Ageratum

Linnaeus

Sp. Pl. 2: 839. 1753

Gen. Pl. ed. 5, 363. 1754

Etymology: Greek a, not, and geras, old age, apparently alluding to long-lasting nature of flowers
Treatment appears in FNA Volume 21. Treatment on page 481. Mentioned on page 460.
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.

Distribution

United States, Mexico, Central America, 2 species widespread as adventives.

Discussion

Species ca. 40 (4 in the flora).

Key

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