Artificial lights can keep houseplants growing in winter

Fluorescent bulbs provide all the artificial light needed to root these cuttings during winter.

Short days put sunlight at a premium. Even houseplants feel it. For indoor gardeners who want plants to keep growing and even flowering this time of year, artificial light is the answer.

Visible light is only a small slice of the electromagnetic wave spectrum. Plants utilize and respond to that part of the spectrum that is visible to us.

Fluorescent light is rich in blue and the shorter wavelengths of red light, important for healthy foliage; incandescent light is rich in “far-red” and is important for flowering.

Plants can be grown indoors with a combination of cool-white, fluorescent bulbs and incandescent bulbs. A good balance of light is achieved with one 15-watt incandescent bulb for every 40-watt fluorescent bulb.

Some 2-by-4s, light fixtures and electrical equipment are all you need to build a version of the research phytotrons (greenhouses) used by botanists. In a home, a small, functional phytotron need take up only a few square feet.

How much light is there under one standard two-tube, 4-foot fluorescent light fixture? The unit of measure for light recalls the days before electricity: one foot-candle (abbreviated fc) is the amount of light a foot away from a candle. On a bright, sunny day outdoors, plants are showered with 10,000 fc. On a cloudy winter day, 500 fc. At 6 inches below the middle of the fluorescent fixture, 900 fc.

The illumination drops by about half for each additional 6 inches distance from the tubes.