Extension Educator, Horticulture
If we have a few more cloudy days, I know I will start to etiolate. Don't worry, etiolation is not a new disease or social crime. Etiolation is the pale spindly growth in plants due to lack of light. You know, the way plants look if you try to grow seedlings on the windowsill. Wait a minute, tall and thin, maybe I should rethink this.
Seedlings grown under too low of light are very poor quality, and probably won't survive outdoors. Luckily, we can use supplemental lighting with good results in growing plants indoors. However, not all lights are created equal. If we make good choices in the type of light, how long the light is left on, and the distance between the light source and the plants, we can have success in growing seedlings even without the sun.
The type of light we choose will effect the intensity or amount of light. The lights we are most familiar are incandescent and fluorescent. Incandescent, or the traditional light bulb, is an inefficient light source producing a lot of heat with not much light. They are also high in red wavelengths, but deficient in blue.
What's this red and blue wavelength stuff? Plants absorb light of many wavelengths, but mostly in the blue and red range. They absorb little in the green range, which is why leaves look green to us.
To get an idea of wavelengths, just look closely at the colors of a rainbow. You can see red and orange on one end and blue and violet on the other end. Light also includes infrared and ultra violet light on the far ends of the rainbow.
Intensity is traditionally measured in footcandles, which is the amount of light an international candle casts over a one square foot surface area from one foot away. To approximate 100 footcandles, hold your hand one foot away from a white piece of paper. If you can see the shadow of your fingers, then the light is at least 100 footcandles.
Because electric lights don't have the intensity of the sun, we generally leave the lights on longer to compensate. However, plants do need some dark time. Maximum time for seedlings is about 16 to 18 hours a day. Distance from the light to the plants effects the amount of footcandles the plants receive. For example, two 40 watt fluorescent tubes placed 1 to 3 inches from the plants deliver about 800 footcandles. At 12 inches away the footcandles drop dramatically to 250 footcandles. Since fluorescent tubes give off little heat, we can place them close to seedlings, about 4 to 6 inches.
Fluorescent lights are the most available, efficient and affordable lights to supplement plants. An inexpensive shop light with a pair of four-foot tubes available at most hardware stores works very well for growing seedlings. New tubes should be purchased each year.
To further confuse the choices, not all tubes provide the same wavelength. A combination of a cool white and a warm white (also listed as day light) is a good combination for growing seedlings, houseplants and African violets. Special grow lights or full spectrum lights are also available. They provide a more complete spectrum or range of wavelengths. Grow lights are also more expensive than regular fluorescent. For you competitive gardeners, the slight increase in growth may be worth the added price.
Starting seedlings indoors does not have to be difficult or expensive. It's a great way to start new varieties not available in stores and get a jumpstart on spring.