Flower:

Dense cylindrical spike held above the surface of the water, 1½ to 3 inches (to 8 cm) long at the tip of the stem and arising from the axils of floating leaves. Spikes have 6 to 10+ whorls of flowers, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a green to brownish, ladle-shaped, sepal-like appendage.

Leaves and stems:

Leaves are mostly submersed, more or less spirally arranged, usually with some floating leaves produced. Floating leaves are firm, elliptic to narrowly egg-shaped, 1½ to 4 inches long, 1 to 1½ inches wide, toothless, rounded to somewhat heart-shaped at the base, pointed to rounded at the tip, on a stalk usually longer than the blade. The upper surface is green, the lower often reddish; 27 to 49 veins flank the conspicuous midrib, a few lateral veins more distinct the the rest.

Submersed leaves are thin, green often turning reddish-brown, broadly elliptic, widest above or below the middle, 2 to 8 inches long, up to 2¾ inches wide, typically arching, often folded, somewhat wavy around the edges, pointed at the tip, tapering at the base, on a stalk up to 2½ inches long, occasionally stalkless. The midvein is flanked by 19 to 49 lateral veins, a few of which are more prominent than the rest. At the base of the leaf stalk is a pale, translucent, membranous appendage (stipule), not connected to the leaf blade, 1½ to 4+ inches long, 2-keeled, the tip pointed to straight across and not shredding. Stems are round, often rusty-spotted, mostly unbranched, elongated at lower depths. Colonies are often formed from creeping 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, reddish-brown when mature.

Achenes are irregularly oval, 4 to 6.5 mm long with 3 obscure, smooth, rounded keels and a short, abrupt beak.

Notes:

Big-leaf Pondweed is one of our more easily recognized pondweeds, with its large, often reddish, strongly arching, ruffled, submerged leaves near the surface, the flowers and fruits held above the water from the axils of floating leaves, often forming large colonies in deeper water. When floating leaves are absent, submersed stems may be confused with Illinois Pondweed (Potamogetan illinoensis), which is typically shorter and stouter, frequently branched, with more lance shaped leaves that are only slightly arching and have only up to 19 veins, only occasionally producing short-stalked floating leaves. Deeper water specimens of White-stem Pondweed (P. praelongus) may also appear similar, however its submersed leaves are generally narrower and short-stalked to stalkless or nearly so, and no floating leaves are produced.