Plants usually yellowish greenish when fresh or dried, hetero-phyllous. Stems 30-200 cm. Leaf blades light yellowish green, narrowly elliptic to lanceolate, rarely ovate, 35-70 × 7-18(-35) mm, apex acute to acuminate; stem leaves 2.1-3.5(-4.2) times as long as branch leaves; distal leaves usually 1-2 mm, not overtopping flowers, or 5-15 mm, surpassing flowers. Inflorescences axillary and terminal, spikelike; cymes crowded toward tips of branches, 2-5-flowered. Pedicels exserted from ocreae, 2.5-6 mm. Flowers: perianth (2.6-)3-4 mm; tepal margins greenish yellow or yellow, rarely white or pink; stamens 3-6(-8). Achenes enclosed in or exserted from perianth, (2.3-)2.5-3.5 mm, shiny or dull, smooth to roughened, sometimes uniformly or obscurely tubercled; late-season achenes 5-12 mm. 2n = 60.

Flowering Jul-Oct. Disturbed places, sandy shores or saline soils and shores; 0-1000 m; Alta., B.C., Man., N.B., N.W.T., N.S., Ont., P.E.I., Que., Sask.; Ariz., Ark., Calif., Colo., Conn., Del., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Mass., Mich., Minn., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., Wis., Wyo.

Subspecies ramosissimum is heterogeneous; some additional elements may deserve recognition. It is closely related to European Polygonum bellardii Allioni, which was collected in south Boston in 1785 (B. L. Robinson 1902). The latter species has semi-open flowers, petaloid tepals with white or pink margins, and eight stamens. A distinct form of P. ramosissimum growing in saline marshes from California has been mistakenly identified as P. patulum Bieberstein (M. Costea and F. J. Tardif 2003b). The morphology of late-season achenes and the branching patterns, which have been emphasized by some authors, appear to have little taxonomic value.

Very local. All of our specimens were found in hard, dry soil on the washed slopes of the banks of streams and sloughs. Some were very near the water and only one grew on the top of the bank. Bicknell (Bull. Torrey Bot. Club. 36: 450. 1909.) says: "I am unable to see that P. exsertum is anything more than a semiviviparous state of Polygonum ramosissimum Michx." My observation is that this character applies to late flowering plants of all of the species of the Section Avicularia which occur in Indiana. On November 14, 1932, I studied in the field several large mats of Polygonum aviculare, and I was able to find only exserted achenes. A study of my herbarium material showed exserted achenes on all of the plants collected late in the fall, some with a few and some with a great number of exserted achenes. Early flowering specimens of Polygonum exsertum show a large number of achenes of the normal form, while plants collected in September usually have few or no normal fruits.