Feeding Houseplants: Less Often Than You Think

Plants make their own food through photosynthesis, using light, water and carbon dioxide. Fertiliser supplies the mineral nutrients they also need — nitrogen, phosphorus, potassium and a set of trace elements — which in nature come from soil and decaying organic matter.

In a pot, that supply is finite. Every watering leaches some out through the drainage hole, and the plant consumes the rest. After roughly two to three months, a fresh bag of potting compost is largely depleted, which is when feeding starts to matter.

This framing explains the most important rule: fertiliser is a top-up for a growing plant, not a treatment for a sick one. Feeding a plant that is failing from low light or root rot adds stress to a plant that cannot process it.

Every fertiliser carries three numbers — the N-P-K ratio, giving the percentage by weight of nitrogen, phosphorus and potassium.

Nitrogen drives leaf and stem growth, phosphorus supports roots and flowering, and potassium regulates general function and stress tolerance. For foliage houseplants, a balanced formula such as 10-10-10 or 20-20-20 diluted appropriately covers essentially everything.

Specialist formulas are mostly marketing for the indoor grower. The genuine exceptions are orchids, which want a weak balanced feed at every watering, and cacti and succulents, which do better on a lower-nitrogen mix that does not push soft growth.

Manufacturer dosages assume vigorous plants in good light. Most indoor plants are neither, so half strength is the sensible default.

Through spring and summer, feed every two to four weeks at half strength for foliage plants in reasonable light. In autumn, taper off. Through winter, stop entirely for almost everything — a plant that is not growing cannot use nutrients, and the surplus accumulates in the medium as salts.

The exception is a plant under a grow light that keeps growing through winter. If it is producing new leaves, keep feeding it at a reduced rate.

Excess fertiliser does not simply pass through. Mineral salts accumulate in the medium, raising its concentration until it starts pulling water out of the roots rather than the other way round. The plant then shows drought symptoms while sitting in wet medium.

The visible signs are a white or yellowish crust on the medium surface and around the drainage hole, brown crispy leaf tips and margins, wilting despite adequate moisture, and sudden leaf drop after a feed.

The fix is to flush. Take the pot to a sink and run lukewarm water slowly through it for several minutes, letting it drain freely — roughly four times the pot's volume. Do this every three to four months as routine maintenance, not only when there is a problem. If build-up is severe, repot into fresh medium instead.

Pale, yellowing leaves are usually a light or water problem, not a nutrient one. Diagnose in that order: light first, then watering and roots, then nutrients.

Genuine deficiency in a houseplant looks specific rather than general. Uniform yellowing of older leaves suggests nitrogen; yellowing between the veins while the veins stay green suggests iron or magnesium, common in plants that have been in the same medium for years.

If a plant has been in the same pot for more than two years and looks tired despite good light and watering, repotting into fresh medium is usually more effective than any amount of fertiliser.