Plants annual, 3-8 dm; roots also often arising from proximal nodes; rhizomes and stolons absent. Stems decumbent to ascending, branched, without noticeable ribs, glabrous. Leaves: ocrea hyaline to brownish, cylindric, 5-12 mm, chartaceous, base sometimes inflated, margins truncate, ciliate with bristles 4-12 mm, surface glabrous or strigose, not glandular-punctate; petiole 0.1-0.3(-0.6) cm, glabrous, leaves sometimes sessile; blade without dark triangular or lunate blotch adaxially, ovate-lanceolate to linear-lanceolate, 2-8 × 1-3 cm, base tapering to cuneate, margins antrorsely strigose, apex acute to acuminate, faces glabrous or sparingly strigose along veins abaxially, glabrous or strigose along midvein and margins adaxially, not glandular-punctate. Inflorescences terminal, sometimes also axillary, erect, uninterrupted, 10-40(-80) × 3-7 mm; peduncle 10-50 mm, glabrous; ocreolae overlapping, margins ciliate with bristles (0.5-)1-4(-6) mm. Pedicels ascending, 1-2 mm. Flowers 1-5 per ocreate fascicle, homostylous; perianth pinkish green proximally, roseate distally, glabrous, not glandular-punctate, scarcely accrescent; tepals 5, connate ca. 1/ 3 their length, obovate, 2.2-2.8 mm, veins not prominent, not anchor-shaped, margins entire, apex obtuse to rounded; stamens 5, included; anthers yellow, elliptic to ovate; styles 3, connate proximally. Achenes included, dark brown to black, 3-gonous, 1.6-2.3 × 1.1-1.6 mm, shiny, smooth.

Flowering May-Oct. Floodplain forests and woodlands, shorelines of ponds, moist roadsides, waste places; 0-300 m; introduced; B.C., N.B., Ont.; Ala., Conn., Del., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.J., N.Y., N.C., Ohio, Pa., S.C., Tenn., Tex., Vt., Va., W.Va., Wis.; e Asia; introduced also in Europe.

Persicaria longiseta is morphologically similar to another Asian species, P. posumbu (Buchanan-Hamilton ex D. Don) H. Gross (= P. caespitosa). Its spread in the United States since its introduction near Philadelphia in 1910 was summarized by A. K. Paterson (2000).