Peppermint is an introduced perennial aromatic herb, growing on square branching stems from one to three feet high. Stems are reddish-green, usually smooth and erect. The stem angles can sometimes have fine hair. It is frequently planted and sometimes escapes to the wild.
The leaves are opposite, dark green, lance shaped, 2x as long as wide, margins sharply but unequally toothed, pointed tip and the based narrowed to a short stalk, which also has some of the reddish color of the stem. The surfaces are usually smooth but the veins on the underside may have fine hair and the underside is densely gland dotted.
The inflorescence is a dense whorl-like cylindric terminal spike, obtuse at the tip and sometimes the terminal cluster is interrupted on the stem. In the mint family this arrangement is called a 'verticillaster' where the flowers look like a whorl arrangement but are actually in cymes that rise from the axils of a pair of opposite bracts. There can be axillary clusters from the upper leaf axils.
Flowers: The calyx of the flower is purple, tubular in shape with 5 linear teeth at the lip. The outside of the calyx is smooth at the base but then hairy and glandular. The flower corolla is white with red tinges and has 2 major lips formed by 5 fused petals. The upper lip of two fused petals is undivided while the lower lip formed from 3 fused petals has three lobes. The flowers are perfect with 4 equal length stamens with pale yellow anthers and a single style with a divided tip. Stamens and style are exserted beyond the corolla throat; all-together - a showy arrangement. The green bracts which separate the flowers are linear to lanceolate in shape and slightly longer than the flower calyx.
Seed: Fertile flowers mature to a dried calyx that contains 4 ovoid, smooth brown nutlets. Seeds are small and need light for to break dormancy; thus should be surface sown. They do not need cold stratification.
Habitat: Peppermint grows from a rhizomatous root system that are widely spreading putting out underground runners to create colonies of plants. In unrestricted space it will be very aggressive. See notes below on control. It grows best in rich soils with partial sun to shade and plenty of moisture.
Names: The genus, Mentha, is named for the classical mythology Greek nymph Minthe, who was unfortunate enough to be turned into a mint plant by Persephone so Mintha could avoid seduction by Hades. Peppermint is a cross between Mentha aquatica and M. spicata. The second part of the scientific name, piperita, refers to being 'peppery'. The author name for the plant classification - 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: The arrangement of the flowers in a terminal spike, and their color, will avoid confusing with other mints where the verticillasters are usually located in the leaf axils and not terminal - see Wild Mint, Marsh Hedge Nettles and Motherwort. One mint that does have a terminal verticillaster is Heal-all, but there the shape of the leaves and the color of the flowers is different.
Above: 1st photo - The inflorescence - the calyx of the flower is purple with glandular hair. Corolla lobes are white with some reddish tinges possible. The 4 stamens and style are exserted beyond the corolla lips. 2nd photo - Plants are leafy, 1 to 3 feet high.
Below: Leaves are opposite with pointed but unequal teeth. The underside (2nd photo) has fine hair on the ribs and is densely gland dotted.
Below: 1st photo - The inflorescence is sometimes interrupted and axillary clusters can arise from the upper leaf axils. The lower section clearly shows that the flowers are in cymes, not in a whorl. 2nd & 3rd photos - Stems are reddish green, 4-angled, smooth except for fine hair on the angles. Leaf nodes may have a deeper color. Leaf stalks are also reddish.
Notes: Being a cultivated and introduced plant, Peppermint is not indigenous to the Garden Area. Eloise Butler first introduced the species in August 1917 with a plant she obtained from a local garden. The only plant census on which it has appeared is that of 2009. There are some references to it being found in the wild in Minnesota, but the U of M Herbarium reports that it has never been collected.
There are two other non-native species of Mentha that have been found in the wild in Minnesota - M. spicata, Spearmint, and Mentha ×gentilis, Heartmint. The only native species of Mentha found in the state is M. arvensis L. var. canadensis, the Wild Mint, which is the only other Mentha in the Garden's collection. The other two species called mints in the Garden - Downy Wood Mint and Hairy Wood Mint - are in the Blephilia genus.
Uses: Peppermint is grown commercially for the production of its oil which is used as flavoring in confectionery, tea, chewing gum, cordials and in cosmetics. The flowering tops and the leaves are used, as both are glandular, and are distilled to produce the oil. It has a high menthol content. There is also a history of medicinal use, particularly for treating irritable bowel, bloating and abdominal pain.
Control: Once a patch of Peppermint is established it is said to be as hard as quack grass to eliminate as all underground root sections must be removed or killed. Removing all leaves immediately will eventually kill the roots. For the home garden, it may be best to plant it in a container. In areas where the plant has been grown extensively such as the Great Lakes region of the U.S., Australia and New Zealand, it has escaped and it is considered invasive.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"