Two plants are commonly known as pennyroyal: European pennyroyal (Mentha pulegium), a perennial native to Europe and Western Asia, and American pennyroyal (Hedeoma pulegioides), an annual that hails from the eastern U.S. Both herbs are low-growing and deeply aromatic, requiring minimal care when planted in the right conditions in the garden.
Partial Shade Is Best
Both European pennyroyal and American pennyroyal will thrive in an area with partial shade or dappled sunlight. European pennyroyal plants being grown for their essential oil should be planted in full sun, advises Plants for a Future. In bright, sunny sites, plants will require more moisture. European pennyroyal grows in U.S. Department of Agriculture plant hardiness zones 6 through 9, while American pennyroyal grows in USDA zones 5 through 9. These low-growing, tidy plants may be grown in a pot as long as it has at least one drainage hole.
Soil Preferences Vary
In its native habitat, European pennyroyal grows in moist meadows and alongside streams. The herb will tolerate most soil types, though it will suffer in dry soils. Water frequently to keep the soil moist at all times. American pennyroyal grows in dry, sandy soils, and needs either rich sandy soil or low to moderately fertile, well-draining soil. If planting multiple herbs, provide 12 inches between rows and 6 inches between individual plants.
Propagation by Seed or Division
Both types of pennyroyal can be propagated easily by seed or division. Sow seeds in a cold frame in spring, or sow directly in the garden in spring or fall. Plants may be divided anytime of year, though they will generally establish more quickly when divided in spring or fall. Replant divisions in their permanent positions in the garden, or plant individual divisions in pots and place in light shade in a cold frame. In the summer, plant divisions in the garden.
Harvest Before Flowering
Pennyroyal is edible, and may be used to make tea and essential oils. To harvest, cut the stems just before flowering. European pennyroyal flowers in late summer or early fall, while American pennyroyal flowers in midsummer or early fall. Though both species are generally considered safe to eat, eating large quantities of plant, particularly essential oil, can be toxic. The essential oil can also cause an allergic skin reaction.