American Water Horehound is a native erect perennial forb growing on square hollow stems up to 40 inches high. They are usually un-branched, with sparse hair or none at all. A vertical groove is on each side of the stem. The plant juice will stain linin and wool permanently.
Leaves are opposite, each pair rotated 90 degrees from the adjacent pairs, elliptic to lance-like, tapering to both a pointed tip and to a short winged leaf stock. Stem leaves are 2 to 4 inches long. The upper leaves are not lobed but have deep coarse irregular teeth. The large lower stem leaves may have shallow lobes at the base of the leaf, but this is not consistent. Leaves reduce in size toward the top of the stem and may be stalkless at the top. The upper surface is a medium green. The under side is a paler color, pitted with glandular dots and can have sparse hair on the main veins. Leaves do not have a mint aroma.
The inflorescence has a number of tight whorl-like clusters of small flowers around the leaf axils in the upper section of the stem, but not at the stem tip. The clusters are not whorls but are called 'verticillasters', where the flowers look like a whorl arrangement but are actually in cymes that rise from the axils of opposite bracts. Only a few flowers in each cyme open at one time.
The flowers are small, 4-parted, with either a very short stalk or none at all (sessile). The green to purplish calyx is short, with 5 lobes in the lower half united into a tube shape, from 2.0 to 3.2 mm long with the upper part of the tube separated into pointed triangular teeth. The corolla has 4 white to pinkish-white petals that are united at their bases to form a tube. The entire corolla varies from 2.5 to 3.5 mm long, just slightly longer than the calyx points. The upper lip of the corolla is more broad while the other 3 are more spreading at the tips. Flowers have 4 stamens, two of which are sterile (staminodes), have thickened tips and remain within the corolla, while the two fertile stamens have yellow anthers (brownish at pollen maturity) and, with the style, are exserted from the corolla. Flowers are not fragrant. Bees and small insects pollinate the plant.
Fruit: Like many mints, the seed cluster has 4 nutlets. These are brown, 3-angled, ovoid, with a depressed center. Nutlets when mature, are shorter than the calyx tube, flattened and indented on the side toward the center of the cluster, without tubercles or teeth at the upper end. Seeds should be surface sown as they need light to break dormancy.
Habitat: American Water Horehound grows from a rhizomatous root system, forming colonies of plants. There are no root tubers. It is a plant of moist meadows, marshes, streambanks and other low grounds that are wet to wet-mesic with full to partial sun.
Names: The genus Lycopus is derived from two Greek words, lykŏs, meaning 'wolf' and pŏus, meaning 'foot', which Stearn (Ref. #37a) maintains is from "some fancied resemblance to a wolf's foot." The species name, americanus, means of or from the Americas. An older name for the plant is Lycopus sinuatus. 'Bugleweed' comes from the shape of the flower. As to the alternate common name of 'Horehound', see the "medicinal notes" at the bottom of the page.
The author names for the plant classification are two-fold: First to classify in 1813 was ‘Muhl’ is for Gotthilf Heinrich Ernst Muhlenberg (1753-1815) a U.S. citizen and German educated botanist who produced several catalogues of plants after retiring as a Lutheran pastor. His work was incomplete for publication so it was validated for publication in 1815 by ‘W.P.C. Barton’ which refers to William P. C. Barton (1786-1856) American botanist, physician and illustrator, Naval surgeon, author of the codes for the operation of the first naval hospitals, and then professor of botany at the University of Pennsylvania where he taught and wrote on Medical Botany.
Comparisons: There are three other Bugleweeds found in Minnesota which have the same general habitat. L. asper, Rough Bugleweed; L. uniflorus, Northern Bugleweed; and L. virginicus, Virginia Water Horehound. Differences are found in the amount of hair, in the leaves, length of the calyx teeth, number of calyx lobes and the root system. Virginicus like americanus lacks tubers. Asper and americanus have longer calyx tips - 1/16 to 1/8 inch and these surpass the mature nutlets. Only virginicus has dense hair on the stem and hair on both leaf surfaces. L. uniflorus has the most shallow leaf teeth and only americnus has the lower leaves that can have lobes also. Asper does not have leaf stalks. Only virginicus has a nutlet cluster where the top is flat not depressed and the species is the only one with 4 calyx lobes. Virginicus is the one you may less-likely encounter as it is known only in 16 counties.
Above: American Water Horehound usually has an unbranched stiff erect stem with the flower clusters in the leaf axils of the upper half of the stem.
Below: 1st photo - The inflorescence is a verticillaster - cymes, placed on opposite sides of the stem at the leaf axil. Individual flowers have a corolla of 4 lobes, slightly longer than the calyx teeth. The two fertile stamens are exserted from the throat. 2nd photo - Drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Below: 1st photo: The calyx is green then purplish with pointed lobes. The square stem is mostly smooth. 2nd photo: The nutlets are 3-angled, ovoid, with a flattened depressed side and rounded on the other sides.
Below: 1st photo - 1st photo: Leaves are elliptic to lance-shaped with deep coarse teeth. 2nd photo - the leaf underside is paler, some sparse fine hair on the mid-vein of this example, and dotted.
Notes: American Water Horehound is indigenous to the Garden. Eloise Butler noted it in her 1907 census. She also planted it in 1932. It has been on all the census reports since. Martha Crone listed it on her 1951 census along with L. virginicus and L. uniflorus. In Minnesota the plant is widespread, found in almost all counties with only a handful of exceptions. In North America it is commonly found, only known to be absent in Nevada and in the far northern Canadian Provinces.
Medicinal uses: The various species of Lycopus have been used in folk medicine for cough remedies. (Ref. #39) It was this use that gave the plants the alternate name of 'horehound' as this references back to the Romans and the Egyptians that used the plant they called 'horehound' for such purposes. Their plant was Marrubium vulgare and since the Lycopus prefers moist environments, we have 'water horehound'. An extract of one species, Lycopus uniflorus, has been studied at the School of Pharmacy, Lebanese American University, for anti-inflammatory, anti-ulcerogenic and antioxidant activities and test results show that the species has such activities. (Saade, Ziadeh, Ramia, Daher, Mroueh - 2009)
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"