Potamogeton epihydrus Raf. ssp. nuttallii (Cham. & Schldl.) Calder & Roy P. epihydrus var. ramosus (C. H. Peck) H. D. House

 Rhizomes present; cauline stems compressed, without dark spots, 10–90 cm long. Turions absent. Nodal glands absent. Leaves submersed and floating, although either type may be absent on certain individuals, alternate to nearly opposite, more or less spiraling to more or less distichous. Submersed leaves sessile; ligules inconspicuous, free from blade, reddish brown, not fibrous, not shredding apically, persistent, convolute, 1–3 cm long, the apex obtuse; leaf blade lax, light green to reddish brown, subulate, not arcuate, 5–22 cm long, 1–10 mm wide, with 3–13 veins, 4–7 rows of lacunae each side of midrib, the base parallel-sided, not clasping, without basal lobes, the margins entire, the apex acute to obtuse, without a bristle tip, not cucullate. Floating leaves petiolate; petiole continuous in color to apex, compressed, 2–12.5 cm long; leaf blade adaxially light green, elliptic to narrowly oblanceolate, 2–8 cm long, 4–20 mm wide, the veins 11–41, the base cuneate, the apex cuspidate to obtuse, without terminal bristle. Inflorescences unbranched, distal only; distal peduncles cylindrical, turgid, axillary, erect, 1.5–5 cm long; spikes emersed, cylindrical, 8–40 mm long, the nodes 5–10, the internodes 1.5–2.5 mm long. Flowers alternate, or whorled; perianth spathulate, clawed, the claw 0.2–0.3 mm long, the blade 0.8–1 mm long, 1.2–1.5 mm wide; anthers 0.5–0.7 mm long; carpels 4. Fruiting gynophore absent. Fruits round obovate, greenish brown, compressed, abaxially angled, laterally angled, 2.5–4.5 mm long, 2–3.6 mm wide.

Flowering summer--fall. Still or flowing waters of lakes, ponds, streams, and rivers; 10--1900; Miquelon; B.C., Man., N.B., Nfld., N.S., Ont., P.E.I., Que., Sask., Ala., Alaska, Ark., Calif., Conn., Colo., Del., Fla., Ga., Idaho, Ill., Ind., Iowa, La., Maine, Md., Mass., Mich., Minn., Mo., Mont., N.C., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., R.I., S.C., S.Dak., Tenn., Utah, Vt., Va., Wash., W.Va., Wis., Wyo; Europe.

Two varieties, Potamogeton epihydrus var. epihydrus and P. epihydrus var. ramosus, have been recognized. These prove not to be distinct. Both varieties often grow in the same body of water in the same population. The wider-leaved plants often occur in more alkaline waters. Two hybrids, P. epihydrus var. nuttallii ´ P. gramineus and P. epihydrus ´ P. nodosus = P. ´subsessilis J.O. Hagström, have been described.

This is a common species of lakes and streams of northern United States and southern Canada. It extends southward in the eastern United States to Louisiana and Alabama. Potamogeton epihydrus is one of our more easily recognized species, by it having floating leaves, linear submersed leaves, and fruits that have an embryo with one full spiral or less. The only other North American pondweed with a similar set of characteristics is P. tennesseensis, which differs from P. epihydrus by the former having long tapering apices of the submersed leaves whereas the latter has blunt to acute apices.