Kelvin light refers to the color of light based on temperature, so speaking about the Kelvin number of a particular light source is another way to talk about the color of that light. Lights with lower Kelvin temperatures tend to be warmer, encompassing the red and yellow hues, while high Kelvin temperatures indicate cooler, blue light coloration. Plants use different colors of light for different growth functions, so the color of light you choose for your plants will affect how they grow.
Incandescent light appears yellow, measuring about 2,700 to 3,000 Kelvin, according to the U.S. Department of Energy. The primary benefit that plants derive from incandescent light comes from the red light provided by incandescent bulbs. Red light encourages flowering, according to the Oregon State University Extension, by helping plants to tell time based on light exposure and flower according to the season. Incandescents also give off a lot of heat, are short-lived, and used on their own, cause plants to grow spindly. For every 300 watts of incandescent light provided, you should provide 1,000 watts of fluorescent or another high-Kelvin source, suggests the University of Missouri Extension.
Fluorescent bulbs provide high Kelvin temperatures -- about 6,500 Kelvin -- appearing blue as a result. Blue light encourages leafy growth, serving another essential light need for your plants. Blue light causes plants to develop full, healthy foliage and also promotes strong, healthy growth of new seedlings. The University of Missouri Extension recommends fluorescent lights as one of the best light sources for your plants, citing not only the high Kelvin temperatures that encourage growth, but also the widespread availability and long life of fluorescent tubes. Aim for cool-white or warm-white fluorescents when making your selection, avoiding fluorescents labeled as white or daylight.
Metal halide (MH) lamps deliver light in a range of temperatures, from 2,700 to 5,500 Kelvin, due to the ability of the manufacturer to blend chemicals in the tube. This quality makes MH lighting some of the best for growing plants because the incorporation of both red and blue light produces flowers and fruit without compromising the vegetative growth of the plant. MH lighting also appears a lot like sunlight, according to the Home Harvest Garden Supply, so plants do not appear discolored under the lights. While the University of Missouri Extension also suggests MH as an excellent light source for plants, they point to difficulty obtaining lights for use in the home as a reason why many homeowners settle for fluorescents.