A plant setting in a sunny window receives less light than it would in a greenhouse or garden. A plant growing in a window gets light from only one side. Plants show sensitivity to one sided light by leaning toward the light. Turning the plant keeps it straight but it doesn't cure the lack of adequate light exposure.
Light intensity varies with the season. Maximum light is available in summer, declines in autumn, and reaches a minimum in winter. During late winter and spring the amount of light increases toward the summer maximum. Watch plants for signs of light deficiency during the autumn or early winter. Deficient plants have pale spindly growth and may be gradually shifted to brighter windows. Moving a plant from a north window, directly to a sunny south window, can result in scorched plants. If a plant must be moved from dim light to direct sun, place the plant to the side of the sunny window. After several weeks, move the plant into more direct sun.
When selecting a window for a particular plant, consider more than the direction the window faces. A window opening onto a large unobstructed area provides more light than if a shade tree was growing just outside the window, although both windows may face south. A window above a heat vent or radiator may provide perfect light exposure, but the temperature and humidity levels may be bad.
Light duration greatly influences plants, as flowering and rest periods are triggered by particular daylengths. Actually it is the length of the dark period, or night, that controls the response. For instance, Poinsettia blooms when given long nights. Blooming is prevented by a few minutes of light, given during the dark period. Some plants enter rest periods during the fall and winter in response to shortening days. Duration can partially compensate for low light intensity. Plants growing in low light may be more tolerant, if the duration of the light is longer. This is often done with artificial lighting. However, plants generally do not do well in continuous light. Altering the daylength may disrupt the blooming cycle of flowering plants, or, it may be used to bring them into bloom at a particular time.
A plant may tolerate low light intensity but this does not mean it requires it. Rubber Tree tolerates low light intensity, but grows best when given at least six hours of sunlight. Do not confuse the term "tolerates low light" with the term "requires low light."
The light requirements for plants are sometimes listed as low, medium, or high. Hold a hand between the source of light and the spot where a plant is to be set. The amount of shadow gives a rough measure of available light. If there is no shadow, or if it is very hard to see, low light exists. If the shadow is somewhat blurred, but definitely there, medium light exists. A sharp and distinct shadow indicates high light conditions.
Plants lacking sufficient light exhibit several symptoms. They will be taller and more spindly than normal and may have smaller, paler leaves. The spacing between leaves will be longer than normal. Flowering plants will fail to bloom normally, or may develop flower buds then drop them. Leaf drop may occur, although leaf drop alone may be a symptom of many things. Plants having variegated leaves may produce entirely green leaves. Occurrence of several symptoms on a plant indicate a need for more light. Foliage developed in low light is very susceptible to scorching in direct sunlight. Spindly growth and the widely spaced pale green leaves are most indicative of light deficiency. Leaf drop, failure to bloom, and slow growth are symptoms of many plant problem, not just lack of light.
Too much light is encountered less often. Too much light, or scorching, causes whitish or bleached spots on the leaves, where the light exposure is greatest. Do not place intolerant plants in strong sunlight.