Ageratina thyrsiflora
Ageratina rothrockii
congested snakeroot
Rothrock's snakeroot
erect (usually forming compact ‘bushlets’), finely and evenly puberulent (hairs minute, bent).
ascending to erect, puberulent to glabrate.
usually alternate, sometimes subopposite (densely overlapping internodes);
petioles 3–20 mm;
blades ovate-lanceolate to triangular, 2.5–6.5 × 1–2.5(–3) cm, bases obtuse to cuneate, margins shallowly and coarsely crenate to serrate to subentire, apices rounded-obtuse, abaxial faces hirtellous.
opposite;
petioles 2–20 mm;
blades (3–5-nerved) lanceolate to lanceolate-ovate, mostly 3–6 × (1.5–)2–3 cm, bases obtuse to truncate, margins serrate to crenate, apices acute to acuminate, sparsely puberulent abaxially, mostly along nerves.
3–8 mm, puberulent.
5–12(–20) mm, puberulent.
2.5–3 mm.
5–7 mm.
white (orange-veined), glabrous.
white, lobes short-villous.
apices acute to obtuse (dark orange-veined), abaxial faces.
apices acute, abaxial faces glabrous or glabrescent, eglandular.
clustered (in dense, terminal aggregates).
clustered.
hispid.
sparsely and finely strigose-hirsute.
on label].
= 85 [ca. 100, fide A. M.
Ageratina thyrsiflora
Ageratina rothrockii
Ageratina thyrsiflora is known from the United States from a single collection dated 1929 from “near Nogales” in Santa Cruz County. It is recognized by its strict, unbranched or few-branched stems with alternate, densely arranged leaves, relatively small heads densely clustered in terminal aggregates, orange-veined phyllaries and corollas, and closely puberulent stems and petioles (hairs minute, sharply upwardly bent).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Ageratina rothrockii is similar to A. altissima, probably its western vicariant, and the two perhaps would be justifiably treated as conspecific.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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