Plants silvery, homophyllous. Stems prostrate to ascending, branched from base, not wiry, 20–70 cm. Leaves: ocrea persistent, 7–15 mm, proximal part cylindric, pruinose, distal parts silvery, margins overlapping, entire or lacerate; petiole 0.5–3 mm; blade bluish green, lanceolate, 10–30 × 2–8 mm, coriaceous, margins revolute, apex obtuse or acute, rugulose when fresh, markedly rugulose when dried, glaucous; middle stem leaves slightly larger than adjacent branch leaves, distal leaves overtopping flowers they subtend. Inflorescences axillary; cymes uniformly distributed, 1–3-flowered. Pedicels enclosed in ocreae, 3–5 mm. Flowers semi-open; perianth (2–)3–4 mm; tube 12–26% of perianth length; tepals ± recurved, overlapping, white, margins white or pink, petaloid, not keeled, oblong-obovate to spatulate, not cucullate; midvein usually unbranched; stamens 8. Achenes exserted from perianth, reddish brown to dark brown, ovate, 3-gonous, 2.5–3(–4) mm, faces subequal, apex not beaked, edges straight, shiny, smooth; late-season achenes common, 3–5 mm. 2n = 40.
Phenology: Flowering Jul–Nov.
Habitat: Coastal beaches, sand dunes, margins of salt ponds
Elevation: 0-10 m
Conn., Del., D.C., Fla., Ga., Md., Mass., N.J., N.Y., N.C., R.I., S.C., Va.
Polygonum glaucum is restricted to maritime beaches along the Atlantic coast from Massachusetts south to Georgia. Over most of its range it is rare and declining; populations on coastal islands of Massachusetts and along the shore of Long Island appear to be secure. Seabeach knotweed appears to be related to P. oxyspermum.