Flower:

Small, tightly packed, flat clusters about 1 inch diameter at the top of the plant, the tips of branching stems, and arising from the upper leaf axils, the flowers in the outer ring of a cluster opening first. Flowers are less than ¼ inch long and tubular with 2 lips, the upper lip notched or not, the lower lip with 3 lobes all about the same size. Color is white, usually with purple spots; the outer surface is finely hairy. Extending from the short tube are 4 white or purple-tipped stamens. The calyx cupping a flower is light green, hairy and has 5 triangular teeth.

Leaves and stem:

Leaves are opposite, up to ~2½ inches (3 to 6.5 cm) long and ½ inch (3 to 12 mm) wide, lance-linear, toothless,  tapering to a blunt or pointed tip, and are stalkless. Surfaces are gland-dotted and hairless except for short hairs along the midvein on the underside. Clusters of small leaves may form in the leaf axils.

Stems are erect, green to reddish, square with short hairs mostly along the angles, usually branched in the upper plant. Mature plants may have many branches and take on a bushy appearance. Colonies may form from short, spreading rhizomes.

Fruit:

The persistent calyx holds 4 black 1-seeded nutlets.

Notes:

I love walking through a patch of Mountain Mint—the leaves are wonderfully aromatic when crushed. In the wild, it is most often found in moist to wet soil, including prairie swales, wet meadows and fens, but may be found in drier conditions as well. It does very well in my suburban garden. A similar species, Narrow-leaved Mountain Mint (Pycnanthemum tenuifolium), has a hairless stem and leaves not more than ¼ inch wide; it is not known to be in Minnesota but is present in Wisconsin.