Description from Flora of China
Polygonum forbesii Hance, J. Bot. 21: 100. 1883; P. reynoutria Makino var. ellipticum Koidzumi; P. yunnanense H. Léveillé; Reynoutria elliptica (Koidzumi) Migo ex Nakai; R. forbesii (Hance) T. Yamazaki; R. yunnanensis (H. Léveillé) Nakai ex Migo.
Herbs perennial, 1-2.5 m tall. Rhizomes well-developed, thick, ca. 1.6 cm in diam., lignified. Stems erect, robust, mostly branched above middle, 0.6-2.1 cm in diam., sparsely pubescent with conic unicellular trichomes, rarely together with 1-4-celled filiform trichomes. Leaves alternate; petiole 0.5-1.8 cm, circular in cross section, with extrafloral nectaries abaxially; ocrea completely surrounding stem, brown, cylindric, 0.5-1.5 cm, scarious, sparsely pubescent with conic unicellular trichomes, peltate multicellular glandular trichomes and 1-4-celled filiform trichomes, apex truncate; leaf blade orbicular to nearly so, 6.6-11.5 × 5.1-9.4 cm, both surfaces sparsely pubescent with peltate multicellular glandular trichomes, mid-vein and main veins sparsely pubescent with cornical unicellular trichomes, base rounded, margin entire, apex abruptly acuminate to cuspidate. Inflorescence terminal or axillary, simple or branched, 2-6.2 cm, pubescent with conic unicellular trichomes, 1-4-celled filiform trichomes and peltate multicellular glandular trichomes, panicle-like, consisting of a series of fascicles of 2-6 spirally arranged pedicellate flowers; each flower fascicle subtended by a bract (ocreola); bracts funnelform, 1.2-1.9 × 1-2 mm, scarious. Flowers functionally unisexual, rarely bisexual, 0.4-1.1 mm, each with bracteoles; bracteoles paired, scarious, fused at base; pedicels 2-3 mm, jointed above middle. Perianth petaloid, white to greenish white, 5-cleft; tepals 5, 3 outer ones keeled, 1.5-2.5 × 1-1.9 mm, with wings developed along main vascular bundle, wings ca. 0.2 wide. Stamens 8, inserted at base of perianth in 2 whorls, outer stamens 3-3.4 mm, inner stamens 3.6-4 mm. Style 1, 0.3-0.5 mm, deeply 3-cleft; stigmas fimbriate. Achenes enclosed in persistent perianth, dark brown, shiny, trigonous, obovate to orbicular, 3.9-4.8 × 2.1-2.9 mm, smooth; perianth wings narrowly tapering to articulated pedicel-joint in fruit. 2n = 66, 88.
The major characteristics that distinguish Fallopia forbesii from other, related species are the following: leaves orbicular to suborbicular, short, rounded at base, abruptly acuminate to cuspidate at apex; achenes relatively large (3.9–4.8 × 2.1–2.9 mm); perianth wings narrowly tapering to the articulated pedicel joint in fruit.
One of us (Li) believes that Fallopia forbesii is conspecific with Reynoutria japonica (see Lauener, Notes Roy. Bot. Gard. Edinburgh 40: 202. 1982).
Anhui, Guangdong, Guangxi, Jiangxi, Shandong, Yunnan, Zhejiang [Korea].