leaf edges and tips — the classic fertiliser burn pattern
• Wilting despite moist soil
• Yellowing leaves, often starting from the edges
• White, crusty deposits on the soil surface (salt build-up)
• Lush, dark green but very soft, floppy growth (excess nitrogen)
• Poor flowering and fruiting despite vigorous leaf growth
• Root damage visible if you remove a plant from its pot

The Nitrogen-Specific Problem

Even without reaching toxic salt levels, too much nitrogen causes its own problems. Excessively lush, soft growth is very attractive to aphids and other sap-sucking insects. Soft tissue is also more vulnerable to fungal diseases like botrytis and mildew. High nitrogen late in the season pushes plants to keep producing leaves when they should be hardening off and preparing for winter, making them more vulnerable to frost damage.

How to Fix Over-Fertilised Plants

For plants in the ground, heavy watering flushes excess salts down through the soil. Water deeply and thoroughly several times over a few days. Remove any granular fertiliser you can see on the soil surface. Stop feeding entirely for at least four to six weeks. If the roots are severely damaged, cut back above-ground growth to reduce demand on the root system.

For container plants, flush the pot thoroughly by running large amounts of water through it. In severe cases, repot into fresh compost, removing as much old compost as possible and trimming any blackened or mushy roots before repotting.

Getting Back on Track

Once plants have recovered, restart feeding at half the recommended rate and build back up gradually. Always follow label instructions, which are set conservatively precisely to avoid this problem. Use slow-release granular fertilisers in preference to heavy liquid feeds if you tend to over-apply.