How to Choose a Grow Light

Measuring Light

The term lumens describes how bright a light is — to the human eye, that is. (Plants and people "see" light differently.) Generally, the higher the lumen rating, the brighter the light.

Intensity describes the amount of light that reaches a surface at given distance from the light source. A super-bright bulb that's 12" from a surface might, for example, provide the same light intensity as a dimmer bulb that's 2" from the surface. When you're growing under lights, you can control light intensity in two ways: by the brightness of the bulb and by how close the bulb is to the plant's leaves.

Duration describes the number of hours of light a plant receives in a 24-hour period. Outdoors, light duration varies with the locale and the season. The farther away you get from the equator, for example, the "shorter" the winter days (the few hours of daylight). Indoors, we control light duration by turning lights on and off or, more readily, with a programmable timer. A rule of thumb is to leave grow lights turned on for about 16 hours per day, giving plants eight hours of "rest."

Most plants benefit by having some time with the lights turned off so they can "rest"; during this time they use some of the energy they captured during the day to fuel growth. Certain plants require specific periods of daily darkness to initiate flower buds and trigger other growth patterns.

Color describes the visual appearence of light. Confusingly, it's described as "color temperature" though it has nothing to do with how hot or cold a bulb gets. Most grow lights provide a balance of cool (blue end of the spectrum) and warm (red end of the spectrum) light. (Although grow lights provide light in colors along the full spectrum, plants use little of the light from the green/yellow part of the spectrum.)

Color is often described using the term Kelvins. A grow light with a 5000 Kelvin rating will appear bluish. One with a 2500 Kelvin rating will appear reddish. In genereral, lights that skew toward the blue end are best for vegetative growth, while those that skew toward red end are good for stimulating plants to produce flowers and fruit.

Seedlings in particular require lots of bright light, and when they don't receive enough of it they get weak and leggy. In most cases, even the sunniest windowsill will not provide the intensity of light they need. A special grow light stand makes it simple to give seedlings the light they need. As your seedlings grow, you can raise the light fixture about once a week to keep the lights a few inches above the foliage.

LED, Fluorescent or Incandescent?

LED grow lights are especially good because they give off very little heat. They also use half the electricity, last 5x longer than fluorescent bulbs, are mercury-free, and won't shatter like glass. Regular incandescent bulbs are not used for indoor growing because they give off too much heat and can burn tender foliage.