Hold on tight: Here come the numbers.
Plants` light requirements are often stated in foot-candles, the amount of illumination equal to the light of one candle per square foot. Low-light indoor plants need 200 to 300 foot-candles for only a few hours a day to stay alive.
Light meters designed to measure foot-candles are available from Chicago Indoor Garden Supply in Streamwood or Charley`s Greenhouse Supply (see main story for addresses).
Suki Kroeber, a member of the Illinois Orchid Society, says her father used a formula to determine whether parts of his greenhouse were too bright or too dim for a particular orchid`s needs. The same method is outlined in the late James Crockett`s book, ”Crockett`s Indoor Garden” (Little Brown, $19.95).
To figure out how much light your plants are receiving, use a light meter or a 35 mm camera with a built-in meter. Set the meter at ASA 25 (film speed) and then set the camera`s shutter speed at 1/60th second. Place a white piece of paper next to a plant leaf and move the camera or meter the same distance from the paper as the paper`s narrowest dimension.
The f-stop reading you get can then be converted to foot-candles. Kroeber`s numbers vary from Crockett`s and both sets of numbers are different from those printed on a Gossen Luna Pro photographic light meter. As a starting point, Gossen`s meter lists the following spot reading conversions:
f/2
130 foot-candles; f/2.8
260; f/4
500; f/5.6
1,000; f/8
2,000; f/16
4,000.
A light`s intensity is measured in lumens, basically the light of one candle. Light bulbs are rated by watts, or how much energy they draw. To figure efficiency of a fixture, a measurement of lumens per watt will tell you how bright the light is per unit of energy consumption.
According to John Bartok of the University of Connecticut, fluorescent fixtures produce 25 to 70 lumens per watt, while a high-pressure sodium lamp produces 100 to 135 lumens per watt. The variables, he says, are the efficiency of the ballast and the type of reflector in the fixture. The age of the fluorescent tubes also can be a variable, because they drop off in brightness by as much as 30 percent within the first month of operation and drop off further between 12 and 18 months.
David Ittel, owner of Chicago Indoor Garden Supply, quotes from George Van Patten`s book, ”Gardening Indoors” (Van Patten Publishing, $14.95; can be ordered by calling 800-345-0096), which rates fluorescents at 22.3 lumens per watt; metal halides at 125 lumens per watt; and high-pressure sodiums at 140 lumens per watt.
Using those figures, Ittel extrapolates: ”If you buy a fluorescent fixture on sale for, let`s say, $14.95, including two 40-watt tubes, you have 80 watts.” That means 12-plus fixtures would be needed to produce 1,000 watts, or roughly a $180 initial investment. Ittel`s metal halide 1,000 watt unit sells for $259.
Using Van Patten`s figures for lumens per watt, the fluorescent fixtures would provide 22,300 lumens, the metal halide 125,000. The effective growth distance (how far away from the light the plant can be) for the fluorescents is around a foot, for the metal halide up to six feet. Bulb life is estimated at one to two years for fluorescents and two to three years for HIDs.
