2. Ageratina rothrockii (A. Gray) R. M. King & H. Robinson, Phytologia. 19: 216. 1970.
Eupatorium rothrockii A. Gray in A. Gray et al., Syn. Fl. N. Amer. 1(2): 102. 1884
Perennials, (20–)40–70(–150) cm (slender, fibrous-rooted crowns, with slender rhizomes). Stems ascending to erect, puberulent to glabrate. Leaves 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. Heads clustered. Peduncles 5–12(–20) mm, puberulent. Involucres 5–7 mm. Phyllaries: apices acute, abaxial faces glabrous or glabrescent, eglandular. Corollas white, lobes short-villous. Cypselae sparsely and finely strigose-hirsute. 2n = 85 [ca. 100, fide A. M. Powell on label].
Flowering (Jul–)Aug–Oct. Rocky slopes and ledges, in oak-juniper, pine-oak, pine, aspen, and spruce-fir woodland; 1700–2400 m; Ariz., N.Mex., Tex.; Mexico (Chihuahua, Coahuila, Durango, Sonora).
Ageratina rothrockii is similar to A. altissima, probably its western vicariant, and the two perhaps would be justifiably treated as conspecific.