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Light is an important factor regulating plant growth, and the closer natural conditions can be simulated, the greater will be the success in growing house plants. Many varieties originate from the floors of great tropical forests, where the sunlight is filtered through the canopy of branches and leaves overhead. As house plants they, therefore, prefer shade or semi-shade. In winter, the deciduous forest trees shed their leaves, and more, though less intense, light reaches the floor of the forest. In winter, when the days are short, such house plants need all the light they can get. During the day they should be moved as close as possible to a window, without endangering them from winter cold penetrating through the glass.
Generally speaking, green-leaved foliage plants prefer shade, and many will tolerate dark areas of a room. Those with coloured leaves require more light, but a north-facing window, which receives the strongest winter sun, is not an ideal position for many kinds of house plants, unless the sunlight is filtered by net curtaining or a venetian blind. Conservatories or glasshouses containing house plants should be shaded in spring and summer as a protection against strong sunlight. Remove all shading, however, during the autumn and winter in a temperature-controlled glasshouse.
Some house plants especially those that flower, such as geraniums, are sun lovers and thrive best with full exposure at all seasons. With these, care must be taken in summer that temperatures behind glass do not become too high. To
prevent this in hot weather, either the window in which the plants are grown should be opened, or lightly shaded.
It is hard to be specific about good or poor light. One rough test for light intensity is to place a white card facing the light source and hold your hand with outstretched fingers about 25cm above it. In theory, if light is 'bright', the hand will throw a definite shadow. If the shadow is faint, light is moderate. This test doesn't work when light is coming in through a wide area or from several directions and is actually coming under the hand. Probably a more reliable test is to say light is bright where room light switched on makes no noticeable difference to the brightness of the position.
Light can be supplemented by artificial lights, especially by fluorescent tubes only half a metre or so above the plants. This works well for foliage plants but to make flower buds, the red rays of incandescent globes are needed. Too many of these too close can create too much heat whereas fluorescent lights are not hot. There are special plant lights on the market.