Narrow-leaf knotweed

Polygonum rurivagum Jordan ex Boreau, Fl. Centre France ed. 3, 2: 560. 1857; P. aviculare var. angustissimum Meisner; P. heterophyllum var. angustissimum (Meisner) Lindman; P. heterophyllum subsp. rurivagum (Jordan ex Boreau) Lindman

Plants green, homophyllous to heterophyllous. Stems 1-10, procumbent to ascending, branched at most nodes, proximal branches divaricate, not wiry, 10-40 cm. Leaves: ocrea (6-)8-12 mm, proximal part cylindric, distal part with veins conspicuous, soon disintegrating into persistent fibers; petiole 0.3-2 mm; blade green, lateral veins strongly raised adaxially, narrowly elliptic to linear-lanceolate, (10-)15-27(-30) × 0.5-4.8(-8) mm, (4.5-)5-15(-19) times as long as wide, apex acute; stem leaves 1.2-2.4 times as long as branch leaves. Cymes uniformly distributed along stems and branches, 1-3-flowered. Pedicels enclosed in or exserted from ocreae, 1.5-3 mm. Flowers: perianth 2.2-3.1 mm, 1.6-2.6 times as long as wide; tube 26-40(-42)% of perianth length; tepals often not overlapping, green with pink or red margins, oblong, ± cucullate, outer tepals not pouched at base; veins branched, moderately to strongly thickened; stamens 7-8. Achenes usually exserted from perianth, blackish brown, ovate, 3-gonous, 2.1-2.6(-3) mm, faces subequal or unequal, flat to concave, apex straight, coarsely striate-tubercled; late-season achenes uncommon, 2.5-4 mm. 2n = 60.

Flowering Jul-Oct. Disturbed places; 0-1000; introduced; Alta., Man., N.B., Nfld. and Labr. (Nfld.), Que., Sask.; Calif., Ind., Iowa, Maine, Mass., N.Y., R.I., S.Dak., W.Va.; Europe.