Flower:

Spike cluster up to 5 inches long at the top of the stem and at the tips of lateral branches, often with a pair of smaller spikes arising from the uppermost leaf axils. There is sometimes a distinct gap between whorls of flowers (an interrupted spike), especially on the lower part of the spike. Flowers are about 1/8 inch long, tubular with 4 lobes, pink to lavender or sometimes white; 4 long stamens extend out of the tube. The calyx holding the flower has 5 teeth, is often glandular, and is hairless except for a short fringe of stiff hairs on the teeth.

Leaves and stems:

Leaves are opposite, up to 3 inches long, to 1+ inch wide, narrowly egg-shaped to elliptic, rounded at the base, mostly pointed at the tip, toothed around the edges, hairless or with a few hairs on major veins on the underside, and stalkless or on very short stalks (to 3mm, 1/8 inch). Crushed foliage has a distinct spearmint scent.

Stems are 4-sided, hairless, light green to red tinged, usually branched. Colonies are formed from stout, whitish underground stems (rhizomes).

Fruit:

The persistent calyx holds 4 tiny, oval, dark brown seeds.

Notes:

Spearmint is the plant famous for flavoring teas, candies and other foods, and has also been used as a medicinal, treating ailments from headaches to flatulence. While the plant may resemble other members of the Lamiaceae (Mint) family, the scent of crushed spearmint leaves is pretty distinctive and should readily identify it, though it is further distinguished by the essentially hairless leaves and stems, leaves that are stalkless or nearly so, and terminal spikes rather than whorls in leaf axils as the related, native Wild Mint (Mentha arvensis) has.

Spearmint has only been recorded in Minnesota a few times, but is more common in other parts of the US. While it hasn't been described as invasive, it certainly has the potential to be problematic. The rhizomes can spread rather quickly, form tangled masses a few inches below the surface and result in a dense monoculture. We grew it in the garden to get photos and it started overtaking the area in just a couple seasons. I dug most of it up but likely missed some rhizomes; we'll have to see how much returns.