American knotweed, renouée faux-buis

Polygonum buxiforme Small, Bull. Torrey Bot. Club 33: 56. 1906

Plants gray-green or bluish green, rarely green, homophyllous or subheterophyllous. Stems 1-3, erect to ascending, ± unbranched, 5-15 cm, or numerous, procum-bent, mat-forming, extensively branched, 20-70(-200) cm. Leaves: ocrea 3.5-6.5(-8) mm, proximal part cylindric, distal part silvery, relatively persistent, with inconspicuous veins, leaving almost no fibrous remains after disintegrating; petiole 0.3-2(-3.5) mm; blade green or gray-green, lateral veins visible but not raised abaxially, lanceolate to elliptic, oblanceolate, or obovate, 6-30(-45) × 3-6(-13) mm, 2.5-5.6(-10) times as long as wide, apex acute to obtuse; stem leaves 1-2.5 times as long as branch leaves. Cymes mostly uniformly distributed, but also aggregated at tips of stems and branches, 2-6-flowered. Pedicels mostly enclosed in ocreae, 1-2.5 mm. Flowers: perianth (2-)2.3-3.4(-3.6) mm, 0.9-1.3(-1.5) times as long as wide; tube 20-36% of perianth length; tepals overlapping, green with white or sometimes pink margins, oblong, apex cucullate, outer tepals pouched at base; veins branched, moderately to strongly thickened; stamens 7-8. Achenes usually enclosed in perianth, light brown to brown, ovate, 3-gonous, (1.8-)2-2.8(-3) mm, faces subequal, concave to flat, apex straight, coarsely striate-tubercled to obscurely tubercled; late-season achenes common, 2.5-5 mm. 2n = 60.

Flowering Jul-Nov. Roadsides, vacant lots, sidewalks, packed and nondrifting sands, borders of marshes and dunes; 0-3500 m; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Ont., Que., Sask., Yukon; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.

Although apparently it has a North American origin, subsp. buxiforme is considered part of the Polygonum aviculare complex because it intergrades with subsp. aviculare (M. Costea and F. J. Tardif 2003).