Conventional wisdom groups seeds into those that need light to germinate and those that sprout better in the dark. Impatiens is considered a light-lover, but gardeners and commercial growers often complain of weak or spotty germination and seedlings with widely varying vigor. At the University of Florida, William Carpenter and his colleagues Eric Ostmark and John Cornell have found a way to get better results when growing impatiens from seed.
Their experiments reveal that once an impatiens seed gets enough light to break dormancy, continued exposure to light actually stunts root growth and inhibits emergence. Looking at six varieties, they found that from 20 to 90 percent of the seed (depending on the variety) will germinate without any light at all. The rest of the seeds must have a relatively short period of bright light to break dormancy, and the exact length of this time varies for individual seeds. Once dormancy requirements are met, however, continued exposure inhibits emergence of the root, a likely cause of differences in seedling vigor later on.
The solution is pretty simple. Sow impatiens seed on the surface of moist soil, and put the seed in sunlight or under bright lights in an area that's about 75° F. After one to two days, cover the seed very lightly with moist potting soil, then cover the containers to exclude light or move them to a warm, dark area. When the leaves emerge, promptly move them back into light to grow. Experiment with the length of time you leave them in light, says Carpenter. The exact time will vary among impatiens cultivars. He also suggests trying this method with other seed that reportedly need light to germinate, like begonias or snapdragons.