It surprises many beginners to learn that temperature is frequently less important than light and moisture. Certainly many plants we now grow in our homes were regarded as ‘stove’ plants not many years ago, and it is only comparatively recently that it has been realized how hardy some of our housepiants really are. Great heat used to be thought necessary for vrieseas. for instance, but we now realize that most species will grow happily at 10 deg C (50 deg F). It is not that the plants have suddenly adapted, but false assumptions were made by previous generations of gardeners. If we look at an average collection of housepiants it is quite likely to contain philodendrons from the tropics of South and Central America, the rubber plant (Flcus elastica) from tropical eastern Asia, a kangaroo vine (Cissus antarctica) from Australia, aspidistras from China and Japan, cacti from places such as Mexico and Texas, and ivies (Hedera helix) from Britain. The amazing thing is that they will all grow happily together in the same home – and it demonstrates the adaptability of plants regarding temperature.

Very few housepiants will tolerate frost, however, and great care will have to be taken in selecting plants for rooms where the temperature is likely to drop below 7 deg C (45 deg F). At that temperature, most housepiants will live, provided they are kept very dry at the roots (some plants, including most cacti, appreciate a cold spell and will flower better for it). Raise the minimum temperature to 10 deg C (50 deg F) and most plants will thrive. A winter night temperature of 15 deg C.

(5S) deg F) may be required for a lew plants. but there is a danger that there will be insufficient light to support good growth during the winter months if the plants are stimulated into too much growth. Examples of housepiants that need 15 deg C (59 deg F) are caladiums. dieffenbachias, codiaeums, and littonias. Although a few housepiants will grow well in a temperature range from 7 deg C (45 deg F) to 15 deg C (59 deg F) – the sweetheart vine (Philodendron scandens) is one -most prefer a higher temperature, and this should be met as far as possible. Do not assume that extra warmth is always good, however – plants from temperate zones, such as ivies and x Fatshedera lizei will not grow very well if kept too warm in winter.

A resting period is vital to many plants. and during this time they should not be stimulated into growth by too much water or warmth. A spare bedroom or light garage with a good window are suitable for resting plants that need a cold period, such as most cacti and succulents, provided there is no danger of exposure to frost. Modern homes tend to allow a wider range of housepiants to be grown than was possible 20 years ago. They have larger windows, which means lighter rooms, and better, more evenly controlled, heating. Even so there are still dangers if the heating system is used only during the day. with the consequent drop in temperature at night. Obviously some drop in temperature is perfectly acceptable – it happens in nature – but with artificial heating in winter the difference can be quite dramatic. And the large windows that are so beneficial from a light aspect can permit a large heat-loss, to the detriment of plants placed on the window-sill. This can be especially pronounced if they are trapped between drawn curtains and the window.

If your home is likely to cool drastically at night, bring your plants into the room and draw the curtains early to retain as much warmth as possible. Both windows and doors can be a source of draughts, which can be extremely harmful to housepiants, not only because they lower the temperature in these areas, but because the turbulent air removes excessive moisture from the leaves in comparison with still air. Draughts may cause leaves and flower buds to drop.


The reason so many plants grow much better in a greenhouse than the home has more to do with light than temperature or humidity.

All housepiants need light – those we grow in shady or dull spots are simply more tolerant of poor light by reason of natural adaptation.

Shade-tolerant plants usually have plenty of green chlorophyll to take advantage of any light they do receive. But even these plants are likely to become drawn and leggy unless the light is of sufficient quality.

Plants with variegated or brightly coloured leaves almost always require good light, and the colour intensity will be diminished if it is not provided. With a few plants new leaves may emerge green if kept in heavy shade, but produce the variegation if transferred to good light (but don’t confuse this with shoots that have ‘reverted’ – these must be removed otherwise the whole plant may gradually go back to the unvariegated state). Good light does not necessarily mean direct sunlight. Magnified through glass. fierce summer sun can be too strong for many plants – that’s why shading is often provided for greenhouses during the summer months. Be especially careful with fleshy-leaved plants, which may suffer from sun scorch. This is particularly likely to happen if the plants are placed beside a sheet of decorative glass – of the type often found in entrance halls and front doors. A pattern of circles, for instance. may act as a magnifying glass and irreparably damage the leaves. A plant that has thrived happily in an adjoining window of plain glass for years may be scorched within a day if placed besides decorative glass in the same aspect in strong sunlight.

