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Rhonda Ferree's ILRiverHort

Rhonda Ferree's Horticulture Blog
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Demystifying indoor grow lights


I'm teaching an Indoor Gardening Series every Wednesday in October for Bradley University's OLLI program. I cover the indoor growing environment and how to properly care for plants grown inside. The most limiting factor for growing plants indoors is proper light. My colleague Candice Miller, University of Illinois Extension horticulture educator, provides the following simple concepts to demystify indoor grow lights.

"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. Plants most use wavelengths in the blue and red parts of the spectrum.

"The two main types of bulbs available 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."

Placement of the light source is also important 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.

"You can often compensate for a lower level of light by providing light for a longer period each day," 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 Miller, Extension Educator, Horticulture, mille116@illinois.edu


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