Pennyroyal (Mentha pulegium) is a spreading plant with upright stems that produce whorls of pale purple flowers in mid to late summer. The small oval leaves are medium green and droop from stalks that can grow to a foot tall. When crushed, pennyroyal's leaves emit a fragrance similar to that of spearmint, but they are not the same plant. Pennyroyal is not a particularly attractive garden plant, but it is easy to grow—perhaps too easy, as you may find that it takes over your garden; the drooping stems will root wherever they touch the ground and it is considered an invasive plant in parts of California and Oregon.
Pennyroyal is generally planted in the spring after all danger of frost has passed but while the soil is still cool. The oils of this plant are toxic to humans and pets.
|Common Name||Pennyroyal, stinking balm, mosquito plant, squaw mint|
|Botanical Name||Mentha pulegium|
|Plant Type||Herbaceous, perennial|
|Mature Size||6–12 in. tall, 3–6 ft. wide|
|Sun Exposure||Full, partial|
|Soil Type||Moist but well-drained|
|Soil pH||Acidic, neutral, alkaline|
|Hardiness Zones||6–9 (USDA)|
|Native Areas||Middle East, Europe, Africa|
|Toxicity||Toxic to humans, dogs, and cats|
Although native to Europe, the Middle East, and Asia, pennyroyal has been a naturalized exotic in North America for centuries. Gardeners new to growing herbs are sure to find success with vigorous pennyroyal plants. They are tolerant of a wide variety of growing conditions, and as long as consistent moisture is present, the plants usually grow like wildfire. The scent of crushed pennyroyal is quite pungent, and its ability to repel nuisance insects like fleas and mosquitoes gives it value in herb, flower, or vegetable gardens.
Pennyroyal is considered an invasive species, mostly in parts of California and Oregon.
Pennyroyal plants grow best in partial sunlight—especially dappled light—but full sun is also tolerated as long as they receive adequate moisture. As a general rule of thumb, aim to plant your pennyroyal somewhere that gets at least six hours of sunlight daily.
Pennyroyal plants grow at their best in moist, rich soil that has been amended with organic matter. That being said, the plant can also grow in clay or sandy conditions—in fact, sometimes a less-than-ideal soil blend can help to keep pennyroyal's rapid growth in check. Pennyroyal has no soil pH preference, growing equally well in acidic, neutral, or slightly alkaline soil.
Regular moisture is an important element to a thriving pennyroyal plant. The surface of the soil should always feel moist to the touch. Pennyroyal grows well in damp areas like stream banks or low-lying depressions in the landscape.
Temperature and Humidity
Pennyroyal grows equally well in areas with cool or hot summers. Humid conditions are better than dry or desert areas for growth—however, pennyroyal can adapt to dry climates provided it gets enough irrigation.
Chemical fertilizers aren't necessary to grow healthy pennyroyal plants. The plants can get all the nutrition they need from soil rich in organic humus from leaf mold, manure, or compost. In fact, too much fertilizer can reduce flowering in pennyroyal plants.
Types of Pennyroyal
There are no named cultivars of pennyroyal; the species plant is the only one commonly sold. However, there is a native plant, Hedeoma pulegioides (American pennyroyal) that is native to eastern North America from the Dakotas to Nova Scotia and south to Georgia and Arkansas. This plant has a similar growth habit and cultural needs to Mentha pulegium, but the flowers are a more true blue rather than lavender in color.
Cutting back your pennyroyal plants is an important part of keeping the plant's growth contained. Pennyroyal spreads indefinitely by creeping stolons (in the same way other mints do), and it can become invasive if not kept in check. Pull up the edges of your pennyroyal plant where it is rooting and spreading, and trim as desired to keep the plant in its designated spot. Do this more frequently in wet, warm weather—at least weekly. Cutting back pennyroyal plants after flowering helps to keep the plant tidy as well.
Pennyroyal is very easy to propagate by vegetative means—the best method is to split off root sections and replant them:
- In spring as new growth is beginning, use a sharp, clean trowel or disinfected garden knife to cut off a section of the root crown, making sure the piece includes both a healthy clump of roots and some growth shoots.
- Immediately plant the section in well-prepared, moist soil.
- Water frequently until the new division is well established.
