This perennial mint relative bears whorls of fluffy lavender-purple blooms in late summer and fall. It is native to parts of Europe and Asia but has become naturalized throughout much of North America. It often is found growing along waterways, a nod to its preference for moist soil. Like other mints, pennyroyal can spread aggressively, so keep it contained. Pennyroyal has been used as an herb for hundreds of years and is sometimes called fleabane due to its insect-repellent nature.