Common Name: Pennyroyal
Scientific Name: Mentha pulegium
Symbol: MEPU
Group: Dicot
Family: Lamiaceae
Growth Habit: Forb/herb
Duration: Perennial

CAUTION: Be extremely careful when handling Pennyroyal. It is known to be a strong abortificant and convulsant when ingested by mammals.

What does it look like?

Pennyroyal has relatively short flowering spikes, usually about a foot or sometimes taller, with bright lavender flowers arranged in circles around the stem. In the mint family, this plant has a peppermint scent (BLM 2003). Pennyroyal can grow low to the ground but also extend upright. The stems are square in cross section and ascend from rhizomes. Branches and leaves grow opposite on the stems. The flowers range from pale to deep pink, to blue or violet, and are clustered in dense whorls at the upper nodes (Bossard et al. 2000).

Flowering usually occurs from June through November and peaks in late summer, but pennyroyal remains a fairly inconspicuous plant during its vegetative growth. Temperatures and soil moisture affect the timing of the blooms, with drier warmer sites flowering earlier. In coastal and estuarine sites, it often flowers through November. After blooms have dried, seed heads remain together throughout the winter and are often distributed by livestock, human contact, vehicles, and such. Alternating temperatures and specific light requirements usually contribute to seed germination. Pennyroyal can also reproduce vegetatively and spread by forming new patches from fragmented stolons (Bossard et al. 2000).

Where does it grow?

Native to Ireland, southern and central Europe, and the Ukraine, pennyroyal is now throughout the world, including the United States. Pennyroyal commonly grows in saturated soils or frequently flooded areas, such as valley bottomlands, seeps. streamsides, marshes, and ditches. Heavy clay or silt are the preferred soil types. It thrives in mild ecological disturbances such as grazing or seasonal deposition of organic debris. Pennyroyal enjoys moderate shade moderate moisture and temperatures (Bossard et al. 2000).

Is it in our watershed?

Pennyroyal has been observed in the Mattole watershed, between Honeydew and Petrolia. However, it has not proven to be a major ecological threat at this time.

Why is it a problem?

Pennyroyal is moderately invasive in wetlands, but its overall ecological impacts have not been well documented. It prospers in areas where native plants once thrived, signifying displacement of some species, particularly vernal pools. It is hard to be certain about its capacity to displace since it tends to grow in areas of frequent disturbances anyways. It is poisonous to livestock and thus a nuisance to ranchers (Bossard et al. 2000).

How do you get rid of it?

There is a general lack of data on the removal of Pennyroyal. Further research and experimentation with various removal techniques are necessary.

Manual Removal: Pennyroyal has underground stems that will resprout if not properly removed. Cover it with black plastic until it completely dies out (BLM, 2000).

Mechanical Removal: It is possible that mowing in late spring or early summer, repeatedly over several years, may deplete reserves but there is no definitive research supporting this technique.

Prescribed burning: This technique may not be effective due to the high soil moisture of pennyroyal habitat. There is need for further experimentation in areas where burns will not affect native species (Bossard, 2000).

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photograph:
Dr. Alfred Brosseau, Saint Mary's College

References:
BLM 2000. Noxious Weeds of the Arcata Field Office. Bureau of Land Management. http://www.ca.blm.gov//arcata/arcweeds.html

Bossard, CC., J.M. Randall and M. Hoshovsky. (eds.) 2000. Invasive Plants of California’s Wildlands. University of California Press, Berkeley, CA. Online version: http://groups.ucanr.org/ceppc/Invasive_Plants_of_California's_Wildlands/