Plants perennial, 5-15 dm; roots also often arising from proximal nodes; rhizomes present, stolons sometimes produced on plants in water. Stems ascending or erect, branched distally, slightly ribbed, glabrous or loosely appressed- to spreading-hirsute near nodes. Leaves: ocrea brown, cylindric, 10-20 mm, chartaceous, base usually inflated, margins truncate, ciliate with bristles 6-12 mm, surface strigose and with loosely ascending to spreading hairs at least proximally, not glandular-punctate; petiole 0.1-0.5 cm, spreading-hirsute, leaves sometimes sessile; blade without dark triangular or lunate blotch adaxially, lanceolate, (3-)6-15(-18) × (1.5-)2-3.2(-4.8) cm, base tapered to truncate, margins appressed-ciliate, apex acute to acuminate, faces sparsely hirsute to loosely appressed hirsute abaxially and adaxially. Inflorescences mostly terminal, erect, uninterrupted, 20-80 × 4-8 mm; peduncle 10-70 mm, strigose; ocreolae overlapping, margins ciliate with bristles (0.6-)1-3(-5) mm. Pedicels ascending, 1-3 mm. Flowers (1-)2-4(-5) per ocreate fascicle, homostylous; perianth greenish proximally, creamy or tan distally, occasionally tinged pink, glabrous, not glandular-punctate, nonaccrescent; tepals 5, connate ca. 3 their length, obovate, 2-3 mm, veins not prominent, not anchor-shaped, margins entire, apex obtuse to rounded; stamens 5, included; anthers pink or red, elliptic; styles 3, connate proximally. Achenes included or apex exserted, brown to black, 3-gonous, (1.5-)2-2.5 × 1.2-1.7 mm, shiny, smooth. 2n = 20.
C. B. McDonald (1980) showed that Persicaria setacea is closely related to P. hirsuta and P. hydropiperoides. Hybrids between P. setacea and P. hirsuta have been produced experimentally but appear to be rare in the wild. Persicaria setacea and P. hydropiperoides occasionally occur in mixed populations but do not hybridize (McDonald). Persicaria setacea sometimes intergrades morphologically with P. hydropiperoides, especially in New England. Specimens of P. setacea without the characteristic ascending or spreading hairs on the ocreae usually can be distinguished from P. hydropiperoides by the extent of adnation of the hairs to the ocreae-up to one-third their lengths in P. setacea, but one-third to two-thirds their lengths in P. hydropiperoides.