Polygonum aviculare subsp. neglectum (Besser) Arcang.
Family: Polygonaceae
Yard Knotweed
[Polygonum neglectum Besser]
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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.

Prostrate to ascending annual, much like no. 8 [Polygonum arenastrum Boreau], but more sparsely branched, and with narrower, linear oblong to lance-linear lvs 8-30 mm; perianth avg a bit larger, to 3 mm, divided half to 3/4 length; achenes avg larger, 2-3 mm; 2n=40, 60. A weed of roadsides, vacant lots, and pastures, seldom on beaches and dunes; Eurasia and much of N. Amer., doubtfully native. (P. franktonii, of maritime and freshwater beaches, dunes, and shores in se. Can.)

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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