Description from Flora of China
Ageratum mexicanum Sims.
Herbs, annual, 30-70(-100) cm tall. Stems erect, simple or branched from middle or lower part; stems and branches purple-red, green, or straw-colored, white tomentose or thinly lanate. Leaves broadly ovate or triangular-ovate; petiole 0.7-3 cm; median stem leaves 2-6 × 1.5-3.5 cm, or length equal to width; upper and axillary leaves smaller; all leaves basally 3-veined or inconspicuously 5-veined, both surfaces sparsely or densely white pubescent, base cordate or truncate, margin crenate-serrate, apex rounded or acute; petioles of upper leaves and axillary branches usually spreading white long tomentose. Synflorescence corymbose, 2-4 cm in diam.; peduncle densely pubescent or powdery pubescent. Capitula 5-15 or more; involucre campanulate, 6-7 mm in diam.; phyllaries 2- or 3-seriate, narrowly lanceolate, 4-5 mm, abaxially glandular pubescent, margin entire, apex long acuminate; corollas tubular, 2.5-3.5 mm; limb purplish; lobes pubescent. Achenes black, 5-angled, 1.5-1.7 mm; pappus of 5 short free scales; scales oblong-lanceolate, 2-3 mm, apex aristate-acuminate, sometimes truncate and 0.1-1.5 mm. Fl. and fr. year-round. 2n = 20, 40.
Ageratum houstonianum is used medicinally. People in Central America (Ecuador) use the plant as an antiphlogistic, to relieve swelling and pain in the throat.
Grasslands, roadsides, slopes in valleys; 100-1500 m. Cultivated and naturalized in Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hebei, Jiangsu, Nanhai Zhudao, Shandong, Sichuan, Taiwan, Yunnan, Zhejiang [native to tropical America; cultivated and naturalized in India, Myanmar, Nepal, Thailand; Africa].