Why indoor plants need a different approach

Outdoor plants can send roots down deep into soil that is constantly replenished by worms, leaf litter and rain. An indoor plant lives in a closed system. Everything it eats must come from the small volume of compost in its pot, and every time you water, some of those dissolved nutrients wash straight out of the drainage holes. A 13cm pot of houseplant compost typically holds enough nutrition to keep a plant going for six to eight weeks after repotting, and not much longer.

That does not mean you should be reaching for the bottle every week. Over-feeding is far more common than under-feeding, and it does more damage. The aim is to top up nutrients at roughly the rate your plant uses them, which changes with the season, the light in your room and whether the plant is actively growing.

When to feed: reading the seasons

In the UK, natural light levels drop sharply from October and rise again from March. Most houseplants respond by slowing down or stopping growth altogether. Feeding a resting plant in December pushes it to produce soft, weak growth that it cannot sustain on limited winter light, and excess fertiliser simply sits in the compost as salts.

• March to May: resume feeding as soon as you see new leaves or shoots. Start at half strength for the first two or three feeds.
• June to August: peak season. Most foliage plants want a feed every fortnight; hungry, fast-growing plants in a bright south-facing room may want it weekly.
• September to October: taper off as growth slows. Flowering types such as Christmas cactus should keep going if they are building buds.
• November to February: pause for the vast majority. Only feed plants in genuine active growth, such as those under grow lights or in a warm, bright kitchen, and then at quarter strength.

A simple rule: feed when the plant is making new growth, and stop when it is not.

What the numbers on the bottle actually mean

Those three figures are the NPK ratio, and they are always in the same order. Nitrogen drives leafy green growth. Phosphorus supports roots, flowers and fruit. Potassium hardens growth, aids flowering and helps plants cope with stress. The numbers tell you the ratio, not the strength.

A balanced feed such as 20-20-20 or 10-10-10 suits general foliage. A tomato feed, high in potassium, is a decent stand-in for flowering houseplants. Orchid feeds are simply weaker and often include more trace elements. For most indoor foliage plants, a ratio leaning towards nitrogen, roughly 3-1-2, is closer to what they actually use.

Strength, dilution and avoiding burnt roots

Root burn happens when the concentration of dissolved salts around the roots is too high. Water is drawn out of the root tissue instead of into it, and the fine feeding roots are scorched. It is not always obvious at first. Watch for brown, crispy leaf tips, a white crust on the compost surface or around the pot rim, wilting despite damp compost, and sudden leaf drop.

• Dilute to at least half the recommended dose. Manufacturers often assume outdoor use, where rain dilutes the feed and roots have room to escape.
• Water with plain water first, then feed, so the roots are already moist.
• Never feed a bone-dry plant, a wilting plant or one that has just been repotted. Fresh compost contains enough food for six weeks.
• Never feed a sick plant. Diagnose the problem first; fertiliser is not medicine.
• Flush the pot every few months with tepid water, letting it run freely through the drainage holes to clear accumulated salts.
• Use rainwater where you can, particularly if your tap water is hard. It reduces the gradual build-up of lime in the compost.

Matching the feed to the plant

• Foliage plants such as pothos, monstera, philodendron and rubber plants: balanced or nitrogen-leaning liquid feed, half strength, fortnightly through summer.
• Flowering houseplants such as orchids, African violets and anthuriums: a higher-potassium feed, and for orchids a dedicated weak orchid feed at quarter strength.
• Cacti and succulents: low nitrogen, diluted to quarter strength, just two or three feeds across the whole summer.
• Citrus: hungry plants that appreciate a high-nitrogen feed with trace elements, given at half strength through the growing season.
• Acid-lovers such as gardenias and indoor camellias: an ericaceous feed, plus occasional sequestered iron if leaves yellow between the veins.

Building a routine that fits your home

Little and often beats a strong occasional dose every time. The easiest method is to tie feeding to watering: keep a small jug, dilute a batch at half strength, and use it on the plants that are in growth that fortnight. Label the jug so nobody mistakes it for water, and use it up within a day or two.

Slow-release granules mixed into the compost at repotting can carry a plant through an entire season, but they are hard to control and release faster in warm rooms. They suit low-maintenance plants well and hungry ones poorly. Liquid seaweed makes a gentle fortnightly tonic if you want something difficult to overdo.

Keep notes for a season. Once you know which of your plants grow steadily through winter and which sulk until April, feeding becomes instinct rather than guesswork, and your plants will reward you with steady, sturdy growth rather than a flush of pale, floppy leaves.