1. Ageratum conyzoides L. ssp. conyzoides (ageratum)

Pl. 264 g, h; Map 1105

Plants annual (occasionally perennial elsewhere), fibrous-rooted. Stems solitary or few, 10–50(–150) cm long, erect or ascending, sometimes from a spreading base, sparsely to densely hairy, sometimes also somewhat glandular. Leaves mostly opposite (the nodes well separated), the uppermost leaves occasionally alternate, short- to long-petiolate. Leaf blades 1–9 cm long, triangular-ovate to ovate, the uppermost sometimes only lanceolate, mostly broadly angled or broadly rounded at the base, rarely truncate, tapered to a bluntly or sharply pointed tip, the margins broadly but often shallowly scalloped or toothed, the upper surface glabrous or sparsely to moderately hairy along the veins, the undersurface moderately to densely hairy, especially along the veins, both surfaces often also somewhat glandular, with mostly 3 main veins. Inflorescences panicles, sometimes reduced to stalked clusters, usually more or less flat-topped. Heads with numerous (more than 25) disc florets. Involucre 3–5 mm long, the bracts 18–30 (the head often also subtended by 1 or a few other narrower bracts), in usually 2 more or less equal, overlapping series, narrowly oblong-elliptic or narrowly oblong-lanceolate, tapered to a sharply pointed tip, noticeably few-nerved or few-ribbed, glabrous or more commonly sparsely short-hairy. Receptacle conical. Corollas purple or lavender-blue (rarely white elsewhere). Pappus of 5(6) more or less lanceolate scales. Fruits 1.3–2.0 mm long, 5-angled, somewhat wedge-shaped in profile (usually slightly and unevenly tapered at the base), glabrous, the angles finely roughened or toothed, dark brown to black. 2n=20, 40. August–October.

Introduced, known thus far only from a single collection from Marion County (native of Mexico, Central America, South America; introduced widely in tropical and warm-temperate regions of the world). Banks of streams.

Ageratums are cultivated commonly as bedding plants in gardens for their beautiful displays of bluish flowers in autumn. A number of cultivars exist. Among native populations in the Neotropics, Johnson (1971) accepted ssp. latifolium (Cav.) M.F. Johnson, which differs in its shorter pappus scales (0.2–0.9 vs. 1.5–3.0 mm).