3. Polygonum achoreum S. F. Blake, Rhodora. 19: 232. 1917.
Blake's knotweed, renouée coriace
Polygonum erectum Linnaeus subsp. achoreum (S. F. Blake) Á. Löve & D. Löve
Plants light green (often covered with whitish powdery mildew), homophyllous or, sometimes, heterophyllous. Stems erect when young, decumbent or prostrate later, moderately branched especially from base, not wiry, 50-70 cm. Leaves: ocreae 5-12 mm, proximal part cylindric, distal part soon disintegrating into brown fibers; petiole 0.3-1.5 mm; blade light yellowish green, elliptic to obovate, 8-35 × 3-15 mm, margins flat, apex rounded; stem leaves 1-2.1(-3) times longer than branch leaves; distal leaves overtopping flowers. Inflorescences axillary, cymes in axils of most leaves and toward tips of stems and branchs, 1-3(-5)-flowered. Pedicels enclosed in ocreae, 1.3-1.8(-2) mm. Flowers closed; perianth 2.6-4 mm; tube 40-55% of perianth length; tepals incurved, yellow-green with yellow to green, rarely pinkish, margins, sepaloid, ± keeled, narrowly oblong, cucullate; midveins unbranched, moderately to heavily thickened, tepals appearing keeled; stamens 5-8. Achenes enclosed in perianth, yellow-green to tan, ovate, 3-gonous, 2.4-3.5 mm, faces unequal, apex not beaked, edges concave or nearly straight, dull, uniformly tubercled; late-season achenes common, 3-5 mm. 2n = 40, 60.
Flowering Jul-Sep. Disturbed areas, roadsides, sidewalks, edges of cultivated fields; 10-800 m; Alta., B.C., Man., N.B., N.W.T., N.S., Ont., Que., Sask., Yukon; Alaska, Colo., Conn., Idaho, Ill., Ind., Iowa, Kans., Maine, Mich., Minn., Mo., Mont., Nebr., Nev., N.Y., N.Dak., Ohio, Oreg., S.Dak., Utah, Vt., Wash., W.Va., Wis., Wyo.
Polygonum achoreum frequently is confused with P. erectum. It can be distinguished by its usually homophyllous leaves, its perianth, which is enlarged at the base and constricted above the fruit, its longer perianth tube, and its yellow-green to tan, tubercled achenes.