Light is probably the most essential factor for indoor plant growth and one of the most problematic. The sunniest location inside often provides less light than shady locations outside. Light is necessary for all plants because they use this energy source to photosynthesize their food. Three variables concerning light are important: intensity, duration, and quality.
Light Intensity. Light intensity (the brightness of the light) influences the synthesis of plant food, stem length, leaf color, and flowering. Plants grown in light that is too low in intensity tend to be spindly with light green leaves (pdf). Similar plants grown in very bright light would tend to be shorter, better branched, and have dark green leaves. The placement of the windows in your home affects the intensity of natural sunlight indoors. Southern exposures have the most intense light; eastern and western exposures receive about 60 percent of the intensity of southern exposures; and northern exposures receive 20 percent of a southern exposure. Indoor plants can be classified according to their general light requirements: very high, high, medium, and low. In general, low-light locations are more than 6 feet (1.8 m) from windows and receive no direct light. Medium-light locations are areas roughly 3 to 6 feet (0.9 to 1.8 m) from windows. High-light areas are within 3 feet (0.9 m) of south-, east-, or west-facing, brightly lit windows.
Light Duration. Day length, or the duration of light received by plants during a 24-hour period, is also important, especially to indoor flowering plants. Poinsettia, kalanchoe, and Christmas cactus initiate flowers only when day length is short (<11 hours of light). Most indoor flowering and foliage plants are indifferent to day length.
Light Quality. Light quality refers to the color of the light, either sunlight or artificial light. Incandescent or fluorescent lights can provide supplemental lighting. Incandescent lights (pdf) produce a great deal of heat and are not very efficient users of electricity. Fluorescent lights vary according to the phosphorus coating used by the manufacturer. Foliage plants grow well under cool-white fluorescent lights (pdf), which are cool enough to position quite close to plants.
Even relatively low light conditions in a home are enough to keep some flowering plants from initiating flowers. It’s important to know that the plant needs a critical length of time in total darkness for flowering to occur.
Light and water guidelines for selected foliage and flowering plants, Table 11.1 (pdf) in California Master Gardener Handbook. UC. Buy Publication
Plants Indoors: Selections for various environmental conditions. UC. Buy leaflet