Flower:

A small dense cluster of 1/8-inch white flowers surrounds leaf axils along much of the stem, blooming from the bottom of the plant up and usually not all flowers in a cluster are open at the same time. Individual flowers are tubular, with 4 spreading lobes about equal in size. There are often tiny pinkish purple spots on the inside of the petals. 2 purple-tipped stamens extend out of the tube. The calyx is hairless, has 5 narrowly triangular lobes each with a sharply pointed tip and is about as long as the floral tube.

Leaves and stem:

The lower leaves are deeply lobed, up to 3½ inches long and 1½ inches wide, with a few scattered hairs along major veins on the underside, and little or no stalk. Leaves are progressively smaller as they ascend the stem and become coarsely toothed to nearly toothless at the top of the plant. The stem is square, mostly hairless, and unbranched.

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. American Water Horehound is most easily distinguished by its deeply lobed lower leaves, which the others all lack. Northern Bugleweed (Lycopus uniflorus) is otherwise distinguished by its short calyx, the lobes not exceeding the fruits; Rough Bugleweed (Lycopus asper) by its (usually) hairy stem and calyx; Virginia Bugleweed (Lycopus virginicus) by its broader, hairier leaves, stamens not extending out of the floral tube, and short calyx. Wild Mint (Mentha arvensis) also has clusters of small flowers in the axils, but it has usually pink to lavender flowers, and a strong mint scent when leaves are crushed. Lycopus species are not aromatic.