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.

Color choices

Fluorescent tubes vary in the color of light they produce. If you've ever experimented with a prism, you know that white-appearing sunlight is actually a combination of colors from violet to red. While some artificial light approaches the color of sunlight, none duplicates it. In all cases, lights produce more or less of certain specific colors.

On the other hand, plants don't need every color in sunlight to grow and flower. For instance, the fact that plants are green means they are reflecting and not using most of the green wavelengths in light. Plants mostly use wavelengths at the two ends of the visible spectrum: the blue end for foliage growth and the red end for flowering.

The earliest fluorescent tubes are now called cool white. These produce an abundance of light in the blue range, but insufficient red light. The so-called warm white tubes complement them by producing more red light.

For many plants, a combination of cool white and warm white tubes will produce good growth. But seedlings need bright light as soon as they germinate, including red and infrared light. Since not enough of this light is produced by either cool or warm white tubes, you'll need to provide supplemental light either from a nearby window or, more conveniently, with fluorescent tubes especially designed for plant growth.

Many different colors of tubes are available. Some are designed to produce specific colors of light, some to approximate sunlight. The color of fluorescent tubes is measured in two ways: color rendering index (CRI) and degrees Kelvin (oK). CRI is a percentage number that measures how closely the light approximates natural sunlight. Tubes with a CRI greater than 90 are considered to be full spectrum.

The color of light is also measured on the Kelvin temperature scale, and this measurement is called the color temperature. Light that is rich in red has a low reading in degrees Kelvin, and light rich in blue has a much higher color temperature. Ordinary incandescent light bulbs produce light of about 2,800 degrees K, while daylight, which is rich in rays from the blue end of the spectrum, has typical readings from 5,000 degrees to 7,000 degrees K. Fluorescent lights range in color temperature from about 2,700 degrees to 6,500 degrees K. A typical warm white tube is about 3,000 degrees K, while a full spectrum tube is over 5,000 degrees K.

Light intensity

It stands to reason that brighter lights deliver more energy to plants than less bright ones. But while most fluorescent lights appear to be about equally bright, here again careful measurements reveal many differences.

For gardeners, the most basic and generally most useful measurement of light intensity is a simple calculation of watts of fluorescent light per square foot.

Experiments and tests have demonstrated that most vegetables and flowering plants need 25 to 30 fluorescent light watts per square foot. Houseplants and seedlings do well with 15 to 20 watts, and germinating seeds need the least 10 to 15 watts per square foot.

A standard 4-foot-long 40-watt fluorescent tube provides 10 lamp watts per square foot. Two 40-watt tubes 6 inches apart supply double that amount.

From this standpoint, the most important choice about intensity is how many tubes your fixture supports. Assuming that most fixtures are about a foot wide or slightly wider, the intensity of light from one that holds four tubes (40 lamp watts per square foot) will be double one that holds two tubes (20 lamp watts).

The light intensity of fluorescent tubes is also measured and rated on another standardized scale called lumens. In short, lumens are a measure of the brightness of the emitted light. When it comes to growing plants, brighter--more lumens--is better.

You can measure the amount of light yourself by using various kinds of meters. Meters made for gardeners measure light directly in footcandles. Footcandles are a measure of the light that is reflected from a surface 1 foot away. One footcandle equals 1 lumen per square foot.

You can also use the light meter in your automatic camera: set the film speed to 100 and the shutter speed to f4. Place a matt white card where the plants will be. With the lens focused on the card only, note the shutter speed. If it indicates 1/250, you have about 250 footcandles, 1/500 equals about 500 footcandles, and so on.

The light that fluorescent tubes produce diminishes rapidly over distance. For instance, 6 inches below two 40-watt cool white tubes, light intensity is about 700 footcandles.

African violets grow well given 600 footcandles for 18 hours a day, so tubes 6 inches above them are just right. Seedlings grow best with higher light intensity, so lights should be 2 to 3 inches above them, close enough to deliver about 1,000 footcandles to the leaves. With insufficient light, they will stretch out and perhaps topple. Leave the lights on for 14 to 16 hours a day. A similar schedule works best for most plants.

Disposal of used tubes

Fluorescent tubes contain mercury, which the EPA classes as a hazardous material. However, current EPA regulations neither list nor exclude the tubes themselves as hazardous waste. It is considered safe to dispose of fluorescent tubes in any municipal solid waste landfill. Fluorescent lamp ballasts made before 1979 likely contain PCBs, a family of chemicals widely used until they were banned in the late 1970s. Unless the ballast is specifically labeled "no PCBs," you should assume it does contain them and only dispose of it at approved recycling centers.

Michael MacCaskey is editorial director at National Gardening.

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