Flower:

Dense cylindrical spike held above the surface of the water, 1 to 2¾ inches (to 7 cm) long at the tip of the stem and arising from the axils of floating leaves. Spikes have 10 to 16 whorls of flowers, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a reddish to orange-brown, ladle-shaped, sepal-like appendage.

Leaves and stems:

Both submersed and floating leaves are produced, more or less spirally arranged. Floating leaves are firm, green, elliptic, 1 to 5+ inches long, ½ to 1¾ inches wide, toothless, pointed at the tip, usually tapering at the base or somehat rounded, with 9 to 21 veins flanking the midrib. The leaf stalk is usually much longer than the blade.

Submersed leaves are thin, bright green to reddish, narrowly lance-elliptic, 3½ to 8 inches long, 3/8 to 1 3/8 inches (10 to 35 mm) wide, pointed at the tip, tapering at the base, often wavy along the edges, on a stalk more than ¾ inch and commonly 2 to 4+ inches long. The midvein is prominent with 2 to 5 narrow rows of large, empty cells (known as the lacunar band) along each side and flanked by 7 to 15 lateral veins.

At the base of the leaf stalk is a pale, membranous appendage (stipule), not connected to the leaf blade, 1 to 3½ inches long, the tip pointed to rounded and not shredding, and may disintegrate by mid-summer. Stems are round, unbranched or few-branched. Large colonies are often formed from spreading 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, red to reddish-brown when mature.

Achenes are irregularly oval, 2.7 to 4.3 mm long, with a narrow, slightly bumpy keel along the back edge sometimes flanked by a pair of less prominent lateral keels. The short, abrupt beak is erect.

Notes:

Large-leaved Pondweed, known as Potamogeton americanus in older references, is fairly common in Minnesota, often forming large colonies, and has a world-wide distribution. It may be found in the quiet waters of lakes and ponds or more often in slow to moderately fast moving rivers and streams, in as little as a few inches of water to 6+ feet deep. It is fairly easily distinguished by elliptic floating leaves, tapering at both ends or only slightly rounded at the base, on stalks usually longer than the blades, and the larger submersed leaves narrowly lance-elliptic, to 8 inches long and about 1 inch wide, tapering at both ends, on stalks more than ¾ inch (2 cm) long, commonly 2 to 4 inches long.

Floating leaves resemble those of some other Pondweeds, but their blades may be heart-shaped or more consistently rounded at the base and/or tip, leaf stalks may be shorter than the blade, submersed leaves may be broader and/or stalkless or shorter-stalked, not more than 2 cm long. While the majority of references state the achenes of P. nodosus have 3 strongly developed, sharp keels, more than one noted the lateral keels are not always present or not always so sharp, and the latter is what we found on achenes that were not quite fully mature as in our photo; that may change as achenes dry down and we'll investigate that further. In any case, the achenes should not be necessary for a positive ID of this species.