Giant knotweed

Polygonum sachalinense F. Schmidt, Mém. Acad. Imp. St.-Pétersbourg Divers Savans 9: 233. 1859; Reynoutria sachalinensis (F. Schmidt) Nakai; Tiniaria sachalinensis (F. Schmidt) Janchen

Herbs, perennial, rhizomatous, 2-4(-5) m. Stems usually clustered, erect, sparingly branched, herba-ceous, stiff, glabrous, glaucous. Leaves: ocrea persistent or deciduous, brownish, cylindric, 6-12 mm, margins oblique, face without reflexed and slender bristles at base, otherwise glabrous or puberulent; petiole 1-4 cm, glabrous; blade ovate-oblong, 15-30(-40) × 7-25 cm, base cordate, margins entire, glabrous or scabrous to ciliate, apex obtuse to acute, abaxial face minutely dotted, glaucous, with hairs along veins distinctly multicellular, 0.2-0.6 mm, tips acute to acuminate, adaxial face glabrous. Inflorescences axillary, mostly distal, erect or spreading, paniclelike, 3-8 cm, axes puberulent to pubescent; peduncle 0.1-4 cm or absent, puberulent to reddish-pubescent. Pedicels ascending or spreading, articulated proximal to middle, 2-4 mm, glabrous. Flowers bisexual or pistillate, 4-7 per ocreate fascicle; perianth accrescent in fruit, greenish, 4.5-6.5 mm including stipelike base, glabrous; tepals obovate to elliptic, apex obtuse to acute, outer 3 winged; stamens 6-8; filaments flattened proximally, glabrous; styles connate basally; stigmas fimbriate. Achenes included, brown, 2.8-4.5 × 1.1-1.8 mm, shiny, smooth; fruiting perianth glabrous, wings flat to undulate, 1.8-2.2 mm wide at maturity, decurrent on stipelike base to articulation, margins entire. 2n = 44, 66, 102, 132 (Japan, Korea).

Flowering Jul-Oct. Disturbed places; 0-500 m; introduced; B.C., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que.; Calif., Conn., Del., Idaho, Ill., Ky., La., Maine, Md., Mass., Mich., Mont., N.J., N.Y., N.C., Ohio, Oreg., Pa., R.I., Tenn., Vt., Va., Wash., W.Va., Wis.; Asia (Japan); introduced in Europe.

Fallopia sachalinensis was introduced as a soil binder and garden ornamental. Like F. japonica, it spreads aggressively and has been declared noxious in California, Oregon, and Washington. It hybridizes with F. japonica, yielding F. ×bohemica. The mid-stem inflorescences of F. sachalinensis usually are shorter than the subtending leaves.