Light is probably the most important factor for indoor plant growth. In most houses, it’s probably the most limiting factor as well. To add to the problem, light levels begin to dwindle in fall as the days become shorter. This means you may need to make some adjustments in the arrangement of your indoor plants during fall and winter.
In a recent article Ross Penhallegon, Horticulturist for Oregon State University, writes that when thinking about light levels for indoor plants, consider light intensity, duration and quality. Light intensity influences the manufacture of plant food, stem length, leaf color and flowering. For example, a plant grown in low light tends to be spindly with light green leaves. A similar plant grown in bright light would generally be shorter, better branched and have longer, darker green leaves.
The distance the plant is from the light source and the direction the windows in a home face determine the light intensity a houseplant receives. Southern exposures have the most intense light. Western and eastern exposures receive about 60 percent of the intensity of southern exposures, while northern exposures get only 20 percent of the light of southern exposures. Light intensity is also affected by the presence and type of curtains, the weather, shade from buildings or trees, the cleanliness of the windows and the reflectiveness of the surroundings.
Penhallegon suggests that you can compensate for low light intensity by increasing the time a plant is exposed to light. Increased hours of lighting allow the plant to make sufficient food to survive and grow. Additional light can be supplied from either incandescent or fluorescent lights or special horticultural fluorescent lights. Any type of light that burns a metal element, including the common household light bulb, is incandescent. Incandescent lights produce a great deal of heat (red spectrum) and are not very efficient users of electricity. Fluorescent lights are cool or blue light. By supplying a combination of incandescent and fluorescent lighting, most of the needed light spectrum is provided.
According to Penhallegon, using only cool white fluorescent tubes provides the best light for the money. Special plant growing fluorescent tubes make plant foliage and flowers appear more attractive, but they are more expensive than the cool white tubes, and in general, plant growth may be no better. Fluorescent fixtures can be suspended about 24 inches above the pots for short plants and progressively higher for taller plants. This will supply good lighting without the danger of heat injury.
Day length or duration of light is also of some importance to only those plants that are sensitive to day length, including poinsettia, kalanchoe and Christmas cactus. Most flowering plants are indifferent to day length. But, writes Penhallegon, all indoor plants need some period of darkness and should be illuminated for no more than 16 hours. When a plant gets too much direct light, the leaves become pale, sometimes sunburn, turn brown and die. During the summer, houseplants need to be protected from too much direct sunlight.
Ivy plants, philodendrons, foliage begonias and peperomias - grown mainly for their foliage - do well in indirect bright light. Tender plants, such as African violets and gloxinias, also should receive indirect light during the summer when the sun’s rays are extremely intense.
On the other hand, most flowering plants require higher light levels for the development of good flowers, explained Penhallegon. These plants grow best where they receive direct sunshine for at least half a day. Placing them near windows with an eastern exposure usually suits them best. Cacti and succulents, grown for their unique forms, require a sunny location. Coleus and crotons must be grown at high light intensities to maintain their decorative foliage colors.