The Effects of Light
Plants use artificial light in the same way they use natural sunlight. Light serves as a source of energy that plants use to convert carbon dioxide and water into sugars or carbohydrates in a process called photosynthesis. The parts of the plant that capture the energy in light--and give plants their green color--are called chloroplasts. Chloroplasts use only blue and red light to carry out photosynthesis, reflecting back green light (which is why we see plants as predominantly green) producing sugars and expelling excess oxygen. The carbohydrates provide energy for growth and processing of nutrients, especially nitrogen, to generate new chloroplasts.
Artificial Light and Natural Light
All life on earth, including plants, has adapted to the light spectrum of the sun. Sunlight contains a balance of red and blue light that cannot be duplicated by one artificial light source because the light spectrum of the sun and its brilliance are based on burning hydrogen and helium.
The spectrum of artificial light is determined by what is burning in the lamp---a material like tungsten, carbon, quartz in a halogen gas atmosphere or the fluorescing phosphor in a fluorescent light. Fluorescent lights tend to give off more ultraviolet light toward the "blue" end of the spectrum and incandescent lights tend toward the infrared end. Neither source approximates the spectrum or intensity of sunlight, making them imperfect sources of energy for earthly plant photosynthesis.
Using Artificial Light
Artificial light may not be the same as sunshine, but just a few hours of artificial light a day can make the difference between a sickly, spindly houseplant and a healthy one--or a garden plant surviving the winter indoors. Too much infrared light will burn plants (much red-spectrum light travels as heat) and too much fluorescent light will encourage "leggy" growth of branches as the ends strain toward the light source.
Combinations of incandescent and fluorescent lights answer this problem. According to the University of Missouri Extension service, a balance of about three fluorescent watts for every one incandescent watt gives plants the right balance of blue and red light. Special plant-growing lamps and tubes also balance colors but may be more expensive to purchase. Another challenge in using artificial light to grow plants is to provide the correct intensity of light. High-light plants like geraniums may need 1,000 foot-candles, or 20 watts per square foot of growing area for 12 to 16 hours a day.
The intensity problem is most conveniently addressed by mounting light sources close to plants to capitalize on all available candle power, choosing only low or medium-light plants for the indoor garden and timing the indoor start of seeds so that seedlings can be moved outdoors as soon as the weather allows in the spring.