9a. Polygonum humifusum C. Merck ex K. Koch subsp. caurianum (B. L. Robinson) Costea & Tardif, Sida. 20: 995. 2003.
Alaska knotweed, renouée du Nord-Ouest
Polygonum caurianum B. L. Robinson, Proc. Boston Soc. Nat. Hist. 31: 264. 1904
Plants frequently reddish or purplish tinged; homophyllous or, rarely, heterophyllous. Stems prostrate, often zigzagged, branched mostly from base, wiry, 2-20(-40) cm. Leaves often opposite at proximal nodes; ocrea 2-3(-4) mm, proximal part funnelform, distal part soon lacerate, nearly completely deciduous; petiole 1-3.4 mm; blade usually reddish or purple tinged, obovate to oblanceolate, 3-12(-25) × (1.5-)2.5-4.5(-8) mm, margins flat, apex rounded to obtuse; stem leaves 1-1.5(-2) times as long as adjacent branch leaves, distal leaves overtopping flowers. Inflorescences axillary; cymes ± uniformly distributed, 2-6-flowered. Pedicels enclosed in ocreae, 0.5-1.5 mm. Flowers closed or semi-open; perianth 1.5-2.3(-3) mm; tube 28-49% of perianth length; tepals partially overlapping, green, margins pink, rarely white, petaloid, not keeled, oblong, ± outcurved, usually not cucullate; midveins unbranched or branched; stamens 5. Achenes exserted from perianth, dark brown to purple, ovate-lanceolate, 2-3-gonous, 1.4-1.6(-2.2) mm, faces unequal, apex not beaked or obscurely beaked, edges straight or concave, shiny or dull, smooth to roughened; late-season achenes common, 2-3.5 mm.
Flowering Jun-Aug. Gravel bars, waste places; 20-700 m; N.W.T., Nunavut, Yukon; Alaska.
Polygonum humifusum often has opposite proximal leaves. In North America, the closest relative of subsp. caurianum is P. fowleri, rare individuals of which produce opposite leaves.
Three specimens of the Asian subsp. humifusum were collected near Nanaimo, Vancouver Island, by J. Macoun in 1883 and 1887 (J. F. Brenckle 1941); the subspecies has not been recollected there. It has stems and leaves that are green, and achenes 2.1-2.7 mm, ± beaked, and exserted 0.9-1.3 mm from the perianth at maturity.