Plants annual, 1-20 dm; roots also occasionally arising from basal nodes; rhizomes and stolons absent. Stems ascending to erect, simple or branched, ribbed, glabrous or appressed-pubescent distally, eglandular or stipitate-glandular distally. Leaves: ocrea brownish, cylindric, 5-20 mm, chartaceous, base inflated, margins truncate, eciliate or ciliate with bristles to 0.5 mm, surface glabrous or appressed-pubescent, eglandular; petiole 0.1-2(-3) cm, glabrous or appressed-pubescent; blade sometimes with dark triangular or lunate blotch adaxially, narrowly to broadly lanceolate, 4-17(-23) × (0.5-)1-4.8 cm, base tapered to cuneate, margins antrorsely scabrous, apex acuminate, faces glabrous or appressed-pubescent, eglandular or glandular-punctate abaxially and occasionally adaxially. Inflorescences terminal and axillary, erect or rarely nodding, uninterrupted, 5-50 × 5-15 mm; peduncle 10-55(-70) mm, glabrous or pubescent, usually stipitate-glandular; ocreolae overlapping, margins eciliate or ciliate with bristles to 0.5 mm. Pedicels ascending, 1.5-4.5 mm. Flowers 2-14 per ocreate fascicle, homostylous; perianth greenish white to roseate, glabrous, not glandular-punctate, accrescent; tepals 5, connate ca. 1/ 1/ 3 their length, obovate to elliptic, 2.5-5 mm, veins prominent, not anchor-shaped, margins entire, apex obtuse to rounded; stamens 6-8, included; anthers yellow, pink, or red, elliptic; styles 2(-3), connate at bases. Achenes included or apex exserted, brown to black, discoid or, rarely, 3-gonous, without central hump on 1 side, 2.1-3.4 × 1.8-3 mm, shiny, smooth. 2n = 88. Flowering May-Dec. Moist, disturbed places, ditches, riverbanks, cultivated fields, shorelines of ponds and reservoirs; 0-1800 m; N.B., Nfld. and Labr. (Nfld.), N.S., Ont., Que.; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., 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., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., W.Va., Wis., Wyo.; South America (Ecuador); Europe (England, Spain). Persicaria pensylvanica is a morphologically variable allotetraploid, with P. lapathifolia probably one of the parents (L. L. Consaul et al. 1991). Three or four varieties (under Polygonum) often have been accepted in North American floras; the characters on which these are based vary greatly within and among populations. J. W. Taylor-Lehman (1987) concluded that Polygonum pensylvanicum is best treated as a polymorphic species without infraspecific taxa, based on specimens primarily from Ohio. The heterostylous Persicaria bicornis often is included in P. pensylvanica. A single chromosome count of 2n = 22 reported by Á. Löve and D. Löve (1982), which could not be confirmed by Consaul et al. because the voucher could not be found, is excluded. Flowers with three styles and trigonous achenes are produced; they are exceedingly rare and probably mostly overlooked. Several Native American tribes prepared infusions and decoctions from P. pensylvanica, which they used as drugs for humans and horses (D. E. Moerman 1998).
Erect, branching annual to 2 m, or seldom prostrate; lvs lanceolate, acuminate; ocreae becoming lacerate, but not fringed-ciliate; racemes numerous, erect, commonly cylindric, blunt, 1.5-3 cm, ocreolae ovate, entire or minutely ciliate; perianth rose or white, scarcely exceeding the achene, 5-lobed to below the middle, the segments with several inconspicuous, branched and anastomosing veins; achenes lenticular, concave on both sides, 2.6-3.4 mm, 85-100% as wide; 2n=44. Fields and waste ground, especially in moist, rich soil; N.S. and Que. to Minn. and S.D., s. to Fla. and Tex. Highly variable, but scarcely divisible into significant vars. Heterostylic southern plants, reaching our range in s. Ill., have been segregated as P. bicorne Raf. (P. longistylum Small), but the populational relationship of these plants to the widespread, homostylic P. pensylvanicum remains to be elucidated.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.