Sp. Pl. 2: 839. 1753.
Gen. Pl. ed. 5, 363. 1754.
Annuals and perennials, mostly 20–120 cm. Stems often decumbent (rooting at proximal nodes), sparsely to densely branched. Leaves cauline; all or mostly opposite; petiolate; blades mostly 1-nerved, deltate to ovate, or elliptic to lanceolate, margins entire or toothed, faces glabrous or ± pilose, puberulent, or strigoso-hispid, sometimes gland-dotted. Heads discoid, in dense to open, cymiform to corymbiform arrays. Involucres campanulate, 3–6 mm. Phyllaries persistent, 30–40 in 2–3 series, usually 2-nerved, lanceolate, ± equal (often indurate, margins scarious). Receptacles conic, epaleate [paleate]. Florets 20–125; corollas white or bluish to lavender, throats ± campanulate (lengths 2 times diams.); styles: bases not enlarged, glabrous, branches ± linear to clavate (usually papillose and dilated distally). Cypselae prismatic, 4–5-ribbed, glabrous or sparsely strigoso-hispidulous; pappi persistent, of 5–6 aristate scales, or coroniform, or 0. x = 10.
United States, Mexico, Central America, 2 species widespread as adventives.
Species ca. 40 (4 in the flora).
|1||Plants colonial; stems and leaves glabrous or glabrate||Ageratum maritimum|
|1||Plants not colonial; stems and leaves hairy||> 2|
|2||Stems puberulent to minutely strigoso-hispid; cypselae glabrous||Ageratum corymbosum|
|2||Stems sparsely to densely pilose (usually in combination with other forms of vestiture); cypselae sparsely strigoso-hispidulous.||> 3|
|3||Peduncles minutely puberulent and sparsely to densely pilose, eglandular; phyllaries oblong-lanceolate, abruptly tapering to subulate tips 0.5–1 mm, glabrous or sparsely pilose, margins often ciliate, abaxial faces eglandular||Ageratum conyzoides|
|3||Peduncles mixed pilose, stipitate-glandular, and viscid-puberulent; phyllaries narrowly lanceolate, gradually tapering to indurate-subulate tips 0.8–2 mm, margins not ciliate or inconspicuously ciliate, abaxial faces stipitate-glandular and sparsely to densely pilose||Ageratum houstonianum|