Think of a rain garden as areas of varying degrees of moisture within the same bed. Since the outer edges of a rain garden dry out faster than the center, plants that work well in the middle might not do as well along the perimeter. In the center (Zone 1), where it's wettest, you need plants that thrive even in mucky soil. Around that, put plants that can tolerate occasional standing water (Zone 2). Plants for the edges and berm should prefer drier conditions, as they seldom stay under water very long (Zone 3). Ideally, rain-garden plants should also survive dry seasons without irrigation, so the best plants are probably the native ones that grow near seasonal wetlands in your area. Using locally native plants also eliminates worries about introducing invasive plants, a particular issue with rain gardens because the growing conditions are ideal for many species.