Mazus reptans

Native to China, Japan, Korea and Taiwan, this low herbaceous perennial spreads by creeping stems that root at their nodes. It forms a dense mat only a few inches tall. Small purple-blue flowers appear in clusters in late spring and summer, with two lips and contrasting white and yellow markings. It suits moist ground between paving stones and can be propagated by division. It has become naturalized in parts of the United States, including Massachusetts and Maine. Photographed in Florida.