Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
The driving force in plant growth is light. Although sunlight usually gives the best results, you can substitute electric lamps with much success. Plants will grow properly as long as you control light intensity, color, and timing.
The light levels provided by electric lamps are lower than direct sunshine, but many plants can adapt. Intensity drops by the inch the further the lamps are away from plants. Adding more lights and moving them closer to plants increases light intensity. If faced with marginal light intensity, houseplant needs can sometimes be met by leaving the lamps turned on longer.
When light strikes a prism, its component colors - red, orange, yellow, green, blue, indigo, and violet- become visible. Each plays a part in plant growth, but blue light is most important to photosynthesis, followed next in importance by red light. Plants stay short, stocky, and leafy when they receive plenty of blue. They stretch, flower, and fruit early under red light. Ordinary cool white fluorescent tubes are a good source of blue light, and make an acceptable sole source of light for foliage plants. Common warm white tubes and incandescent bulbs provide the red end of the spectrum.
Special plant lights casting a purple glow became popular several years ago. Their popularity stems from a red-blue mix of color suitable for most plants and from the healthy lustre of illuminated foliage. These fluorescents go by names such as Plant-Gro, Plant-Light and Gro-Lux and cost much more than ordinary fluorescent bulbs. They are nice to have, but not essential. A cool white and a warm white combined do about the same for plant growth.
Incandescent bulbs release too much heat to be a valuable sole source of light. Their light comes from the red-orange end of the spectrum, causing leggy growth and rapid maturity.
The days can be long in an indoor light garden. Plants grown for flowering indoors under electric lights have high energy needs and grow well with 14 to 16 hour days. Flower and vegetable seedlings destined for an outdoor garden also require 14 to 16 hour days. The lower energy demands of foliage houseplants can be met with 12 to 14 hours of light.
The distance that you place lamps from your plants varies with the types of plants you are growing. Some suggested distances from foliage to bulb using 48-inch fixtures holding two 40-watt fluorescent tubes are 3" to 6" for garden transplants, 4" to 6" for flowering houseplants, 3" to 6" for cacti, and 4" to 10" for foliage houseplants.
If plants fail to flower, stretch too much, or show pale color, they probably need more light. Leaves appear bleached when receiving more light than they can handle, but this is rare when using fluorescent lights.
Finally, keep lighting equipment clean and in good shape. A layer of dust can cut light intensity dramatically. New fluorescent lamps have the brightest light. The lifespan of 40-watt tubes is 9,000 hours, a little over one year of constant use or nearly two years at the normal day lengths. Dark bands at the ends of a tube warn that it is time for replacement. Even though the bulb still lights up, don't keep using it. If you do, you'll deprive your plants of the full amount of light they need to flourish.
(Prepared by Ellen Silva, Extension Technician, Consumer Horticulture, Virginia Tech, Blacksburg, VA 24061-0327.)