Q: Last spring I had lots of luck starting seed of coleus, impatiens, zinnias, marigolds, and vinca with just one fluorescent light. I want to add more this year, and I'd like to know which lights are best. — Kristin Fahey, Akron, Ohio
A: Stick with ordinary fluorescent lights. They are readily available, reasonably priced, and work well for seedlings. Combining a “warm” white tube with a “cool” white in the same fixture will give the same results as a pair of special “grow lights.” The best are probably 4-foot-long shop lights. Since light intensity is reduced at each end of any fluorescent tube, longer tubes generally give you more useful light per foot. I recommend the 4-foot units because, given their widespread use in workshops and garages, they are usually a better buy than longer ones (indeed, they’re often on sale in hardware stores and home centers). Each unit holds a pair of tubes and will illuminate a growing area about 8 inches wide. Keep the lights just 2 or 3 inches above the tops of the seedlings, and leave them on for 16 hours each day. When fluorescents are farther away, the energy reaching the plants is reduced, and the seedlings will stretch toward the light and become weak-stemmed. The need to raise the lights as the seedlings grow often leads gardeners to hang their seed-starting lights with chains; they move the chains a link or two to make small adjustments. The main advantage of the fluorescent light stands sold in garden catalogs is that they make it even easier to adjust the height of the light fixtures.
Over time, the energy a fluorescent tube delivers to plants drops off significantly, long before you can notice a visible decrease in light. A good rule of thumb is to replace your seedling lights after 12 to 18 months of service. They’ll still have lots of life left for other uses around your home, and your seedlings will grow better with fresh fluorescent tubes. Once seedlings reach a height of 8 inches, it’s difficult to keep them growing well under fluorescents. The lower leaves become hidden from the lights and thus can’t get enough energy. But with most annuals, there’s no need to wait that long before moving them outdoors. Four to eight weeks of growing (after the first leaves appear) is plenty for nearly all annual flowers and vegetables. If you want to tack a few more weeks onto your seed-starting effort, the best strategy is to build a cold frame (an unheated bottomless box with a transparent adjustable lid). With a frame, plants can go outdoors at least two weeks before it’s warm enough to plant them in open ground. And it’s the perfect place to help them adjust to the intense light of full sun.