Q: I want to start a rock garden. Where can I find some plants that are beyond the ordinary? — Martin Lanneau, Natchez, Miss.
A: A good way to acquire unusual plants is through seed exchanges run by plant societies. For a modest membership fee, these groups send publications filled with top-notch information and, in late autumn and winter, offer swap lists of available seeds. The North American Rock Garden Society, the Scottish Rock Garden Club, and the American Penstemon Society are among the best.