For lush, green foliage, feed plants every two weeks using a fertiliser with a high nitrogen content. Most generic house plant fertilisers are rich in nitrogen and will keep foliage plants looking good. If the leaves look yellow, then a nitrogen-rich feed will give them a boost, as will any feed containing magnesium (Mg). Reduce feeding to monthly in autumn and stop completely in winter.
Orchids and flowering plants
Give flowering plants a generic house plant fertiliser every two weeks in spring, then change to a weekly feed that is rich in potassium in the run up to flowering.
Tomato fertilisers contain extra potassium, but are often too strong for houseplants, so dilute by half the recommended dose. Orchids do not appreciate heavy doses of fertiliser and can be killed by too much, so feed weekly with a generic houseplant fertiliser diluted to half or even quarter strength. If using commercially available orchid fertilisers, follow the dosage instructions on the pack.
Cacti and succulents
Most cacti and succulents can survive without supplemental feeding, but they will grow and flower more profusely if fed. Feed them with a specialist cactus feed to provide the right balance of nutrients. Avoid fertilisers with too much nitrogen, as they can promote soft, weak growth. Apply cactus fertiliser every two weeks in spring and summer only.
Citrus plants are extremely heavy feeders. Specialist citrus feeds are available, but a generic houseplant feed will suffice every two weeks in spring and summer. These plants often need more magnesium (Mg) than is available in regular feeds, so add a solution of Epsom salts monthly, especially if the leaves begin to turn yellow.
During winter, stop feeding entirely unless your plant is flowering, in which case a monthly dose of houseplant feed will help.
Some houseplants prefer ericaceous or acidic soil. These include African violets, gardenias, ferns and azaleas. Such plants should be fed using a specially-formulated ericaceous fertiliser. You don’t need to feed them in winter.
Plants such as sundews, pitcher plants and the Venus flytraps generally grow on soils that contain very few nutrients. They’ve developed the ability to catch insects to supplement their diet and don’t require additional fertiliser. It’s also wise to avoid feeding them too many bugs as even in the cleanest of homes, they’ll generally catch what they need without help.