Post category: House Plants


House plants are attacked by quite a few kinds of pests because these appreciate the warm conditions, soft growth and the absence of predators.

Greenflies are the most common houseplant pest, greenflies, can appear on any plant at any time of year. Black, sooty moulds may grow in the greenflies excretions.

Red spider mites cause yellowing and curling of leaves and are difficult to control. Busy lizzie is often attacked.


Red spider mite damage


When a plant infested with white fly is touched, tiny white flies fly out. Though not as common as the other two, whitefly can cause severe stunting too.

Scale insects are protected by waxy scales and these sap-suckers can reduce growth and cause leaf-fall, especially on evergreen plants such as citrus. Scale insects can be controlled by dabbing with cotton wool dipped in methylated spirits.

Caterpillars and earwigs sometimes eat holes in houseplant leaves. Affected plants are usually near an open window. Damage is usually slight and not worth controlling.

Vine weevil grubs eat the roots of many house plants, especially cyclamen and woody plants. The adult weevil crawls into the house and lays eggs in the compost. Sudden wilting, although plants have been watered, and the presence of white grubs in the compost are the usual indicators.

Tiny black mushroom flies crawling on the surface of the compost, lay eggs that can feed on the roots as tiny larvae. These are an indication of over-watered compost and can be controlled by allowing the compost to dry out for a time.

Among the specific houseplant insecticides are: Derris, Malathion, Tumblebug, Kerispray, Bio Sprayday and Picket. Take the plants outdoors for spraying and spray all affected plants at the same time to avoid re-infestation.

Repeat if necessary. Alternatively, plants can be immersed, pot and all, in a bucket of diluted spray material. Use rubber gloves.

Root-rot is the most common disease of house plants, usually caused by over-watering. Affected plants often die.

Leaf and stem-rots are caused by various fungi, but mainly grey mould or Botrytis. Avoid over-watering or misting in the dormant season. Place plants in a warmer, brighter place. If they do not recover, discard them.


Soak-water the plant before potting. If the new pot is made of clay, put it in to soak too. If an old pot is used, wash off all traces of old potting compost. Place a piece of broken pot, or a flat stone, over the drainage hole if it is large.

Put some moist compost into the pot. Gently knock the plant out of the old pot, supporting the root ball on one hand, the pot with the other and tapping the pot edge against the edge of a bench or similar surface.

Try out the root-ball for depth in the new pot. Set it deep enough to allow 2-4 centimetres of rim space for watering, and one centimetre or so to cover existing root-ball with new compost. The bigger the pot the greater should be the watering rim space.

Fill in around the root-ball with new compost and firm gently. Clean the sides of the pot and water the compost lightly. Keep the plant out of direct sunlight for a few days after potting.

March/ April is the best time for potting because the plant gets the full benefit of the growing season. However, potting can continue through the summer until August.

Avoid potting in autumn and winter. There is generally no advantage in potting this late in the year – plants will not grow during that period and there is a risk of root rot diseases developing during the winter months.

Plants need to be re-potted when they outgrow their pots, which depends on the vigour of the plant. Every two or three years is enough for most house plants. The best indication that a plant needs re-potting is the compost drying out very quickly after watering, or the pot toppling over at a touch. Move gradually up through the pot sizes – do not put small plants into large pots.


Types of pots


Indoor plants are grown in a pot or container of some sort. The compost in the pot provides a medium for supplying moisture and nutrients, and it also supplies anchorage for the plant. As plants grow, they need to be moved into larger pots.




Ordinary soil is not suitable for house plants because frequent watering eventually cakes it into a hard mass, that admits neither water not air. An open compost is necessary for house plants, and compost for houseplants should be peat-based or contain a fair proportion of peat.

Peat-based composts include Shamrock Seed and Potting Compost, Erin Potting Compost, Bio Potting Compost and Levington Compost, Use lime-free compost Brown Gold, for lime-haters. Mixing these composts with garden soil, unsterilised, helps to deter vine weevils.




Plants need carbon dioxide from the air for growth. The old notion that it is dangerous or unhealthy to have plants in the bedroom at night is not true. Plants marginally increase the carbon dioxide level at night, having reduced it during the day – another person sleeping in the same room increases the carbon dioxide level many times more.

Roots need oxygen to live and for the uptake of nutrients. Air is excluded by excessive water in wet compost, causing the roots to die and eventually to rot. The roots ‘drown’ for lack of air, but this will not happen if the compost is open enough to allow air to reach the roots. Open, well-aerated compost is essential for healthy growth.




Air-borne dust is another pollutant that affects plants. House air invariably carries a lot of dust from open fires, clothing and furnishings. A thick layer of dust not only spoils the look of the plant but it reduces the amount of light reaching the leaves, which reduces growth.

