Gardening Articles: Flowers :: Annuals

The Facts of Light

by Michael MacCaskey

Light. The word is so common that we often don't think much about what it does. There is the light created by the sun, and the light of spiritual clarity. But how often do we think about how light is the beginning of all life on earth?

Inside each cell in green plants are microscopic worlds called chloroplasts. These trap and convert some of the energy in light, mix it with water and carbon dioxide, and turn it into a simple sugar. It is this sugar that plants use to grow, flower, and set seed. That sugar is also what we and other animals harvest from some plants to power our own activities.

The best light for plant growth is sunlight. But almost since the invention of electric lights, researchers and gardeners have used it to study plants and grow them. Artificial sunlight has the great advantage of being controllable. Whereas some days might be too sunny or cold or windy to provide the right light for delicate seedlings, the intensity and duration of artificial light can be fine-tuned.

For plants, the simplest and easiest artificial light to use is fluorescent light. Short of a greenhouse, a fluorescent light setup is the best way to start seeds or root cuttings indoors. Fluorescent light gardens are similarly ideal for growing and blooming compact plants like African violets, some orchids, and many other kinds of indoor plants. This article is about fluorescent lights, how they work, what the different numbers on their labels mean, and what you need to know to garden successfully with them.

Fluorescent lights

These are the most common and least expensive lights that gardeners can use to grow plants. They are more efficient than common incandescent lights for plant growth, giving more light per watt of electricity with less heat. By virtue of their light quality and cool operating temperature, they are by far the preferred way to establish seedlings.

Fluorescent lights work in two steps. First, electrons stream between electrodes at each end of the tube and produce ultraviolet light, which is in turn absorbed by phosphors coating the tube's inner wall. Those phosphors (substances that emit light when excited by radiation) convert and reradiate the ultraviolet light as visible light. The mix of phosphors determines the color of the light produced.

Fluorescent tubes range in power from 15 to 215 watts, but high wattage tubes are increasingly rare. Most useful for indoor gardeners is the 4-foot-long 40-watt "bipin" tube, with two contact points at each end. In general, the cost of a tube varies according to how common it is. For instance, a 4-foot cool or warm white tube costs as little as $2, while a specialty full-spectrum tube for plants costs about $20.

Tubes' useful lifetimes are surprisingly variable, ranging anywhere from 12,000 hours (about 18 months) to 34,000 hours (almost four years). Usually the least expensive cool white tubes are the shortest-lived. If maximum intensity is critical, replace tubes before they burn out, at about 70 percent of their rated life. At that point, most tubes are producing about 15 percent less light than when new. According to this schedule, you should replace a tube rated at 20,000 hours and used 14 hours a day after 33 months.

The cost of electricity is easy to calculate if you know how much your local utility charges per kilowatt-hour. Four 40-watt tubes operating for 13 hours consume about 2 kilowatt-hours of electricity.

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