Annuals. Stem 10-65(-70) cm tall, erect, not rooting at nodes, with abundant 0.2-1.1 mm patent to retrorse nonglandular trichomes and scattered 0.2-0.7 mm patent glandular trichomes. Stipules lanceolate to subulate, distinct. Leaves 1-3 alternate but opposite at inflorescence; petiole with usually 0.2-1.1 mm patent nonglandular trichomes and scattered glandular trichomes; leaf blade 2.2-6.5 cm, palmately cleft, ratio of main sinus/middle segment length = 0.7-0.9, pilose with appressed nonglandular trichomes; segments 5(or 7), rhombic, 2-5 mm wide at base, 5-12-lobed in distal half, ratio of second sinus/middle segment length = 0.27-0.31. Cymules in dense aggregates at apex of each branch or solitary in forks, 2(or 3)-flowered; peduncle 0-2.5(-6) cm. Pedicel 3-11 mm, with 0.2-0.7 mm patent or sometimes retrorse but not appressed nonglandular trichomes and 0.3-0.6 mm patent glandular trichomes; bracteoles lanceolate. Sepals 5-6.5 mm, mucro 1.5-2 mm, ratio of mucro/sepal length = ca. 0.3, outside with 0.1-0.6 mm antrorse to patent nonglandular trichomes and 0.2-0.5 mm patent glandular trichomes, inside glabrous. Petals white or pale purple, 5.5-6 mm, erect to patent, inside glabrous, margin basally ciliate, apex rounded or slightly notched. Staminal filaments whitish, lanceolate to lanceolate with an abruptly narrowed apex, abaxially sparsely pilose and basal half ciliate, trichomes 0.1-0.3 mm; anthers yellowish, 0.4-0.6 mm. Nectaries 5, hemispheric, glabrous. Stigma pinkish. Fruit 1.9-2.5 cm, erect when immature; mericarps smooth, with a basal callus, with 0.5-1.8 mm patent to antrorse nonglandular trichomes and sometimes 0.4-1 mm patent glandular trichomes; rostrum 1.5-1.9 cm, with a 1-2 mm narrowed apex; stigmatic remains 0.7-1.4 mm. Seeds 2-2.2 mm. Fl. Apr-Jul, fr. May-Sep. 2n = 46-48, 52.

This species has distinctive mericarps that are blackish when ripe and that have long patent trichomes. Presence of glandular trichomes is rather variable along plant. The whole plant is used in Chinese medicine for relieving rheumatic pains and colds.

This species is also a naturalized weed in South America, N Europe, and Japan.

Plains, weedy areas; sea level to 800 m. Naturalized in Anhui, Chongqing, Fujian, Guangxi, Hubei, Hunan, S Jiangsu, Jiangxi, Sichuan (Yanyuan), Taiwan, Yunnan, Zhejiang [native to North America].