Flower:

A small dense cluster of 1/8-inch white flowers surrounding leaf axils along much of the plant; usually not all in a cluster are open at the same time. Individual flowers are tubular with 4 spreading lobes of approximately equal size, sometimes the upper lobe is 2-parted or merely notched. 2 purple-tipped stamens extend out of the tube. The calyx has 5 broadly triangular lobes, sharply pointed at the tips and shorter than the floral tube.

Leaves and stem:

Leaves are up to 3 inches long and 1 inch wide, hairless or minutely hairy, coarsely toothed around the edges, generally lance-elliptic, pointed at the tip, wedge-shaped at the base, on a short stalk. Attachment is opposite with leaf pairs at right angles to the pair above and below. Stems are square, hairless or minutely hairy, and unbranched.

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 do not extend past the fruits.

Notes:

There are several Lycopus species in Minnesota, all with similar clusters of small, white, tubular flowers around the leaf axils, most growing in the same type of habitat at the same time, often next to each other. Northern Bugleweed is most easily distinguished by its short calyx, relative hairlessness, and coarsely toothed but unlobed leaves; it is easiest to separate from the others (especially L. asper) when fruits are present. American Water Horehound (Lycopus americanus) is distinguished by its deeply lobed lower leaves; Rough Bugleweed (Lycopus asper) by its hairy stem and calyx, the calyx lobes as long as the floral tube and much longer than the fruits; Virginia Bugleweed (Lycopus virginicus) by its broader, hairy leaves and lack of tubers. 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.