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FNA | Family List | FNA Vol. 5 | Polygonaceae | Polygonum

6. Polygonum glaucum Nuttall, Gen. N. Amer. Pl. 1: 254. 1818.

Seabeach knotweed

Plants silvery, homophyllous. Stems prostrate to ascending, branched from base, not wiry, 20-70 cm. Leaves: ocrea persistent, 7-15 mm, proximal part cylin-dric, 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.

Flowering Jul-Nov. Coastal beaches, sand dunes, margins of salt ponds; 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.


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