Polygonum pensylvanicum is an annual herb, erect or reclining, to 2 m tall, branched near base; nodes often reddish, leaf sheath encircling stem above node, appearing 2-parted, with a thicker, short, cup-shaped lower portion extending from the base of the leaf stalk, encircling the stem, above this is a longer, membranous, papery sheath, margin not bristly, but becoming shredded.
Leaves alternate, lance-shaped, to 3-20 cm long, 1-6 cm wide, on stalks to 1.5 cm, leaf tips long-pointed, lower surface punctate-covered with minute dots (Yatskievych 2006).
Flowers pink to white, 0.3-0.4 cm, 5-lobed divided to below middle; inflorescence branched, of spike-like racemes, 1.5-3 cm long, often covered with glandular hairs (use lens); blooming June-Oct. (Hough 1983).
Fruit dry (achenes), flat, concave on both sides, about 0.3 cm, brown to black, shiny (Mitchell and Deans 1978). Seeds eaten by many birds and small mammals; fruiting Aug.-Nov. (Hough 1983).
Wetland status: FACW.
Frequency in NYC: Frequent.
Habitat: Moist to wet open areas and edges in rich, moist disturbed soil (Hough 1983), pH 4-8.5; intolerant of salt and shade (USDA, NRCS 2006).