Flower:

Cylindrical spike held above the surface of the water, 1 to 1½ inches (to 4 cm) long at the tip of the stem, sometimes arising from the upper leaf axils. Spikes have 3 to 5 whorls of flowers, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a green to brown, ladle-shaped, sepal-like appendage.

Leaves and stems:

Leaves are all submersed and more or less spirally arranged along the stem; no floating leaves are produced. Blades are light to dark green, linear-oblong, ½ to 4 inches long, up to 3/8 inch (to 1 cm) wide, rounded to blunt at the tip, toothed and usually wavy along the edges, with a prominent midvein flanked by 1 or 2 pair of lateral veins. Leaf bases are stalkless. At the base of the leaf is an inconspicuous brownish, membranous appendage (stipule), not connected to the leaf blade, and may disintegrate early. Glands at the leaf nodes are absent. Stems are flattened, usually branched.

Numerous vegetative buds (turions), also known as winter buds, develop in late spring at branch tips and in leaf axils. Turions are tightly wrapped in several leaves forming an egg-shaped, bud-like structure and flanked by short, roundish leaves on opposite sides of the branch (2-ranked), 1 to 4 leaves per side. Turions drop off in summer, overwinter, and shoot up the following spring. Dense mats may result.

Fruit:

Fruit is a dry seed (achene), brown to reddish-brown when mature, though fruit is not often formed.

Achenes are irregularly oval, 5 to 6 mm long, with a conspicuous, knobby keel along the back edge and a longer, curved tooth at the base. The beak is erect and nearly as long as the body.

Notes:

Curly Pondweed is highly invasive, at its worst forming dense monocultures that impede water flow, impair recreational activites, degrade or destroy fish habitat, crowd out native aquatic plants, and can impact waterfront property values. When it dies back in mid-summer it releases large amounts of phosphorus, feeding algae blooms and sucking oxygen from the water, potentially impairing fish populations and drinking water quality. The USGS has an excellent fact sheet with more information about its spread, impacts, and control measures.

Curly Pondweed is easily recognized by its stalkless, linear-oblong leaves that are wavy/crinkly and toothed along the edges, achenes with a long beak and knobby keel along the back edge, and the presence of turions at branch tips and in leaf axils. Floating leaves are absent. There is some resemblance to the submersed leaves of other Pondweeds, but the toothed submersed leaves and no floating leaves should distinguish P. crispus from the rest.

In Minnesota, it was first observed in Duluth in 1906 but the earliest herbarium records are from 1937 in Lake Minnetonka, Hennepin County, the Lanesboro fish hatchery, Fillmore County, and along the Mississippi River in Winona County. It is now much more widespread than the state distribution map indicates, found in nearly every county. See EDDMapS for a more accurate (though probably still incomplete) picture of its presence in MN.