Description from Flora of China
Helxine sagittata (Linnaeus) Rafinesque; Persicaria sagittata (Linnaeus) H. Gross ex Nakai; P. sagittata var. sieboldii (Meisner) Nakai; P. sieboldii (Meisner) K. Ohki; ?P. sieboldii var. brevifolia Kitagawa; ?Polygonum belophyllum Litvinov; ?P. paludosum (Komarov) Komarov; P. sagittatum var. boreale Meisner; P. sagittatum Linnaeus subsp. sieboldii (Meisner) V. N. Voroschilov; ?P. sagittatum var. paludosum Komarov; ?P. sieboldii var. pratense Y. L. Chang & S. X. Li; P. sagittatum var. sibiricum Meisner; P. sagittatum var. sieboldii (Meisner) Maximowicz ex Komarov; P. sieboldii Meisner; Tracaulon sagittatum (Linnaeus) Small; T. sibiricum (Meisner) Greene; T. sieboldii (Meisner) Greene; Truellum sagittatum (Linnaeus) Soják; T. sibiricum (Meisner) Soják.
Herbs annual, scandent, 0.3-2 m tall. Stem light green to green, often becoming reddish purple at maturity, simple to extensively branched, glabrous, angulate, with recurved prickles along angles, often decumbent at base, usually producing fibrous roots at lower nodes. Leaves petiolate, upper ones often shortly petiolate or subsessile; petiole 0.5-4 cm, glabrous, retrorsely prickly along angles; leaf blade green adaxially, light green abaxially, broadly lanceolate to oblong, 2-8.5 × 1-3 cm, thin, abaxially glabrous, abaxially usually retrorsely prickly near base of midvein, adaxially glabrous to densely pubescent with appressed simple multiseriate hairs, base sagittate to deeply cordate with small rounded or triangular lobes, margin ciliate or eciliate, apex acute to subacute or obtuse; ocrea oblique, 0.5-1.3 cm, scarious, glabrous, often weakly ciliate at tip, lower ones often lacerate. Inflorescence terminal or in axils of upper leaves, elongate, 3-15 cm, often slender, usually branched, usually glabrous but sometimes with a few recurved prickles in lower portion; inflorescence bracts resembling ocrea but much reduced, 1.5-3 mm; each inflorescence branch terminated by a dense headlike cluster of many flower fascicles. Flower fascicles usually 2- or 3-flowered, closely spaced; bracts lanceolate to narrowly elliptic, 3-5 mm, glabrous, apex strongly acuminate; bracteoles narrowly elliptic to elliptic, 3-4 mm, glabrous, often ciliate at tip; pedicel short, 1-1.5 mm, glabrous, included in bracteoles. Perianth white to greenish white, often reddish, 3-5 mm, 5-parted; tepals broadly elliptic, not becoming fleshy, apex obtuse. Stamens 8, inserted at base of perianth in 2 whorls; inner stamens 3, ca. 2 mm; outer stamens 5, 1-1.5 mm. Style 1, 3-cleft to middle, ca. 0.5 mm, included; stigmas 3. Achenes dark brown to black, dull to shiny, 3-4 mm, usually ovoid, sharply trigonous, punctate to smooth, apex acute.
Polygonum sagittatum s.l. is an extremely variable species and known from both Asia and North America. Some authors have pointed out, on the basis of differences in achene surface and leaf margin, that North American populations are separable from Asian ones, and have treated them as two distinct varieties of P. sagittatum, or as two distinct species: P. sagittatum in North America and P. sieboldii in eastern Asia. In Park’s previous taxonomic monograph of P. sect. Echinocaulon (Syst. Bot. 12: 167–179. 1987; and Mem. New York Bot. Gard. 47: 1–82. 1988), P. sagittatum and P. sieboldii are recognized as conspecific mainly based on flavonoid chemistry and also the observation that North American plants are well within the range of variability of eastern Asian ones. However, the recent molecular analyses of North American and eastern Asian populations of P. sagittatum (Park, in prep.) show that these disjunct populations are genetically somewhat divergent from each other. The degree of genetic divergence, however, strongly suggests that they can be recognized either as a single polymorphic species (P. sagittatum s.l.) or two distinct varieties of P. sagittatum, but they can hardly be treated as two distinct species.
In moist grassy situations, often in partial shade, occurring naturally in meadows, pastures, margins of swamps, ponds, and along stream banks, common as a weed along roadsides and other disturbed sites; 100-2200 m. Anhui, Fujian, Gansu, ?Guizhou, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Nei Mongol, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Yunnan, Zhejiang [N India, Japan, Korea, Mongolia, Russia (Far East) (var. sieboldii); E North America: (var. sagittatum)].