NGA Articles: The Facts of Light

The Facts of Light (cont)

By: Michael MacCaskey

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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