Common pests and problems on houseplants

Watch on Hoyas, succulents, orchids and citrus

Mealybugs look like small blobs of white waxy fluff in leaf joints, stem forks and the crown of a plant. They are often mistaken for cotton wool or mould. Pull one apart and there is a pale, slow-moving insect underneath. They leave sticky honeydew and stunt new growth over months rather than killing a plant outright, though seedlings and cuttings can decline quickly. Dab them weekly with a cotton bud dipped in surgical spirit.

Watch on Bay, citrus, ficus and orchids

Scale insects are brown, yellowish or black domes, two to six millimetres across, fixed along stems and the undersides of leaf veins. They stay in place when pushed and lift off whole when scraped, which separates them from growths on the plant. Soft scales leave honeydew, so the leaves below turn sticky and then black. Armoured scales can weaken a plant without leaving sticky leaves. Scrape them off and wash the leaves down.

Watch on Figs, calatheas, palms and ivy

Spider mites make glossy leaves look dull and finely speckled with pale dots that will not wipe away. Turn a leaf over and watch for tiny moving specks. Fine webbing on the shoot tips means numbers are high. Hot dry air near a radiator or a sunny window helps them breed quickly, and a plant can lose most of its leaves within a month. Raise the humidity and shower the plant.

Watch on Monsteras, alocasias, calatheas and anthuriums

Thrips leave silvery or bleached patches on leaves, with tiny black dots of excrement. New leaves may emerge puckered and scarred. The insects are slender, about a millimetre long, and run rather than fly when disturbed. Silvering plus black specks is the key sign. An established plant can grow out of light damage, but thrips can carry untreatable viruses, so protect the rest of the collection. Hang a blue sticky trap and treat the leaves and compost together.

Watch on Soft new shoots, especially plants brought in from the garden

Aphids form dense green, black, pink or grey clusters on soft new shoots, leaf undersides and flower buds. They sit still when disturbed, unlike whitefly, and the white flecks among them are shed skins rather than a second pest. Most arrive on plants brought in from the garden in autumn. They leave sticky honeydew and curl the new leaves, though an established plant usually loses little. Rub them off by hand or rinse them under the tap.

Watch on Pelargoniums, poinsettias, hibiscus and herbs

Brush the plant and a cloud of tiny white insects lifts off, circles and settles again. Aphids and scale do not do this. The adults sit under the youngest leaves, so a plant can look clean from above. Most of the population is the flat translucent young stage stuck to the leaf underside, which is why swatting the adults changes little. They leave honeydew and slowly yellow the foliage. Wash the undersides, or release Encarsia wasps under glass.

Watch on Cyclamen, African violets, begonias and ivy

Cyclamen and broad mites leave old leaves looking normal while new leaves come out small, thickened and tightly curled at the edges, often with a bronzed underside. The mite is around 0.2 mm and hides in folded growth, so you usually cannot see it. Severe distortion with no visible insect is the key sign, though it is often mistaken for thrips or virus. Isolate the plant early and cut out the worst growth.

Watch on Any pot kept constantly damp, ferns and calatheas most

Fungus gnats are small dark flies, two to four millimetres long, that drift up from the compost when you water or move a pot. They stay near the pot rather than on the plant, which separates them from leaf pests. The larvae eat fungi and dead matter in wet compost and do little harm to an established plant, though they can kill seedlings. Let the surface dry between waterings.

Watch on Container plants, primulas and chrysanthemums

A plant with root aphids may wilt in afternoon sun and recover overnight, then eventually fail to recover. The leaves can look clean while the compost stays damp. Knock the plant out of the pot and look for grey, cream or yellowish aphids on the roots, often under a white or bluish waxy powder. Spraying the foliage will miss them. Wash the roots clean, cut away the worst damage, and repot into fresh compost.

Watch on Cyclamen, begonias, primulas and potted succulents

Vine weevil causes two separate problems. Adults cut neat semicircular notches from leaf edges at night. This looks untidy but rarely matters. The grubs eat the roots of a container plant through autumn and winter until it collapses. Tip out a failed plant and look for plump white legless grubs with an orange-brown head and chewed roots. Water on nematodes in early autumn while the compost is warm, and pick adults off after dark.

Watch on Damp pots and seed trays, rarely the plant itself

Woodlice are grey and armoured, and scatter when a pot is lifted. Millipedes are slow and cylindrical, and coil into a spiral when touched. Both usually eat tissue that is already rotting, so they are often present after another pest or rot has damaged the plant first. In a pot they appear in damp compost and seldom trouble a healthy plant, though they can graze seedlings and soft cuttings. Look for the original damage before treating either.

Watch on Succulents, orchids, peace lilies and figs

Root rot makes a plant wilt while the compost is wet, which separates it from drought. The roots have died and can no longer take up water, so watering again makes it worse. Lower leaves yellow and drop, the compost smells sour, and a lifted root ball shows brown, soft roots whose outer layer slides off when pinched. It is one of the most common ways a houseplant dies. Cut away the rot, repot into a smaller pot with an open mix, and water less.

Watch on Any leaf sitting below scale, aphids or whitefly

Sooty mould makes leaves look black, often after they have been sticky for a week or two. Run a finger over a leaf and the black coating lifts away, leaving sound green leaf underneath. The mould lives on honeydew and does not feed on the plant, so it is a sign of sap-sucking insects above rather than an infection. Check for aphids, scale, whitefly or mealybugs. Deal with the insect and the mould stops coming back.

Watch on Pelargoniums, peperomias and succulents

Oedema makes small raised blisters on leaf undersides. They are clear and watery at first, then dry into rough tan or rust-brown corky scabs, sometimes with matching ridges on stalks and stems. They will not rub off because they are part of the leaf. Oedema happens when roots take up water faster than the leaves can lose it, often in dull, still winter conditions. Water less, give more light and move some air.

Watch on Monsteras, alocasias, peace lilies and philodendrons

Guttation leaves clear drops on leaf tips and edges first thing in the morning. The drops are usually gone by mid-morning and may leave a small white crust where they dried. The plant is pushing sap out under root pressure overnight, which is normal, and the crust is often mistaken for scale insects. It wipes off with a damp cloth. Heavy guttation every morning can mean the compost is holding more water than the plant is using.

Watch on Calatheas, marantas, ferns and spider plants

Hard water leaves a hard white or buff crust on the compost surface and the outside of terracotta pots. It feels gritty rather than fluffy and returns after you scrape it off. On the plant, it can cause brown crisp leaf tips with a narrow yellow band between the brown and the green. Lime-hating plants may yellow between the veins while the veins stay green. Tap water carries dissolved calcium and magnesium that build up in a pot, and fertiliser adds more salts. Flush the pot through and switch to rainwater.