Plants require a variety of foods, including nitrogen, phosphorus, potassium and small quantities of many other minerals. Potting composts suitable for houseplants contain all these essential ingredients in carefully formulated mixtures, but this ready-made food supply does not last forever.

Potting composts contain only a limited amount of plant food, which quickly becomes used up by growing plants. The nutrients in soil-less composts normally last for about six to eight weeks, while those in soil-based mixtures linger slightly longer because the soil element also contains foods. After this, you need to start supplementary feeding if the plant is actively growing.

Plants that are resting, or dormant, do not need food: unused fertiliser can accumulate in the potting compost, to possibly toxic levels.

 

Dietary health check

Fertiliser

Whenever you water your plants, assess their condition and learn to recognise the first signs of starvation or over-feeding before their health begins to suffer. Feeding is not a remedy for ill health and can actually make matters worse.

Always check first for other possible causes, such as disease or being pot-bound, before assuming that a plant is suffering from starvation. After treating an ailing plant for a pest or disease problem, spray the leaves with a diluted liquid fertiliser, which can be absorbed quickly as a tonic if the plant needs it.

 

Forms of fertiliser

Fertilisers are available in different formulations—some kinds specifically for indoor plants, while others are for general use in the garden. Always read the manufacturer's instructions to make sure the one you choose is appropriate.

Use specialist fertilisers for orchids, bromeliads, and cacti and other succulents. Foliage houseplants prefer a balanced fertiliser, whereas flowering houseplants will perform better if you feed them with one that is high in potash.

  • Concentrated liquid feeds, soluble powders and crystals are the most convenient to use. The manufacturer's recommended amount is mixed into water and given to the plant at watering time. The same fertilisers but at greater dilutions can be sprayed on the leaves for very quick results.

    You need to repeat liquid feeding regularly, especially for very vigorous plants. Check the manufacturer‧s recommendations on feeding intervals, but also bear in mind the needs of each plant. Before applying liquid fertiliser, make sure the compost is moist, as the roots can be burnt if conditions are dry.
     

  • Clusters, pins and spikes impregnated with fertiliser are pushed into the compost, where they remain active for up to a year, slowly dissolving whenever the plant is watered. Although convenient to handle, they tend to concentrate food in one part of the compost unless several are evenly distributed around the pot.

 

Top-dressing

Exhausted compost can be refreshed by top-dressing, which provides an immediate supply of nutrients in a layer of fresh compost. The technique is particularly good for large plants growing in pots 25cm (10in) or more in diameter, because they are difficult to repot.

In general, the best time to top-dress is at the start of the growing season. Use an old table fork or a small scoop or trowel to remove a 2.5–5cm (1–2in) layer from the top of the old compost, taking care not to damage roots or stems. Replace with a layer of fresh compost, firm gently and water thoroughly.

 

When to feed

Always check how often and how much food a particular houseplant needs. The majority of plants will stay healthy if fed regularly during the main growing season, from mid-spring to early autumn, but pay attention to the needs of particular plants.

For example, a strongly growing busy lizzie (Impatiens) needs feeding every two to three weeks during spring and summer, while a less vigorous plant such as clivia can be happy with a single feed in spring and only two or three feeds in summer.

  • Upper leaves look small, yellow, pale or lacklustre.
  • Growth is slow, weak or non-existent during the main growing season.
  • Lower leaves turn yellow and drop when growth should be active.
  • Plants produce few or no blooms, and these flowers are smaller than usual or malformed in some other way.

Symptoms of over-feeding

  • Stunted or twisted growth occurs in summer, and thin weak stems develop in winter.
  • White crusty deposits develop on the surface of potting compost and around the rim of clay pots.
  • Unbalanced growth develops, because a fertiliser stick or cluster has produced a concentration of food in one place, and so damaged nearby roots.
  • Healthy leaves wilt, but not owing to the plant being too dry or too wet.
  • Leaf edges turn yellow or brown, in combination with one or more of the above symptoms.

Top tip: If a plant needs feeding when the compost is already very moist, apply a dilute foliar feed to the leaves or insert one or more fertiliser clusters in the potting compost. Either method avoids the risk of waterlogging the roots by adding yet more liquid.

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