For younger plants that haven't begun to spread, you can easily root a tip cutting in moist soil or a vase filled with water.
How to Grow Pennyroyal From Seed
Seeds collected from the dried flower heads (or purchased from commercial seed sellers) should be planted in small containers filled with ordinary potting soil. Plant six or seven seeds per pot, barely covering them with 1/8 inch of potting mix (they need light to germinate). Give them a head start indoors by planting four to six weeks before the last frost date.
At a temperature of around 68 degrees Fahrenheit, the seeds will sprout within about 21 days. Transplant the seedlings outdoors after all danger of frost has passed, spacing them 18 to 24 inches apart.
Potting and Repotting Pennyroyal
Pennyroyal has a cascading growth habit that works well spilling over the edges of large patio containers or hanging baskets of any material. Pennyroyal plants will grow in pots of any size and any material, in a standard commercial potting mix. Make sure the container has good drainage holes. They are pretty tolerant of being root-bound. Take care that the plant doesn't escape through the drainage hole and into the landscape if you're trying to keep it contained.
Within its hardiness range, pennyroyal requires no winter protection against cold, but you should cut back the dried stems to ground level after they die back. Potted specimens should be moved to a sheltered location for the winter.
How to Get Pennyroyal to Bloom
Mature plants (at least two years old) generally bloom robustly with blue-purple flowers in mid to late summer. Failure to bloom is usually traced to lack of sunlight, or to soil that is too rich or has been overly fertilized. Top-dressing with compost is the only feeding these plants need, and applying chemical fertilizer often causes them to withhold flowers.
Common Pests and Plant Diseases
As a natural pest-repelling plant, pennyroyal is troubled by few insects, though leafhoppers and caterpillars may be occasional problems.
Powdery mildew and mint rust can trouble plants growing in crowded, damp, shady conditions. Keep your pennyroyal plants disease-free with adequate sun and spacing, and by planting them in well-draining soil. If fungal diseases continue to be an issue, consider thinning out your plant to improve airflow.
Common Problems with Pennyroyal
The most common complaint with pennyroyal involves its tendency to spread in an uncontrolled manner. When it infiltrates pasture areas, for example, it often displaces native grasses and reduces the ability of the pasture to feed grazing animals.
Unfortunately, the best method of eradication is also quite labor intensive—tilling up the soil and carefully removing all traces of the rhizomatous roots. Non-selective herbicides like glyphosate will also kill pennyroyal; they are best applied after the plant flowers but before it goes to seed. But avoid using chemical herbicides in boggy or wet locations, and if you do use chemicals, make sure they are approved for wet conditions.
Does this plant really repel insects?
Yes, the pennyroyal plant is good for repelling insects. This is one of the most pungent of the mint species, and the odor emitted by its leaves is especially useful for repelling mosquitoes and fleas.
How do I harvest and use pennyroyal leaves?
The plant is used frequently in natural pest repellents. Make use of the pruned stems from your pennyroyal wherever unwanted insects are a problem. You can lay freshly cut stems around pest-prone areas, or dry the stems and leaves to use in pest-repelling potpourri mixtures.
How can I use pennyroyal in the landscape?
Pennyroyal is a traditional cottage garden plant, but it is not a particularly attractive specimen. It is mostly grown as an insect-repelling plant. Should you decide to grow it, pennyroyal's spreading habit is good for filling in bare spots in the garden or large containers, and its profuse flowers will attract butterflies and other pollinators.
How long does pennyroyal live?
Its habit of steady spread means that a colony of pennyroyal plants will continue indefinitely. However, you may find that the plants travel through the garden (and into surrounding areas) as overgrown stems die back at the same time that spreading rhizomes extend into new locations.
Are there other mint species that are more attractive?
Yes. While most mints are grown for their scent, some are used more for their ornamental value. For example, buddleia mint, Mentha longifolia (also known as horse mint) has long, purple, nectar-rich flowers and beautiful silvery foliage. It is suitable for USDA zones 5 to 10. However, like most mints, this is also an aggressive grower.
Pennyroyal. Invasive Plant Atlas of the United States.
Pennyroyal Oil. Poison Control.
Pennyroyal. Invasive Plant Atlas of the United States.