There are several types of lights available to grow plants. What works best depends on the individual needs of the plants, the growing space size and budgetary concerns about efficiency and set-up costs. Modern technology is constantly giving indoor gardeners new options to maximize their growing space and to diversify the types of plants able to be grown.
High intensity discharge lights, or HID, include metal halide and high pressure sodium lamps. These are usually used for plants that need full sunlight conditions or to cover a large space with the fewest number of lights. Because of the intensity of the light they produce, these lights are capable of penetrating to the lower leaves of plants or underwater, which lesser lights can't do to the same degree. HID lights produce a lot of heat.
T12 and T8 Fluorescents
Standard T12 fluorescent tubes work well to grow lower light loving plants such as foliage tropicals. Fluorescents are not nearly as hot as most other types of lighting; however, more bulbs are needed because the light they produce is less intense than other kinds of lamps.
T8 tubes put out slightly more light per watt than T12s and are a smaller diameter, so more tubes can be fit into the same space and generate more light with the same amount of energy.
T5, also known as compact fluorescent or power compact, is one of the best options for plant lighting on a smaller scale. These bulbs are more affordable than HID and produce more usable light per watt of power burned than T12 and T8 fluorescent tubes. The drawback to T5s is they produce more heat than other fluorescents and are a little more expensive to set up. Common shapes are straight tubes, U-shaped tubes and screw-in base spirals.
LED plant lights are just beginning to hit the retail market. Arrays of LEDs comprising of red spectrum, blue spectrum and full spectrum white diodes are available; some even have a screw-in base to retrofit existing pendant fixtures. LEDs show real prospects in eliminating the heat problems associated with other types of lighting, like HID lamps. They also are a possible solution for spot lighting and supplemental lights for a growing area.
Incandescent bulbs can be used to grow plants; however, they make better heaters. Most of the energy used is released as heat rather than as light. Where heat is not an issue, bulbs with a directional reflective surface built in will work for pendant-type fixtures. Incandescent bulbs are great for use in dark corners or hallways where a plant might not get enough light from a nearby window and a standard screw-in base fixture already exists.