Light intensity falls very dramatically even a short distance from the window. This can be demonstrated most effectively with a light meter of the kind used by photographers. The same device will also emphasize the strong directional effect of the light. This will lead plants with a strong light requirement to turn towards the source and consequently produce drawn or eneven growth. Fven plants on the window-sill may benelit from a quarter of a turn of the pot each day, to encourage bushy, symmetrical growth. But don’t do this with plants just coming into flower as it may cause bud drop.

Some groups of plants, such as bro-meliads and orchids naturally live perched among forest trees or in other situations where they receive only filtered light. Because

they, are naturally adapted to these conditions, they can be grown away from direct light for most of the year. Other groups, on the other hand, revel in as much sun as they can get – the terrestrial cacti and succulents being good examples.

As a general rule, (lowering houseplants need more light than foliage plants. This is because good light is needed to produce a hormone responsible for flower initiation: the hormone, called llorigen. is only produced in sufficient quantity when the light intensity and certain other factors are right for the particular species. Some plants, such as a few of the desert cacti, have never ever Ilowered in this country because the light intensity is never great enough to initiate flowering in those species. If a plant is failing to flower, it may benefit from more light; if an African violet (saintpaulia) is not flowering well, for instance, it may bloom more prolilically if placed in better light. It is not only the quality of the light that affects flowering, but also the length of time for which it shines. With many houseplants this is not critical and they will bloom regardless of the day length. other factors having a stronger influence on flowering. But some other popular houseplants. such as the Christmas cactus (Sehlumbergera x buckleyi), kalan-choes and chrysanthemums, are profoundly influenced by day length. All-the-year-round chrysanthemums only flower as the days shorten and nights lengthen – a phenomenon commercial growers can use to produce blooms at will at almost any time of the year. By using artificial light at certain times and enforced darkness at others. the plants can be hoodwinked to flower out of season.

for this reason, and because a growth retardant is also used to keep them dwarf, all-the-year-round pot chrysanthemums can not be kept to flower in the same way for the next year. They can. however, be planted outside to grow into taller plants and to bloom at their natural time.

It is not normally practicable to advance flowering by artificially increasing the dark period at home, but it might be worth experimenting with kalanchoes. which can be induced to flower earlier by covering them with black polythene at the right time. Being short-day plants they normally flower in winter, but can be induced to flower in mid or late autumn if covered to ensure the plant only receives nine or ten hours of daylight during early and mid summer. Try to avoid the temperature rising above 15 deg C (59 deg F). As most people want their Christmas

cactus to flower at Christmas, the problem is usually one of ensuring that flowering is not delayed. Artificial light in the evening is going to delay flowering – even though it may only be switched on for a short time it will be sufficient to affect flowering. For that reason it is often best to keep the plant in a garage or spare room where artilicial lighting is likely to be used infrequently, until the buds start to form.

At the very first sign of flower buds. move it to its flowering position then leave il undisturbed. The flower buds will grow in relation to the light source, and turning or moving the plant will cause them to attempt to readjust – and almost certainly drop in the process.

Artificial light

Most domestic lighting is perfectly satisfactory for working and reading by. and even for displaying plants effectively, but ordinary light is not so suitable for growing plants, for efficient photosynthesis and healthy growth, plants need a different type of light. fortunately, lamp manufacturers produce special fluorescent tidies that produce light of the right quality, and these can be obtained from good electrical shops or from aquarists’ suppliers (they are used to illuminate aquariums, where light of the right type is necessary for healthy growth of aquatic plants). These can be used to grow plants in otherwise unsuitable places.

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