Plants that hop around

All but the tidiest, most obsessively controlling of gardeners look forward to those plants that hop around. At first they are placed near the front of the border. Next they find their way out onto the path, into the gravel or the cracks in the paving. And the first time this happens one thinks: Now my garden is beginning to show that extra panache–this is maturity in miniature, and these will be my signature plants.