Plants produce energy for growth through the process of photosynthesis, by which their chlorophyll uses light energy to convert carbon dioxide and water into food. Plants in natural environments respond to the different types of sunlight during their growing seasons. People growing plants in greenhouses or other indoor environments must attend to the light needs of their plants to encourage optimal growth.
Visible light in the blue spectrum causes plants to grow healthy and abundant vegetation. During the summer months when the sun is high in the sky, plants receive a lot of light in the blue wavelengths. Therefore, growers choose lights such as cool white fluorescent lights for helping seedlings to sprout and to develop healthy vegetation in their early stages of growth. Plants that do not have enough blue light often get lanky instead of maintaining a compact shape.
Lights in the red spectrum encourage plants to produce flowers because this is the type of light that is more common in the late summer when the sun is lower in the sky. Plants still require light in the blue spectrum, in addition to the red lights, to flower. In addition, some plants will only flower when they have at least 12 hours of darkness, and others will flower only when they have at least 12 hours of daylight, according to the University of Arizona. Flowering is important not only in plants grown for their flowers, but also in plants grown for their fruit or vegetables that follow the flowers.
Green light is essentially useless to plants because their green leaves reflect most of the light in the green spectrum instead of absorbing it. If a person were to try to grow plants under lights that emitted wavelengths only in the green spectrum, the plants would not grow at all. That is why lights designed specifically for growing plants indoors do include green light.
Some lights produce more heat than others, and this heat can have adverse effects on plants. For example, growing plants under incandescent bulbs is generally ineffective because these bulbs get very hot. The heat will damage the plants when the bulbs are placed within a foot or two of the plants, which is the distance required to benefit maximally from the light itself. Fluorescent, LED, metal halide and other types of grow lights do not produce as much heat as incandescent bulbs.
Lights vary in the intensity of light they produce, and growers can also vary the intensity by moving the lights closer to or farther from the plants. The University of Alaska recommends that growers provide about 25 to 30 watts of light per square foot of plants, although 10 to 20 watts may be adequate for seedlings. Less intense lights may not produce growth rates as high as those produced by high-intensity lights.