Leaves should be cleaned from time to time, especially at the start of the growing season, by spraying with clean water or sponging large leaves. Leaf shine products, such as Bio Leaf Shine and Kerishine, can be used to enhance the shine of leaves, but only on naturally shiny-leaved plants. Do not use household polish, cleaners, oil or milk on house plants.


When to feed 


House plants depend entirely on whatever nutrients are in the compost, or are added to it subsequently. Potting composts have enough food for about two months. After that, plants must be fed for continued good growth.


Dieffenbachia needs regular feeding


Which feeds to use


Soluble plant food formulations are easy to use and safe for the plants. Solid fertilisers too easily lead to overfeeding and it is best to stick to special houseplant feeds. There are many brands: Phostrogen, Kerigrow, Baby Bio, Hygeia Plant Food, Miracle Gro, Algoflash, Shamrock and J. Arthur Bowers.

Some brands are high in nitrogen, which is the first figure in the analysis given on the packet or bottle, for instance 10:2:4. These are suitable for foliage plants. Flowering plants need a more balanced feed, such as the formulae 7:7:7 or 5:4:4.

Most kinds of plant food contain trace elements. Use slow release plant food tablets, or spikes, to reduce the frequency of feeding.


Too much feed


In extreme cases, too much plant food can cause a plant to wilt and die, especially when excessive plant food is applied to a dry pot. Constant over-feeding can cause stunting and brown edges to the leaves.

Quite often, over-feeding causes rapid, dark-green leafy growth at the expense of flowering; this is typical of geraniums. Over-feeding can give rise to a white salt deposit on the compost surface. However, in a ‘hard’ water area, this simply may be lime in the water that rises to the surface of the compost.


Too little feed


When plants are short of nutrients, growth is weak with small, pale leaves and thin stems. Often the leaves have yellowish blotches, or markings, between the veins.

On poorly fed plants, the flowers sometimes have poor colour, and the flower heads tend to be small. Stems and leaves can take on a purple tinge, especially in peat composts. Older leaves, or young leaves, can go yellow, and fall off or shrivel. Stems are spindly.




The method of watering has an influence on the frequency. Soaking the pots to half their depth in a basin of water will ensure that a full complement of water is absorbed. Allow to drain afterwards. Plants watered by this method

Watering from the top of the pot is easier because the plants can be watered in situ. Fill the rim space with water. If it soaks away quickly, fill it again. It can be quite difficult to re-wet dried out posts by this method as much of the water escapes between the compost and the pot.

A good compromise is to soak-water every third or fourth watering during the growing season. In winter, top-watering is best, because a full soak-watering might leave the compost too wet, with little prospect of drying quickly enough.

Always use water at room temperature, as cold water can cause plants to drop their leaves. Use only ‘soft’ water for azaleas, citrus, camellias, stephanotis, and indoor heathers. If the water supply is hard – ‘fur’ in the kettle – use rainwater, or melted water from defrosting the fridge.




Wet compost is cold, and this can slow down or even stop growth. On over-watered plants, new leaves can be small and weak. In winter, cold, wet compost exhausts the plant’s food reserves, the leaves turn yellow and fall off, or parts of leaves turn brown, often without drying out.

Do not stand plants in a saucer or tray of water, especially in winter. It is very common for entire plants to die if rotting of the roots follows over-watering. A heavy pot is a good indicator of too much water.


Too little water


Wilting is the most dramatic manifestation of water shortage. The plant cells, empty of moisture, deflate like a balloon and rigidity goes from the leaves and soft stems. Following wilting, plants seem to recover upon watering, but later, brown patches often develop.

Plants that frequently run short of water, even without wilting, suffer considerably, because parts of the root system may die. Growth is often affected – new leaves will be small.

Flowering can be hastened but the flowers tend to be small and often shrivel without opening. Dry plants look ‘hard’ – often with a bluish or greyish tinge. A light pot is a good indicator of dry compost, even though the surface looks moist.


Summer needs


Water is a critical need of house plants for good growth, and for keeping them cool by transpiration – the plant version of sweating. It follows that more water is needed in the warm months. Water is lost from the compost in the pot and through the pot itself if it is made of clay.


Gloxinia rots when over-watered


The ‘Touch Rule’ is a simple test. Place a finger firmly on the compost in the pot. If it feels dry and hard, the plant may be in need of water. If it feels cool and soft, the plant has enough moisture. If it feels cool and wet and moisture remains on the skin, the plant is over-watered.


