The best lights for growing plants depend largely on what kind and how many plants you want to grow and how much you are willing to spend. Light comes in waves of particles. The distance between the peaks and troughs of these waves, called wave length, is different for each band in the color spectrum. Shorter waves, which plants need in order to grow, are bluer. Longer waves, which plants need in order to blossom and fruit, are redder.
Metal halide lights produce an abundance of blue light that promotes plant growth. Blue light is given off by the sun in the spring and summer. Metal halide lamps produce a lot of heat. They also use a lot of electricity and are expensive to run. Metal halide lights are good for growing vegetables and when you want to encourage the growth of stalks and stems.
High pressure sodium bulbs give an orange-red light similar to that given by sunlight in the late summer and fall. This light triggers hormones in plants that encourage them to produce buds, flowers and fruit.
High pressure sodium lights don't give as much light as metal halide lights, but they are cheaper to run and last twice as long. They are often used as streetlights. Greenhouses often start seedlings and encourage their growth with sunlight and use high-pressure sodium lights to get them to fruit.
CFL (Compact Fluorescent Lights) are those curling, screw-shaped fluorescent lights that are widely replacing incandescent bulbs. Traditional long, fluorescent tubes are designated by the letter T. The standard tube, called T12, is 1 ½ inches wide. T5 tubes, more recently on the market, are 7/8 inch wide.
Both CFL fluorescent lights and fluorescent tubes are now on the market that yield full spectrum light, meaning they give all visible wavelengths from blue through red.
CFL and T12 full spectrum tubes produce more full spectrum light and are best for germinating seeds and growing seedlings. Some T5 fluorescent tubes also produce spectrum light, but less of it; they are useful for plants that are budding or flowering.
Fluorescent lights give off less heat than metal halide lights, which means they can be placed closer to the plant for more intense light. They are cheaper to operate than either metal halide or high pressure sodium.
LED (light-emitting diode) lights are cheap to operate; they are the lights used to light alarm clocks, watches, electronic signs and blinking tickers. They have an advantage over older neon lights in that they can be easily changed with a computer, letting business owners change messages quickly. They are also used as grow lights.
LED lights are available that contain two bands of blue and two bands of red light, best absorbed by chlorophyll. The grow lights you see giving off a curious pink or violet light are LEDs. There have been claims that certain kinds of LED lights are better than metal halide lights, but those assertions have not been proven by systematic study.