Maximum time for seedlings is about 16 to 18 hours a day. Distance from the light to the plants effects the amount of footcandles the plants receive. For example, two 40 watt fluorescent tubes placed 1 to 3 inches from the plants deliver about 800 footcandles. At 12 inches away the footcandles drop dramatically to 250 footcandles. Since fluorescent tubes give off little heat, we can place them close to seedlings, about 4 to 6 inches.
Fluorescent lights are the most available, efficient and affordable lights to supplement plants.
An inexpensive shop light with a pair of four-foot tubes available at most hardware stores works very well for growing seedlings. New tubes should be purchased each year. To further confuse the choices, not all tubes provide the same wavelength. They provide a more complete spectrum or range of wavelengths. Grow lights are also more expensive than regular fluorescent.