Demystifying indoor grow lights - U of I Extension

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Demystifying indoor grow lights

This article was originally published on November 26, 2012 and expired on January 15, 2014. It is provided here for archival purposes and may contain dated information.

URBANA -- Using artificial lighting to grow or start plants indoors may seem like a complicated task, said a University of Illinois Extension horticulture educator, but you can succeed if you understand just a few simple concepts.

"The first thing to consider is which type of bulb to use," said Candice Miller. The light that plants use includes wavelengths that humans can see (visible light) and some of the wavelengths that humans can't see, such as microwaves and infrared light. Plants can capture the wavelengths in the blue and red parts of the spectrum between 400 to 700 nanometers to use during photosynthesis.

"The two main types of bulbs available for purchase are incandescent and fluorescent. Incandescents are not particularly good because, while they serve as a good source of red rays, they a poor source of blue, produce too much heat for most plants, and are about one-third as efficient as fluorescent bulbs," Miller said.

She said that fluorescent bulbs are the better choice if only one type of bulb will be used. Cool white fluorescent bulbs are more efficient and supply a small amount of red rays in addition to orange, yellow-green and blue rays but still do not provide quite enough light for optimum plant growth. Fluorescents made especially for growing plants can also be purchased but are usually more expensive. These have a higher output in the red range to balance the blue output.

"To lower the cost and still have optimal growth, a home gardener may consider using a combination of specialty grow lights and cool white fluorescent bulbs," said Miller. "Use one special plant-growing tube to each one or two cool white tubes."

Some fluorescent bulbs are labeled with "T" numbers. These refer to the diameter, in eighths of an inch, of tubes. "So a T8 tube is 8 eighths, or 1 inch, in diameter. Older tubes are T12, with most new bulbs T8 or even T5. These newer bulbs tend to be much more energy efficient and even with lower wattage can produce more light due to new technology and materials in their production," Miller said.

The quantity of light and placement of the light source are also important factors to consider. Fluorescent tubes give off little heat, so they can be placed as close as 2 to 6 inches above seedlings or plants. Incandescent and similar bulbs give off more heat, so they need to be kept a foot or more above plants. Most stands that hold the fixtures are adjustable to allow the light to be moved up and down.

The quantity and duration of light needed will vary from plant to plant. Light for growing is commonly measured in foot-candles. Low-light plants such as Chinese evergreen and peace lily need between 50 and 250 foot-candles. Medium-light plants such as African violets, begonias, dracaena, dumb cane, flame violet, and seedlings need 250 to 1,000. High-light plants such as most herbs and orchids need over 1,000 foot-candles.

"Light quantity and light duration are related, so you can often compensate for a lower level of light with a longer duration," Miller explained. "For medium- to high-light plants, 16 to 18 hours of light at day is recommended. For low- to medium-light plants, 12 to 14 hours of light could be used."

"Whichever lights you choose to use, be sure not to keep them on continuously, as plants need periods of darkness. A simple timer can be used to turn lights on and off."

Source: Candice Hart, Extension Educator, Horticulture,

Pull date: January 15, 2014