Native to Europe, the Middle East, and Asia, pennyroyal plants have been present in North America for centuries, to the point of naturalization. A member of the mint family, pennyroyal is best planted in early spring will grow quickly, spreading aggressively if not kept well-contained. The plant features long, weed-like stalks with vibrant green leaves and tight purple blooms. When crushed, pennyroyal's leaves emit a fragrance similar to that of spearmint.
For centuries, pennyroyal plant has been used as an herbal preparation to deter pests like fleas and gnats. Pennyroyal also had a history of medicinal uses, although that trend has faded with new knowledge of its potential toxicity. Aside from its pros and cons as a herb, pennyroyal is very easy to grow. Its profuse lavender flowers attract butterflies, and its spreading habit fills in bare spots in the garden or container.
|Botanical Name||Mentha pulegium|
|Common Names||Pennyroyal, American pennyroyal, stinking balm, mosquito plant, squaw mint|
|Plant Type||Herbaceous perennial|
|Mature Size||6–12 in. tall, 3–6 ft. wide|
|Sun Exposure||Full sun, partial shade|
|Soil Type||Moist but well-drained|
|Hardiness Zones||6–9 (USDA)|
|Native Areas||Middle East, Europe, Africa|
|Toxicity||Toxic to humans, dogs, and cats|
How to Grow Pennyroyal
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 plants grow best in partial sunlight, 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.
Regular moisture is an important element to a thriving pennyroyal plant. The surface of the soil should always feel moist to the touch. Additionally, 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.
Is Pennyroyal Toxic?
In spite of its historical medicinal uses, we now know that pennyroyal is not safe to take internally. The plant's oils are highly toxic to both humans and animals, and even a small amount can result in a serious reaction. If you notice any of the below symptoms, contact the proper poison control center or emergency services promptly.
Symptoms of Poisoning in Humans
- Abdominal pain
- Liver failure
- Kidney failure
- Loss of consciousness
- Organ failure
Symptoms of Poisoning in Animals
- Bloody nose
- Difficulty breathing
- Coughing up blood
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 mint does), 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. Additionally, cutting back pennyroyal plants after flowering helps to keep the plant tidy as well.
Making new pennyroyal plants is as simple as digging up a piece of the plant and replanting it in moist soil. Pennyroyal roots along its stem as it spreads, so if you remove a plant piece that is already forming roots, the propagation process will be a quick and easy matter of division. 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
Pennyroyal seeds germinate in one to three weeks in temperatures that range between 70 degrees Fahrenheit to 90 degrees Fahrenheit. You can direct sow the seeds outdoors in spring, or give them a head start indoors four to six weeks before the last frost date.
Potting and Repotting Pennyroyal
Pennyroyal plants will grow in pots of any size and are pretty tolerant of being root bound. To pot or repot your plant, use a commercial potting soil and choose a pot with a drainage hole at its base. Take care that the plant doesn't escape through the drainage hole and into the landscape if you're trying to keep it contained.
Although it's no longer recommended to consume pennyroyal, the plant is still 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.
Common Pests & Diseases
As a natural pest repelling plant, pennyroyal is troubled by few insects. However, 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.