Short days put sunlight at a premium. Even houseplants feel it.

Mostly, they just sit and wait for better growing conditions. But 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, which runs from the very short gamma and X-rays (with wavelengths measured in millionths of a meter) to the long radio waves (with wavelengths measured in kilometers).

Plants utilize and respond to that part of the spectrum that is visible to us (390 to 780 millionths of a meter), except for a portion in the middle. That portion, which is green, is reflected rather than absorbed. That's why grass is green. Plants can't “see” it; we can.

Make light for plants

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

Without belaboring all the pros and cons of different lights, let's just say that plants can be grown to perfection indoors with a combination of run-of-the-mill, cool-white, fluorescent bulbs and screw-in incandescent bulbs.

A good balance of light is achieved with one 15-watt incandescent bulb for every 40-watt fluorescent bulb.

Plant growth is almost as good if you use fluorescent bulbs alone, especially if only foliage houseplants are grown. But incandescent bulbs, besides affecting flowering, cast a light that gives plants a warmer, more pleasant appearance.

Still in the experimental stage, but with a lot of potential for offering specific wavelengths and low energy use, are LED lights.

Some 2-by-4s, light fixtures and electrical equipment are all you need to build a stepped-down version of the phytotrons — research greenhouses — used by botanists.

These phytotrons are plant growth chambers within which light, temperature, and humidity can be carefully controlled and studied.

You almost need sunglasses to look into or enter phytotrons, where every inch of reflective wall and ceiling is covered with fluorescent and incandescent lights. Or, in some newer installations, LEDs.

In a home, a small, functional phytotron need take up only a few square feet in the corner of a basement, spare room or even a closet.

Use your phytotron as a recuperation area for light-starved plants, for bringing houseplants into flower before moving them for display, and for growing seedlings.

With attention to design and craftsmanship, you could build a permanently displayed habitat of healthy plants.

But it's not sunlight

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. Dust and age further diminish the light.

Plants vary in their light needs.

Cast-iron plant, baby's tears and other foliage plants grow happily with less than 250 fc.

Real flowers, so welcome this time of year, can be coaxed from flowering maple (this is Abutilon species, no relation to the tree maples outside), crown-of-thorns, African violet, and oxalis with 600 fc.

Put on your sunglasses and crank the intensity up to 1,400 fc for flowers on tuberous begonias, and Christmas cherries and peppers.

Artificial lights will make your plants feel that you have taken them on a winter trip to the Caribbean.