By Matt Gibson
When in bloom, American pennyroyal attracts a number of pollinators and other beneficial insects, including carpenter bees and dagger bees.
Keep in mind, however, when using European pennyroyal that it can be a bit invasive, like its mint relatives, and may spread out into unwanted areas if it is not regularly cut back or contained with strong, deep borders to block its roots from spreading out. American pennyroyal, on the other hand, is not invasive, but still has pest deterring potential as a companion plant in garden beds.
Varieties of Pennyroyal
There are two different kinds of pennyroyal, both of which are toxic, and both of which are used as flea deterrents and pest repellants, and both of which have potential as a protective companion plant for pest prone plants in the garden. European pennyroyal is the common name for the Mentha pulegium species, which is a member of the mint family of plants, and considered the true species of pennyroyal. Like its mint family relatives, European pennyroyal is invasive, and needs to be cut back regularly, or contained by planting in containers or in locations that have deep, strong borders to prevent spreading.
American pennyroyal, is the common name for the Hedeoma pulegioides species, which is not a member of the mint family, but still equipped with a fresh, minty fragrance. Hedeoma pulegioides is commonly called American pennyroyal, but is also known as tickweed, mosquito plant, American false pennyroyal, mock pennyroyal, squaw mint, and stinking balm. Both varieties of pennyroyal are low-growing plants with fuzzy stems. The American pennyroyal has a square stem and grows only six inches to one foot high.
The two varieties can be identified by the number of stamens they have. European pennyroyal has four stamens, while American pennyroyal possesses only two stamens. The American pennyroyal has small, thin leaves, and is rather ordinary until around July, when the plant starts to bloom, producing pale blue clusters of small, pretty, minty-smelling flowers.
Growing Conditions for American Pennyroyal
Plant American Pennyroyal in a location that receives partial sun and open shade, with dry to average soil. American pennyroyal will also tolerate full sun, but will need average or greater water provisions when grown in full sunlight locations. American pennyroyal will also tolerate thin, rocky soils, and will adapt to most soil conditions.
How to Plant American Pennyroyal
Plant American pennyroyal seeds outdoors in autumn or store them in the fridge to plant outdoors in early spring or start indoors under grow lights. Once seedlings emerge, they can easily be transplanted into the garden in mid-spring. Seeds require light to germinate but will grow rather quickly after sprouting. Plant American pennyroyal seeds in prepared beds after danger of frosts have passed. Plant seeds on the soil surface and moisten the soil with a light mist from a spray bottle. Keep the soil in the seed beds moist throughout germination and sprouts should start to emerge in two weeks.
Start new American pennyroyal plants from seed each spring by sowing indoors under grow lights on the surface, or just below the surface of a moistened seed-starter soil mix. The optimal temperature for germination is 70 degrees Fahrenheit. Moist stratification speeds up germination. Starting seeds indoors is preferable, but you can also start seeds outdoors by planting them on the surface, or just beneath the surface of the soil in the fall to the end of winter. Once American pennyroyal plants are established outdoors, they will reseed themselves naturally and easily. Once indoor seedlings sprout, they can be transplanted outdoors very easily.
Care for American Pennyroyal
American pennyroyal plants do not require much care after the seedlings become established. Provide water in cases of extreme heat or drought. Divide established pennyroyal plants once every three years during the early weeks of spring. Pinch off the terminal ends of the plant to promote a more compact shape.
How to Propagate American Pennyroyal
American pennyroyal is propagated from seed, cuttings, or rhizome division in the spring. For more information on how to propagate via seed, refer to the section above titled, “How to Plant American Pennyroyal. Divide established plants every three years in the early weeks of spring for optimal form and production.
Garden Pests and Diseases of American Pennyroyal
Due to the strong scent of American pennyroyal’s foliage, it is a natural deterrent for most garden insects and pests. American pennyroyal has no known problems reported with specific pests or common garden diseases.
How to Harvest American Pennyroyal
The flowering tops and leaves of American pennyroyal are typically harvested when the plant is in flower. Considering the toxicity of the oils that are extracted from the flowers of the pennyroyal plant, harvesting and storing pennyroyal should be left in the hands of experienced professionals.
American pennyroyal is not the most beautiful plant that you could grow in your garden. It’s not a showstopper, or an eye-catching garden front piece. Even when it is in bloom, it’s not going to take anyone’s breath away. It’s not going to provide you with food throughout the growing season, or save you money on produce.
However, American pennyroyal can grow in poor soils, it attracts beneficial insects and deters pests. It can help with erosion control and can improve soil conditions in the locations where it is planted. Though it will most likely never win any garden merit awards, American pennyroyal is a very versatile plant that comes with several benefits that make it a worthwhile addition to the modern garden.
Toxicity Warning: Don’t Consume Pennyroyal
For decades, products were created using pennyroyal to make flea deterrents and other pest repellants, a vapor to treat nasal congestion from colds, and an infusion used to detoxify the human body. Though pennyroyal is still used to make flea deterrents and pest repellants, recent developments, such as the death of two infants in California who were given pennyroyal tea, have caused scientists to reconsider ingesting pennyroyal in any form, as it is toxic, and potentially lethal to humans.
In 1994, a college student died from drinking teas made with only 2 teaspoons of pennyroyal oil over a two day period. In 1992, a healthy dog passed away from licking a small amount of pennyroyal oil off of its fur. It should have come as no surprise that pennyroyal should not be ingested in any form, as it was used as an abortion remedy over 2000 years ago.
These fatal incidents involving the use of pennyroyal essential oil, which was originally formulated for therapeutic and homeopathic uses, and to use as an insect repellant, has caused the oils to be taken off the market, as well as other products containing pennyroyal that were intended for human ingestion.
The active ingredient in both the American and the European species of pennyroyal, called pulegone, has been used as a flavoring agent and perfume ingredient for ages due to its pleasant mint-like smell and taste. Recent information has led to more restrictions on the use of products containing pulegone, especially products designed for human consumption, as it has been proven to be highly toxic. Pulegone is still used in pest repellants, but is no longer used as a flavoring agent or cold remedy. Unfortunately, some lessons have to be learned the hard way, as was the case with the severity of pennyroyal’s toxicity.
Household Uses of Pennyroyal
Though ingesting any form of pennyroyal should be avoided, the plant has several household uses that should not be overlooked. The dried leaves of pennyroyal plants can be placed around the bedding areas of your household pets. The essential oil of pennyroyal used to be applied to flea collars but has been found to be harmful to pregnant cats, causing them to have miscarriages, and has since been removed from the market. For the safety of your pets, only use the fresh or dried leaves of the pennyroyal plant for pest maintenance.
For the same reason dogs have been known to roll around in a pennyroyal patch, putting crushed pennyroyal stems in your pockets, or wrapped inside your handkerchief, can deter gnats and mosquitos.
Though more research is needed, pennyroyal might also be beneficial in the garden. Some gardeners have reported having success underplanting their eggplant crops with pennyroyal to deter flea beetles. Pennyroyal plants might be useful as companion plants when paired with pest prone crops in the garden.