Common knotweed

Polygonum aviculare var. eximium (Lindman) Ascherson & Graebner; P. aviculare subsp. heterophyllum (Lindman)Ascherson & Graebner; P. aviculare subsp. monspeliense (Thiébaut-de-Berneaud ex Persoon) Arcangeli; P. aviculare var. vegetum Ledebour; P. heterophyllum Lindman; P. monspeliense Thiébaut-de-Berneaud ex Persoon

Plants green, heterophyllous (often lacking larger proximal stem leaves at maturity). Stems 1-3, ascending or erect, sometimes decumbent, basal branches divar-icate, (10-)25-75(-100) cm. Leaves: ocrea (5.5-)6-10(-14) mm, proximal part cylindric, distal part silvery, hyaline, soon lacerate and disintegrating into fibers; petiole 1-6(-8) mm; blade green, lateral veins visible but not raised abaxially, elliptic to oblanceolate, 18-50(-60) × (6-)10-20 mm, 2-4.5 times as long as wide, apex acute or obtuse; stem leaves 1.4-4 times as long as branch leaves. Cymes aggregated at tips of stems and branches, 3-8-flowered. Pedicels mostly exserted from ocreae, 2-5 mm. Flowers: perianth (2.3-)2.8-4.7(-5) mm, 1.8-2.8 times as long as wide; tube (15-)20-37% of perianth length; tepals overlapping, green with pink, red, or white margins, oblong, flat (cucullate in fruit), outer tepals not pouched at base; veins branched, thickened; stamens 7-8. Achenes enclosed in or barely exserted from perianth, brown to dark brown, ovate, 3-gonous, (2.1-)2.7-3.7 mm, faces subequal, concave, apex straight, striate-tubercled; late-season achenes uncommon, 3.5-5 mm. 2n = 40, 60.

Flowering May-Oct. Fields, uncultivated areas, waste places, roadsides; 0-3500 m; introduced; St. Pierre and Miquelon; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., La., Maine, Md., Mass., Mich., Minn., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., Wyo.; Eurasia.