11d. Polygonum aviculare Linnaeus subsp. neglectum (Besser) Arcangeli, Comp. Fl. Ital. 583. 1882.
Narrow-leaf knotweed, renouée négligée
Polygonum neglectum Besser, Enum. Pl., 45. 1822; P. aequale Lindman subsp. oedocarpum Lindman; P. aviculare subsp. rectum Chrtek
Plants green, homophyllous or sometimes heterophyllous. Stems usually 1-7, procumbent to ascending, sometimes erect, mostly branched from base, (5-)15-60 cm. Leaves: ocrea (3-)4-8 mm, proximal part cylindric, distal part with inconspicuous veins, eventually disintegrating and leaving few or no fibrous remains; petiole (0.3-)1-3(-5.2) mm; blade green, lateral veins visible but not raised abaxially, narrowly elliptic or oblanceolate, (7.6-)12.2-34(-40) × 1.5-6.8(-8) mm, (3.4-)4.2-9.2 times as long as wide, apex acute or obtuse; stem leaves 1-2.7(-3.3) times as long as branch leaves. Cymes uniformly distributed along stems and branches, 1-3(-5)-flowered. Pedicels mostly enclosed in ocreae, 1.5-3 mm. Flowers: perianth (1.9-)2.3-3.4 mm, 1.6-2.6 times as long as wide; tube 28-48% of perianth length; tepals overlapping, spreading slightly as achene matures, green usually with pink or red, rarely white, margins, oblong, ± cucullate, outer tepals not pouched at base; veins branched, moderately to strongly thickened; stamens 7-8. Achenes exserted from perianth, dark brown, ovate, 3-gonous, 1.2-1.8 mm, faces unequal or, less often, subequal, flat to concave, apex with straight edges or somewhat bent toward narrow face, striate-tubercled or, rarely, obscurely so; late-season achenes uncommon, 2-3.7 mm. 2n = 40, 60.
Flowering Jun-Nov. Disturbed places; 0-1500 m; introduced; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que., Sask.; Ala., Alaska, Ariz., Ark., Calif., Conn., Ga., Ill., Ind., Kans., Ky., La., Maine, Md., Mass., 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., Wyo.; Europe.
Subspecies neglectum has been reported from Colorado, Idaho, Iowa, and Wisconsin; those reports have not been confirmed.