Longleaf Pondweed

Sometimes confused with Potamogeton natans L., but submerged leaves not reduced to narrowly linear phyllodes, and petioles without discoloured joints at the tips. Common from hills to the plains of our area.

Six hybrids, Potamogeton gramineus ´ P. nodosus (= P. ´ argutulus Hagström), P. illinoensis ´ P. nodosus (= P. ´ faxonii Morong), P. nodosus ´ P. richardsonii (= P. ´ rectifolius A. Bennett), P. natans ´ P. nodosus (= P. ´ schreberi Fischer [P. ´ perplexus A. Bennett]), P. alpinus ´ P. nodosus (= P. ´ subotusus Hagström), and P. epihydrus ´ P. nodosus (= P. ´ subsessilis Hagström), have been described.

Potamogeton nodosus is a common floating-leaved species throughout much of the United States and southern Canada. When both submersed and floating leaves are present, it is very easily recognized by the petioles of the submersed leaves being longer than 5 cm.

Perennial, rhizomatous, aquatic. Stem branched, leafy, terete, simple. Leaves heterophyllous, submerged leaves petiolate, lanceolate or broadly lanceolate-oblong, veins 9 or more, thin translucent, 6-10.5 cm long; floating leaves broadly ovate, elliptic, coriaceous, of firm texture, 4-16 cm long, 2-5 cm broad, entire. Stipules free, keeled, lanceolate 2-15 cm long, 8-10 mm broad. Spikes cylindric, 4.5-11 cm long, 1.5-2.5 mm broad; peduncle 7-11 cm long. Flowers in whorls, sessile, small. Perianth segments small, obtuse, entire, c. 1.5 mm long. Stigma short. Fruitlets 3-4 mm long, 2-3 mm broad, obliquely obovoid, spongy, ventral margin convex, shortly beaked.

Rhizomes present. Cauline stems terete, without spots, to 100 cm; nodal glands absent. Turions absent. Leaves both submersed and floating, or floating absent, ± spirally arranged. Submersed leaves petiolate, lax; stipules persistent to deliquescent, conspicuous, convolute, free from blade, light brown, not ligulate, 3--9 cm, not fibrous, not shredding at tip, apex round to acute; petioles 2--13 cm; blade light to dark green, linear-lanceolate to lance-elliptic, not arcuate, 9--20 cm ´ 10--35 mm, base acute, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, acute, without sharp awl-like tip, lacunae in 2--5 rows each side of midrib; veins 7--15. Floating leaves: petioles 3.5--26 cm; blade adaxially light green, lenticular to elliptic, 3--11 cm ´ 15--45 mm, base cuneate to rounded, apex acute to rounded; veins 9--21. Inflorescences unbranched, emersed; peduncles not dimorphic, terminal, erect to ascending, cylindric, 3--15 cm; spikes not dimorphic, cylindric, 20--70 mm. Fruits sessile, red to reddish brown, obovoid, abaxially keeled, laterally ridged, 2.7--4.3 ´ 2.5--3 mm, keel well developed, lateral ridges with blunt to sharp tips; beak erect; sides without basal tubercles; embryo with 1 full spiral. 2n = 52.

Warm temperate and tropical regions of Europe, Africa, Asia and America.

Distribution: Warmer regions of Europe, N. and Cent. America, Africa and Asia.

Alta., B.C., N.B., Ont., Que., Sask.; Ala., Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Mexico; West Indies; Central America; South America; Eurasia.

600-3000 m

Fl. Per.: April-August.

Flowering summer--fall.

Clear to turbid waters of lakes, streams, rivers, and sloughs; 0--3300m.