Flower:

Small dense clusters of 1/8 to 1/6-inch white flowers surround leaf axils along much of the stem, blooming from the bottom of the plant up and usually not all flowers in a cluster open at the same time. Individual flowers are tubular, with 4 spreading lobes about equal in size. There may be a few tiny pinkish purple spots on the inside of the petals. 2 stamens about as long as the petals extend out of the tube. The calyx is hairy, about as long as the floral tube, and has 4 or 5 narrowly triangular lobes with sharply pointed tips.

Leaves and stems:

Leaves are up to 4 inches long and to 1 inch wide, coarsely toothed, lance-oblong, pointed at the tip, rounded or tapered at the base, and stalkless or nearly so. Attachment is opposite, with leaf pairs at right angles to the pair above and below.

Leaf surfaces are hairless to variously hairy, or hairy just along the veins on the underside. Stems are square and also variously hairy, from minutely hairy to having long spreading hairs just along the angles, and may become smooth or nearly so with age.

Prostrate stems (stolons) root at the nodes and terminate in a tuber, which produces a flowering stem the following year.

Fruit:

Fruit is a set of 4 nutlets forming a square, each nutlet containing a single seed. The calyx lobes are much longer than the fruits.

Notes:

There are several Lycopus species in Minnesota, all with similar clusters of small, white, tubular flowers at the leaf axils, most growing in the same type of habitat at the same time, often next to each other. Rough Bugleweed is most easily distinguished by it slightly larger flowers (1/6 inch vs. 1/8 for the others), the calyx as long as the floral tube, some degree of hairiness, and its stalkless, coarsely toothed but unlobed leaves; it is easiest to separate from the others (especially L. uniflorus) when fruits are present. American Water Horehound (Lycopus americanus) is otherwise distinguished by its deeply lobed lower leaves; Northern Bugleweed (Lycopus uniflorus) by the short calyx lobes, not longer than the fruits; Virginia Bugleweed (Lycopus virginicus) by its broader, stalked leaves and short calyx lobes. Wild Mint (Mentha arvensis) also has clusters of small flowers in the axils, but has usually pink to lavender flowers, and a strong mint scent when leaves are crushed. Lycopus species are not aromatic.