Herbs, annual, 50-100 cm tall, sometimes less than 10 cm, with inconspicuous main root. Stems robust, ca. 4 cm in diam. at base, simple or branched from middle, stems and branches reddish, or green toward apex, white powdery puberulent or densely spreading long tomentose. Leaves often with axillary abortive buds; petiole 1-3 cm, densely white spreading villous; median leaves ovate, elliptic, or oblong, 3-8 × 2-5 cm; upper leaves gradually smaller, oblong, sometimes all leaves small, ca. 1 × 0.6 cm, both surfaces sparsely white puberulent and yellow gland-dotted, basally 3-veined or obscurely 5-veined, base obtuse or broadly cuneate, margin crenate-serrate, apex acute. Capitula small, 4-14, in dense terminal corymbs; peduncle 0.5-1.5 cm, powdery puberulent; involucre campanulate or hemispheric, ca. 5 mm in diam.; phyllaries 2-seriate, oblong or lanceolate-oblong, 3-4 mm, glabrous, margin lacerate; corollas 1.5-2.5 mm, glabrous or apically powdery puberulent; limb purplish, 5-lobed. Achenes black, 5-angled, 1.2-1.7 mm, sparsely white setuliferous; pappus scales 5 or awned, 1.5-3 mm. Fl. and fr. year-round. 2n = 20, 38, 40.

Ageratum conyzoides is used medicinally in China to treat a variety of conditions, including common colds, headaches, boils, eczema, bleeding wounds, and burns.

Valleys, forests, forest margins on slopes, riversides, grasslands, field margins. Cultivated and naturalized in Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hainan, Henan, Jiangsu, Jiangxi, Nanhai Zhudao, Shaanxi, Sichuan, Taiwan, Yunnan; only cultivated in Hebei and Zhejiang [native to tropical America; widespread weed throughout Africa, India, Nepal, and SE Asia].