Growing Herbs Under Lights
by National Gardening Association Editors
The brightness of a light is measured in either lumens or foot candles. Lumens refers to the amount of light at the source, foot candles to the amount of light falling on a given area. As you move farther away from a light, the lumens stay the same, but the foot candles decrease. For perspective, a typical bright but overcast day is about 1,000 foot candles, and a bright summer day at noon is about 10,000 foot candles.
Fluorescent lights at 6 inches provide 700 foot candles, and at 12 inches the light drops to 450 foot candles. Fluorescent efficiency, 60 to 80 lumens per watt, though superior to that of standard incandescent, is far below that of HID lamps. Cost is $30 to $60 for the fixture and tubes, and about $2 to $4 per month for electricity.
High-Intensity Discharge Lights
A 400-watt HID lamp delivers about 1,000 foot candles of light 3 feet below the bulb and can sufficiently illuminate 25 square feet. Cost of electricity is $4 to $8 per month, depending upon lamp size and your electricity rates. (Operate HIDs only during off-peak hours to reduce energy cost.)
Most gardeners prefer metal halide lamps because the light they produce is good for healthy growth and flowering and pleasant to work around. Efficiency is 120 to 130 lumens per watt. Bulbs are available in 150-watt ($200) to 1,000-watt ($350) versions.
High-pressure sodium lamps are the most efficient type of HID lamp, producing about 140 lumens per watt. The light they produce is reddish, so it's less pleasing to work around, but is preferred for some flowering plants. Bulbs are available in 150-watt ($160) to 1,000-watt ($400) versions.
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