This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
8: 294. 1870.
Aquatic, perennial, glabrous or pubescent; stem floating or submersed, simple or sparingly branched, 4°-20° long. Leaves oblong, elliptic or elliptic-lanceolate or narrowly lanceolate, 1 1/2'-4' long, petioled, obtuse or subacute at the apex, slightly inequilateral, rounded or narrowed at the base, sometimes ciliate; ocreae cylindric, those of the branches often longer than the internodes, their limbs sometimes spreading, usually glabrous; raceme terminal, usually solitary, 1/2-1' long, dense, erect, oblong or ovoid; calyx rose-color, 5-parted; stamens 5, exserted; style 2-cleft, exserted; achene orbicular-oblong, 1 1/2" long, biconvex, black, smooth and shining, or granular.
In ponds and lakes, Quebec to Alaska, New Jersey, Kentucky, Colorado and California. Europe. Ascends to 2000 ft. in the Adirondacks. Ground-willow. Willow-grass. Red shanks. Heartsease. July-Aug.
Polygonum amphibium var. emersum Michx. Fl. Bor.
Perennial by long creeping or horizontal root-stocks, glabrous or strigose-pubescent; stem erect or assurgent, commonly simple, channeled, enlarged at the nodes, 1°-3° high. Leaves ovate-lanceolate or oblong-lanceolate, or the upper sometimes narrowly lanceolate, 2i'-8' long, acute or usually acuminate at the apex, rounded or cordate at the base, petioled, the lateral nerves prominent, sometimes forking; ocreae cylindric, becoming loose, not ciliate; racemes 1 or 2, erect, 1'~3' long, linear-oblong, dense; calyx dark rose-coior, 5-parted; stamens 5, exserted; style 2-cleft, exserted; achene broadly obovate or orbicular, 1 1/2" long, very convex, lenticular, black and slightly granular, but shining.
Polygonum lapathifolium L. Sp. PI. 360. 1753. P. incarnation Ell. Bot. S. C. & Ga. 1: 456. 1817. P. lapathifolia S. F. Gray, Nat. Arr. Brit. PI. 2: 270. 1821. P. lapathifolium incanum Koch, Syn. Fl. Germ. 711. 1837. P. lapathifolium nodosum Small, Mem. Torr. Club 5: 140. 1894.
Annual, stem simple or much branched, erect or ascending, swollen at the nodes, 1°-3° high, the peduncles and pedicels often glandular. Leaves lanceolate or oblong-lanceolate, 2'-10' long, usually broader than those of the preceding, attenuate to the apex, acuminate at the base, short-petioled, ciliate, glabrous or pale-pubescent, inconspicuously punctate; ocreae cylindric, ribbed or striate; racemes panicled, 1'-4' long, drooping, narrow, rather dense; calyx pink, greenish or white, 5-parted; stamens 6; achene broadly oblong or ovoid, lenticular, 1" long, brownish or black, slightly reticulated but shining, its faces concave.
In waste places, throughout temperate North America. Naturalized from Europe. Sometimes a troublesome weed. Native also of Asia. Willow-weed. June-Sept.
Persicaria tomentosa (Schrank) Bicknell (Polygonum tomentosum Schrank) has been separated from P. lapathifolia by the pubescent leaves and slightly larger flowers and fruits.