Flower:

Cylindrical spike, held above the surface of the water, ½ to 1+ inch (to 3 cm) long at the tips of branching stems. Spikes have 5 to 9 whorls of flowers, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a green, paddle-shaped, sepal-like appendage.

Leaves and stems:

Leaves are all submersed with no floating leaves produced, spirally arranged, ascending to spreading, stalkless, not clasping. Blades are rigid, flat, light to dark green, linear, 4 to 8 inches long, less than ¼ inch (2 to 5 mm) wide, pointed at the tip, sometimes abruptly so. The midvein is conspicuous yellow, with 15 to 35 lateral veins.

At the base of the leaf is a translucent, membranous appendage (stipule) that is white to brownish, not connected to the leaf, ½ to 1½ inches long, and becomes shredded at least at the tip. Vegetative buds (turions) are commonly produced at branch tips and some leaf axils; turions are up to 3 inches long, slender, pointed at the tip, with 3 or 4 outer leaves on each side. Glands at the leaf nodes are rarely present.

Stems are branching, quite flat, and without spots. Plants lack rhizomes.

Fruit:

Fruit is a dry seed (achene), light green to olive at maturity. Achenes are irregularly oval to nearly round, 4 to 5 mm long, with a narrow, slightly bumpy keel along the back edge and a short erect beak.

Notes:

Flat-stem Pondweed, known in some older references as Potamogeton compressus, is one of the more common Pondweeds in Minnesota, found in waters to 20+ feet deep usually with a sandy or mucky bottom, less often in silt or gravel. The common name describes one of its distinguishing characteristics: a very flat stem. Of the few Pondweeds with a flat stem and linear leaves, this has the largest leaves (both length and width) than any others, 4 to 8 inches long and more than 2 mm wide, with numerous parallel veins and a pointed tip. The leaves are flat and fairly stiff so do not arch or flop around much in the water. No floating leaves are produced.