Annuals, perennials, or sub-shrubs, 20-150 cm (fibrous-rooted). Stems erect, sparsely to densely villous. Leaf blades ovate to elliptic-oblong, 2-8 × 1-5 cm, margins toothed, abaxial faces sparsely pilose and gland-dotted. Peduncles minutely puberulent and sparsely to densely pilose, eglandular. Involucres 3-3.5 × 4-5 mm. Phyllaries oblong-lanceolate (0.8-1.2 mm wide), glabrous or sparsely pilose (margins often ciliate), eglandular, tips abruptly tapering, subulate, 0.5-1 mm. Corollas usually blue to lavender, sometimes white. Cypselae sparsely strigoso-hispidulous; pappi usually of scales 0.5-1.5(-3) mm, sometimes with tapering setae, rarely 0. 2n = 20, 40.

Flowering Jul-Aug. Disturbed sites, mostly coastal; 0-20 m; introduced; Ala., Calif., Conn., Fla., Ga., Ky., Md., Miss., Mo., N.C.; South America; introduced, Mexico; West Indies; Central America; Pacific Islands (Hawaii).

Ageratum conyzoides is apparently native to South America. North American plants were escapes and naturalized from cultivation.