Houseplants suffer from fewer problems than plants growing outdoors, and any pest or disease symptoms are usually noticed at an early stage. If you inspect plants regularly, disorders can be identified and treated before they become serious.

Pests and diseases

ill houseplant

The main source of pest and disease infection is from new plants when they are brought into the house, so check plants thoroughly before buying. Houseplants that have spent the summer outside also need careful inspection before bringing inside, and any dead or dying foliage should be removed.

Always wash the outsides of pots before giving them a place indoors. If you can see signs of pests on a plant, pick or wash them off, or spray with an insecticide, then quarantine the plant for a week or two before mixing it with others.

When a houseplant is infected with a pest or disease, treat it promptly because a problem can quickly spread in congenial indoor surroundings. Always use a product suitable for houseplants and follow meticulously the manufacturer's instructions.

The most frequently found pests include:

  • Aphids: Tiny, soft-bodied insects, often found in colonies on shoot tips and the undersides of leaves. Sponge off with water or an insecticidal soap solution, or spray with an insecticide.
  • Caterpillars: Tell-tale signs include partly eaten leaves and dark droppings. These pests often cling to the undersides of leaves, and can be picked off by hand and destroyed.
  • Red spider mite: Minute insects that turn leaves yellow or mottled, eventually covering them with fine webbing. They like dry conditions and are often resistant to chemicals. Spray or wipe leaves with an insecticidal soap solution and increase humidity by misting or other means.
  • Mealy bugs: Patches of white fluff conceal these tiny greyish insects. Spray with an insecticidal soap every two weeks until clear of infestation.
  • Scale insects: Small raised blisters hide and protect these tiny insects, often found along stems and veins on the underside of leaves. Wipe off scales with a cottonbud.
  • Vine weevils: These white grubs feed on roots so the plant wilts, while dark adult weevils eat notches from the leaf edges. Both are difficult to control. Search out and destroy grubs among the roots, then repot in fresh potting compost, or water plants with a biological control (Heterorhabditis) when the temperature is 12°C (54°F) or more.

Top tip: Clean the outside of plant pots that have spent the summer outside before bringing them indoors. This group includes busy lizzie (Impatiens), shrimp plant (Justicia brandegeeana) and stromanthe.

Treat infested plants with a systemic insecticidal spray. For safety, do this in an isolated spot outside, away from other plants. If you have to spray indoors, wrap a plastic bag round the plant to create a closed environment and make sure the room is well ventilated.

 

Diseases

Diseased Houseplant

Serious diseases are rare but often difficult to treat, so it is better to discard badly affected plants. Isolate a diseased plant immediately and check others frequently for symptoms.

  • Rotting or decayed stems: Usually the result of excessive watering, especially in autumn and winter when plants need less water. Plants can sometimes be saved by cutting out affected areas and drying out the compost.
  • Moulds and mildews: Fluffy or powdery, white or grey patches occur in moist or cool conditions. Treat by removing infected foliage and spraying with an appropriate fungicide.

Top tip: Yellowing or unsightly damaged or dead foliage should be promptly pulled away, or cut with sharp scissors, as it can harbour disease.

 

Other problems

Unhealthy Houseplant

One of the most common problems is overwatering rather than under-watering. Symptoms of this include wilting flowers and buds falling off, and pale or brown-edged leaves.

Ill health may also be caused by draughts or fluctuating temperatures, insufficient light or too much direct sun, or the wrong humidity level. Hard water can cause the leaves of lime-hating plants such as azaleas to turn yellow.

Adjusting the growing conditions can often lead to prompt recovery, so review your care of a plant before assuming it is diseased. Bear in mind, however, that some plants, including annuals and bulbs, and older stems of bromeliads and cyperus, naturally die down after flowering, and some leaves of evergreens turn yellow and brown with increasing age.

Scale insects are one of the most common houseplant pests, particularly those with leathery or succulent leaves, and cacti. Remove them with a cottonbud and isolate the affected plant from its neighbours, which you should check regularly in case the pest has spread.

 

Holiday care

Before you go away, especially during summer, ask someone to look after your houseplants and leave a note of any special treatments that might be needed. If you have to leave the plants completely unattended, move them away from sunny windows and remove all flowers and buds.

Keep the compost moist by using self-watering pots, watering wicks or capillary mats. To set up a mat, place flat-bottomed pots with drainage holes on an upturned seed tray covered with a capillary mat. Keep the mat wet by trailing one end into a container with water in it. Raise the container initially so that the water level is above that of the pot bases, but once the water is flowing, lower the water level.

Alternatively, water the plants thoroughly, gather them together in a sink or bath, and pack wet screwed-up newspaper between the pots to create a moist microclimate. During winter, when most plants are relatively inactive, ensure the compost is moist, then position plants away from windows and draughts, in a temperature just above their preferred minimum.

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