Polygonum caespitosum var. longisetum is a weedy annual herb to 1 m tall, (more commonly 30 cm or less), stems slender, often pinkish, or reddish, branched, reclining.
Leaves alternate, thin, dark green, narrowly elliptic, tapering at both ends, almost stalkless, margins and veins below finely fringed (ciliate, use lens); node sheaths with long bristles 0.5-1.0 cm, as long as the membranous part (Uva et al 1997).
Flowers purple-rose, about 0.2 cm, in bristly, terminal spikes 2-4 cm long, 0.5 cm wide, loosely flowered, lower flowers often distant, sheath-like bracts below flowers overlapping, bristles 0.3 cm long, often longer than flowers; blooming and fruiting June-Nov. (Hough 1983).
Fruit an achene, 3-sided, shiny, dark, about 0.2 cm, eaten by many birds and small mammals (Martin et al. 1951).
Wetland status: FACU-.
Frequency in NYC: Common.
Habitat: Moist, shady or part-shade in disturbed areas, trail edges, damp, shady roadsides. Often many plants together. Intolerant of either drought or flooding (Sultan et al. 1998).