Adding too much fertiliser, applying it too frequently, or feeding during periods of dormancy can damage or kill roots, cause leaf burn, promote rapid weak growth, and ultimately weaken the plant. Understanding how fertiliser works and when to apply it is as important as understanding the nutrients themselves.
Fertiliser works by providing soluble nutrients in the compost that roots absorb along with water. When fertiliser concentration in the compost becomes too high, the osmotic pressure around the roots reverses: instead of drawing water in, roots lose water to the surrounding compost solution. This is called fertiliser burn or salt toxicity, and it physically damages root cells. Affected roots turn brown and mushy at the tips, reducing the plant's ability to take up water and nutrients even when adequate water is present.
The signs of over-fertilising are sometimes mistaken for other problems. Brown crispy leaf tips and edges that appear to start at the ends of the leaf and work inward are a common sign, similar in appearance to underwatering but accompanied by other clues.
Yellowing lower leaves, a white crusty deposit on the compost surface or on the pot exterior (salt accumulation from fertiliser residues), wilting despite moist compost, and stunted or distorted new growth are all signs that fertiliser levels may be too high.
When Over-Fertilising Is Most Likely to Happen
The most common cause of over-fertilising is applying fertiliser during autumn and winter when houseplant growth slows or stops. Plants in low-light conditions absorb nutrients far more slowly than actively growing plants in spring and summer. Fertiliser applied in winter accumulates in the compost because the plant is not using it, building up to damaging concentrations. Feeding should generally be reduced or stopped entirely from October to February for most houseplants.
Applying fertiliser at a higher concentration than the label recommends is another frequent cause. The temptation to add a little more in the hope of faster growth usually produces the opposite effect. Using undiluted liquid fertiliser rather than diluting it correctly, or adding granular fertiliser in excess of the stated amount, leads directly to salt build-up.
How to Fix Over-Fertilising
The primary remedy for salt build-up from over-fertilising is flushing the compost thoroughly with plain water. Water heavily and allow excess water to drain freely, repeating two or three times to wash accumulated salts out through the drainage holes. This is most effective in a pot with good drainage. Do not fertilise again for at least four to eight weeks after flushing, and when you resume, use a half-strength dose.
If the root damage is significant and the plant is wilting despite flushing, repotting into fresh compost may be the best course of action. Remove the plant from its pot, gently shake off as much of the old compost as possible, trim any visibly dead or mushy roots, and repot into fresh potting soil. Fresh compost removes the accumulated salt environment entirely. Hold off fertilising for at least six to eight weeks after repotting to allow roots to recover.
How to Fertilise Correctly
Most houseplants benefit from feeding once a month during the active growing season (March to September in the UK) at the dilution recommended on the fertiliser label or slightly weaker. Applying a balanced liquid fertiliser with roughly equal proportions of nitrogen, phosphorus, and potassium covers the nutritional needs of most tropical houseplants. Specialist formulations for orchids, cacti, or leafy tropicals are worth using for those specific plant types. Always water the compost before applying liquid fertiliser to avoid applying nutrients to dry compost, which increases the risk of root burn.
Granular slow-release fertilisers are a lower-risk option as they release nutrients gradually over several months. However, they can still accumulate if applied at the same time as regular liquid feeding, so choose one approach rather than combining both. Worm castings incorporated into the compost at potting time provide a gentle, low-risk background nutrition that is difficult to over-apply.