Even with a bright sunroom, sun-loving herbs will need 12 to 16 hours a day of supplemental light. You have two choices: fluorescent or high-intensity discharge (HID).
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.
Home and school gardeners have used fluorescent lights for starting seeds and growing plants for many years. Because their light intensity is low, they are best for seedlings and low-growing herbs so that even the lowest leaves are no more than 8 inches from the tube. A standard 4-foot unit with two 40-watt tubes will light an area about 8 inches wide. A variety of special tubes for growing plants are available, but a combination of standard cool- and warm-white tubes is also effective. Verilux tubes approximate the color of natural light. Vita-Lite ″power twist″ tubes produce somewhat more light per watt, and the quality of light is balanced for optimum plant growth.
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.
Serious herb growers, including commercial producers of fresh-cut herbs, rely on high-intensity lamps. Our stock plants grew fabulously through the winter under these lights. They produce a much greater intensity of light, meaning the bulb can be several feet above the plants and still deliver adequate intensity to lower leaves. There are two types: metal halide and high-pressure sodium. Both utilize large, long-lived, and relatively expensive bulbs filled with various combinations of rare gases and a remote ballast.
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 to 1,000-watt 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 to 1,000-watt versions.