Description from Flora of China
Persicaria hydropiper (Linnaeus) Spach; ?P. hydropiper var. diffusa Kitagawa; P. hydropiper var. vulgaris (Meisner) Ohki; P. vernalis Nakai; ?Polygonum hydropiper var. longistachyum Y. L. Chang & S. X. Li; P. hydropiper var. vulgare Meisner; P. schinzii J. Schuster.
Herbs annual. Stems erect, 40-70 cm tall, much branched, glabrous, dilated at nodes. Leaves with peppery taste; petiole 4-8 mm; leaf blade lanceolate or elliptic-lanceolate, 4-8 × 0.5-25 cm, both surfaces glabrous, densely brown punctate, sometimes appressed hispidulous along midvein, base cuneate, margin ciliate, apex acuminate; cleistogamous flowers present in many leaf axils; ocrea tubular, 1-1.5 cm, membranous, sparsely appressed hispidulous, apex truncate, shortly ciliate. Inflorescence terminal or axillary, spicate, pendulous, interrupted below, usually lax, 3-8 cm, slender; bracts green, funnel-shaped, 2-3 mm, margin membranous, sparsely shortly ciliate, each 3-5-flowered. Pedicels longer than bracts. Perianth greenish, white or pink above, 5(or 4)-parted, brownish pellucid glandular punctate; tepals elliptic, 3-3.5 mm. Stamens 6, rarely 8, included. Styles 2 or 3. Achenes included in persistent perianth, black-brown, opaque, ovoid, biconvex or trigonous, 2-3 mm, densely small pitted. Fl. May-Sep, fr. Jun-Oct. 2n = 18-22.
Riverbanks, streamsides, wet valleys; sea level to 3500 m. Anhui, Fujian, Gansu, Guangdong, Guangxi, Guizhou, Hainan, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Nei Mongol, Ningxia, Qinghai, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Xinjiang, Xizang, Yunnan, Zhejiang [Bangladesh, Bhutan, India, Indonesia, Japan, Kazakhstan, Korea, Kyrgyzstan, Malaysia, Mongolia, Myanmar, Nepal, Russia, Sikkim, Sri Lanka, Thailand, Uzbekistan; Australia, Europe, North America].