Flower:

Cylindrical spike held above the surface of the water, ½ to ~1¼ inches (to 3+ cm) long at the tip of the stem and arising from the axils of floating leaves. Spikes have 5 to 10 whorls of flowers, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a green to orange-brown, ladle-shaped, sepal-like appendage.

Leaves and stems:

Both submersed and floating leaves are produced, though floating leaves are sometimes absent. Floating leaves are firm, green, elliptic to somewhat spatula-shaped, widest at or above the middle, 1½ to 3+ inches (4 to 7+ cm) long, 3/8 to 1 inch (1 to 2.5 cm) wide, toothless, rounded to blunt at the tip, tapering at the base to a stalk shorter than to about as long as the blade. Blades have 9 to 13 veins. Color is green to reddish.

Submersed leaves are alternate, green to reddish, lance-linear to oblong-elliptic, widest at or below the middle, 2 to 7+ inches (4.5 to 18+ cm) long, 2 to 10 mm (to 3/8 inch) wide, tapering at both ends, blunt to pointed at the tip, and stalkless. The prominent midvein is flanked by 1 to 6 pairs of narrow rows of large, empty cells (known as the lacunar band). Edges are flat to slightly wavy.

At the base of the leaf is a membranous appendage (stipule), not connected to the leaf blade, pale brown to reddish, up to 1 inch (2.5 cm) long, the tip somewhat rounded and not shredding. Stems are light brown to reddish, somewhat flattened, unbranched or few-branched. Colonies are often formed from long rhizomes. Vegetative buds (turions) are not produced. Glands at the leaf nodes are absent.

Fruit:

Fruit is a dry seed (achene), the flowering spikes forming densely packed seed heads, though it is not unusual to have a fair number of aborted fruit. Achenes are greenish-brown when mature, generally oval, 2.5 to 3.5 mm long, longer than wide, with a smooth keel along the back edge that may or may not be flanked by a pair of obscure lateral keels. The beak is less than 1 mm long and curved.

Notes:

Alpine Pondweed is a circumpolar species native to parts of Europe, Asia and North America. In Minnesota it is mostly found in our north-central and northeast counties, usually in cold, shallow water, either quiet or flowing, with mucky, sandy, gravelly or even clay bottoms. We encountered it in about a foot of water in the Brule River in Wisconsin, where the river flowed at a pretty brisk pace.

It is distinguished by elliptic floating leaves up to ~3 inches long, rounded to blunt at the tip, tapering at base to a stalk usually shorter than the blade, and have 9 to 13 veins; submersed leaves are stalkless, flexible, narrowly lance-linear to oblong-elliptic, 2 to 7+ inches long, up to 1 inch (2.5 cm) wide, with 7 to 9 veins including a prominent midvein flanked by noticeable lacunar bands. Mature fruit is 2.5 to 3.5 mm long, longer than wide, with a smooth keel along the back, sometimes with obscure lateral keels. The floating leaves are sometimes absent, especially in deeper water. One other distinctive characteristic is both floating and submersed leaves can be quite reddish, more noticeable when dry.

Overall it most closely resembles Potamogeton epihydrus, but its subersed leaves are narrower, more ribbon-like, the midvein plus lacunar bands take up about a third the width of the blade, and floating leaves are longer-stalked. Floating leaves can resemble those of some other Pondweeds as well, but their blades may be heart-shaped or more consistently rounded at the base, more sharply pointed at the tip, leaf stalks may be longer than the blade; submersed leaves may be stalked, more broadly elliptic or ribbon-like, have wavy edges, have broader or no lacunar bands; and fruit may be more round in outline with a bumpy keel and/or more distinct lateral keels.