Winter needs


In the October to March period, house plants need only a fraction of their summer requirements. Using the ‘Touch Rule’, keep the plants between moist and dry – certainly not wet. Ideally, the compost should be dry on the surface and moist just below. Plants maintained slightly on the dry side resist cold and disease much better.

Watering will be necessary only every seven to thirty days in winter. The winter flowering house plants make an exception to the general guideline. Cyclamen, azalea, poinsettia, potmums and christmas cactus should be kept just moist in winter.


Heated rooms 


Coleus likes warm conditions


Although house plants need warmth for growth, the main concern is to provide conditions warm enough for healthy survival through winter. Many house plants come through winter in such bedraggled condition that they are thrown out. Because indoor temperatures are usually higher than outdoors, most of the plants used as house plants are native to warm climates.


Partly heated rooms



Busy lizzie is quite tolerant

Rooms that are heated for part of the day in winter are suitable for the majority of house plants. The temperature usually stays above 10° to 12° Celsius, and this is adequate for most house plants. Flowering house plants stay in flower longer in cool conditions.


Unheated rooms


Rooms that are not heated at all are suitable for only a limited range of house plants. On a cold night, temperature levels can fall to 5° Celsius in such rooms, and even close to freezing on occasion. These are harsh conditions for house plants and many types will lose condition.

Plants that can survive unheated rooms include geraniums, rubber plant, fatsia, ivy, silk oak, spider plant, mother of thousands, cacti and succulents, cyclamen, primula and cape primrose. Note, that behind the curtains of unheated rooms, plants might be exposed to frost.

Freezing conditions often occur in greenhouses and conservatories in winter, and since very few house plants can survive sub-zero temperatures, greenhouses are not suitable for overwintering house plants unless artificially heated.


Too much heat


Excessive heat is usually localised – caused by heaters or radiators. Any plant in direct contact with a heat source will be scorched and, even though the heater can be some distance away, brown marks could still appear on the leaves nearest to the source. If the air in the room is generally too warm and dry, brown edges often develop on the leaves over a period of time.


Too little heat


The main symptom of a problem with low temperatures is leaf drop. Plants drop leaves for several reasons – too much water, too little water, root damage, old age – but in winter, if leaves drop off suddenly, it is usually from cold. They are often still green when they fall. Some plants sacrifice part of the leaf, which later goes brown.

Many plants can adapt to low temperatures over a period, but even normally hardy plants can suffer if they get a sharp shock. This can be caused by a sudden move from a warm room to a cold one, or by draughts causing low temperatures at floor level.

Avoid sudden changes and, if plants are suffering move them for a while to a warm room to recover. A first sign of trouble, very often, is listless appearance – a slight droopy look – not quite wilting. Plants can tolerate cool conditions better if they are getting good light and are not over-watered.


Low light 


Most indoor situations have some light. This ranges from very little to quite a lot – even within a average-sized room. There are plants to suit any situation where daylight reaches.


Croton needs good light


In really poor light situations, leave the plants in position for only a couple of months at a time, rotating them between good light and dull conditions. Otherwise consider installing artificial lighting of sufficient power to allow plants to grow in shaded corners. The lights should be aimed at the plants, and placed fairly close to them. Use ‘cool’ lights that will not scorch the leaves.

Green foliage plants tend to turn pale yellow-green, and variegated foliate plants tend to lose their variegation if there is too little light. Flowering plants flower badly, or not at all. Plants become lanky, with smaller leaves on long leaf stalks and with bigger gaps in between, and they turn towards the source of whatever light there is.


Good light


The majority of house plants need good light, such as the window-sill of an east, or west-facing room and near, but not on, the window-sill of a south-facing room. Flowering house plants, variegated foliage plants and green foliage plants all need good light – the needs of flowering plants being greatest and those of green foliage plants least.

Many house plants – mostly the foliage kinds – cannot tolerate the very bright light in south-facing windows and porches. Some plants revel in it, such as geraniums, cacti, succulents, yucca and bougainvillea. 

Fading is the tell-tale sign of too much light. Flowers lose their colour and often dry out at the edges of the petals. Foliage plants often fade to yellowish green, and variegated plants, though they enjoy some sun, may fade to whitish yellow. Ferns go brown from scorching, and other plants can develop brown spots on the leaves too. Danger time is summer.


Flowering plants


Flowering house plants can be very short-lived – in some cases they just one step up from cut flowers – or they can be quite long-lived. Some types, such as cineraria and primula are disposed of after flowering finishes.


Hippeastrum makes a good house plant


Others, such as begonia and cyclamen, have a bulb or tuber that is retained to flower again; and some such as geranium and azalea that stay green and leafy after flowering, are grown on as foliage plants until they flower again.