Pennyroyal (Mentha pulegium) is a creeping mint with small oval leaves and a slightly medicinal scent. It enjoys spots with full sun to light shade, provided it gets regular watering. Pennyroyal blooms in mid-summer sending up stalks with successive tiers of tiny lavender flowers. Like most members of the mint family, pennyroyal will grow to fill the space it can reach. About twice a year give it a haircut and pull back the runners sneaking off to parts unknown. It is considered hardy in Zones 6-9.
If you need an herb that can tolerate being stepped on, pennyroyal is for you. Pennyroyal has long been a favorite groundcover in herb garden design. Plant it between flagstones as it stays low and will spread between the cracks. Left on its own, it will grow to six inches high but foot traffic will keep it low and help it to form a dense mat.
In Medieval times, the branches of pennyroyal were used as a strewing herb to deter fleas and ants. A tea made from pennyroyal is an old-fashioned remedy for colds and menstrual problems. Pennyroyal was also an ingredient in spring puddings. In fact, one of its archaic names is pudding grass. Do not ingest the concentrated oil of pennyroyal. It can be poisonous and has been implicated in miscarriages (human and animal).