Polygonum arenastrum is an annual herb, to 10 cm tall, taprooted, freely branched, sprawling to reclining, mat-forming; stipules (ocreae) sheath-like, membranous, 2-parted, becoming ragged.
Leaves alternate, stalk short, jointed at juncture with basal sheath; blade 0.5-2 cm long, 0.3-0.9 cm wide cm, elliptic, dark bluish-green, often deciduous late in season, leaves.
Flowers green with white or pink margins, 1-5 in axils, tubular, petal lobes 5, to about as long as tube (corolla divided to middle), stamens 5, styles 3; mature flowers to 0.26 cm long; self-fertile (autogamous), apparently not visited by insects, (Mulligan and Kevan 1973); blooming and fruiting June-Sept. sometimes to Dec. (Hough 1983).
Fruit dry, an achene, 0.2 cm (to 0.4 cm late in season), 2 sides flat or convex, third side narrowly concave, dark brown, pitted and grooved, seeds eaten by birds (Mitchell and Dean 1978). Seeds eaten by mourning doves and other seed-eating birds (Yatskievych 2006).
Wetland status: FACU.
Frequency in NYC: Occasional (probably more common than indicated).
Habitat: Open, disturbed areas. More common in ball fields, curbs, roadsides, lawns.
Note: Much like P. aviculare but leaves and achenes smaller and without large, early season leaves). Polygonum buxiforme, a rare native plant, is very similar but found in sand dunes and coastal sandy soil.