Recommended Watts for a Plant Light

Plants manufacture their own food using sunlight or, if grown indoors, artificial lighting. Lack of sufficient light causes straggly, light-green growth and dropping of the lower leaves. Too much can burn leaves, but is rarely a problem indoors. The amount of light that reaches a leaf depends on the wattage of the bulb, its distance from the plant and the number of hours the light is on. Twelve to 18 hours daily is recommended.

Fluorescent bulbs are the most commonly used plant lights. Inexpensive and readily available, they produce more light per watt of electricity than incandescent bulbs. Forty watt bulbs are usually used, though higher and lower wattages are available. The light from cool-white bulbs is more blue, from warm-white bulbs more red and a combination of both types is best for most plants. A fixture with a reflector will give more light per 40 watt bulb than one without.

Low-light plants can thrive with two 40 watt bulbs, without a reflector, placed one to two feet above them. You could make do with one bulb and some light from a window. Medium-light plants would need two bulbs as close as six inches.

Sun-loving flowers and vegetables are the most difficult to grow indoors. They need a minimum of four bulbs, plus reflector, six inches away from the leaves. Even with lamps of a higher wattage, the light diminishes considerably with distance so the lower leaves of a tomato, for instance, would be in shade.

Seedlings should be grown with the plant light only two or three inches above them to avoid legginess and toppling over.

For best light, plants need to be positioned toward the middle of a fluorescent fixture since the ends of the bulbs emit less light than the center.

High intensity discharge (HID) lights are more expensive than fluorescents, produce heat and use a lot of electricity but can give you up to 1,000 watts of light, a necessity for growing tomatoes and other plants that need full sun outdoors to flower or ripen fruit. The two types are metal halide and high pressure sodium. Metal halides have strong blue wavelengths and are often used for foliage plants, lettuce, herbs and other plants that don't need to flower. Sodium has a high amount of the red light needed for flowering and fruiting. The best lighting systems have both types of bulbs for broad-spectrum growth. Newer bulbs provide both red and blue wavelengths. Use 150 to 400 watt lamps for low- and medium-light plants, 600 or 1,000 watts for sun lovers.

Light emitting diode (LED) lights have the advantage of lower energy consumption, longer lifetime and lower heat output than incandescent or HID lights. Most LED lights, however, have a light intensity that is too low for growing plants, though there are LED lights on the market now that rival HID lamps for effectiveness. A 300 watt LED light will give the same light as a 1,000 watt HID lamp, best for fruiting vegetables. Use a 90 to 150 watt LED light for medium-light plants.