How to Grow and Care for Pennyroyal

Although native to Europe, the Middle East, and Asia, pennyroyal (Mentha pulegium) has been present as a naturalized exotic in North America for centuries. It is a spreading plant with upright stems that produce whorls of pale purple flowers in mid to late summer. The small oval leaves, medium green in color, droop from the stalks. When crushed, pennyroyal's leaves emit a fragrance similar to that of spearmint. Pennyroyal is not a particularly attractive garden plant, though it has been used for centuries as an herbal preparation to deter pests like fleas and gnats. It is very easy to grow—perhaps too easy, as this foreign exotic is now so widely naturalized that its invasiveness can no longer be controlled. You may find that it takes over your garden, as the drooping stems will root wherever they touch the ground.

Pennyroyal is generally planted in the spring after all danger of frost has passed but while the soil is still cool. While this plant has been used to flavor teas, this is now considered a bad idea. In concentrated form, the oils of this plant are highly toxic to humans and to pets.

Pennyroyal Care

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.

Light

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.

Soil

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.

Water

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.

Fertilizer

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.

Pruning

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.

Propagating Pennyroyal

Pennyroyal is very easy to propagate by vegetative means—the best method is to split off root sections and replant them:

1. In spring as new growth is beginning, use a sharp trowel or 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.
2. Immediately plant the section in well-prepared, moist soil.
3. 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. 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.

Overwintering

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.