Northern knotweed

Polygonum aviculare var. boreale Lange, Consp. Fl. Groenland. 1: 105. 1880; P. arenastrum Boreau subsp. boreale (Lange) Á. Löve; P. boreale (Lange) Small; P. heterophyllum Lindman subsp. boreale (Lange) Á. Löve & D. Löve

Plants green, homophyllous or subheterophyllous. Stems 1-7, prostrate to ascending, mostly branching from base, 6-50(-90) cm. Leaves: ocrea 3.5-7 mm, proximal part cylindric to funnelform, distal part soon disintegrating, nearly completely deciduous; petiole 7-9 mm; blade green, lateral veins visible but not raised abaxially, obovate-spatulate or oblanceolate, (12.5-)16-44(-55) × (4-)6-18(-22) mm, 2-4(-5.5) times as long as wide, apex obtuse to rounded; stem leaves (1.1-)1.3-2.5(-3) times as long as branch leaves. Cymes uniformly distributed, rarely crowded at tips of stems and branches, (3-)4-7-flowered. Pedicels mostly exserted from ocreae, 2-5 mm. Flowers: perianth 3.3-5.5 mm, 1.6-2.8 times as long as wide; tube 25-35(-39)% of perianth length; tepals overlapping, green with white or pink margins, obovate, flat or curved outward in fruit, outer tepals not pouched at base; veins branched, thickened; stamens 6-8. Achenes enclosed in perianth, dark brown, ovate, 3-gonous, (2.5-)2.7-4(-4.2) mm, faces subequal, concave, apex straight, coarsely striate-tubercled; late-season achenes uncommon, 4-5 mm. 2n = 40.

Flowering Aug-Oct. Sandy, gravelly, or rocky coastal areas; 0-500 m; Greenland; Nfld. and Labr. (Labr.); nw Europe.