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.

FNA 2006, Kearney and Peebles 1969, McDougall 1973

Duration: Perennial Nativity: Native Lifeform: Subshrub General: Perennial herbs or subshrubs, to 70 cm tall or more, herbage puberulent to glabrate, plants arising from fibrous-rooted crowns with slender rhizomes. Leaves: Leaves opposite, lanceolate to lance-ovate, to 6 cm long, with 3-5 nerves, margins serrate to crenulate, acute or acuminate at the tip, sparsely pubescent below. Flowers: Heads discoid, disks white, with white, protruding stamens, corolla lobes short-villous, involucres 5-7 mm high, phyllaries with acute tips, the abaxial faces glabrous to glabrescent, borne on peduncles 5-12 mm. Fruits: Achenes to 1 mm, sparsely and finely strigose-hirsute. Ecology: Found on rocky slopes and ledges in pine-oak, oak-juniper, aspen, and spruce-fir, woodlands from 6,500-7,500 ft (1891-2286 m); flowering August-October. Distribution: Arizona, New Mexico, Texas; Mexico. Notes: Look for this species under Eupatorium rothrockii in older texts. The keys to this species are the short-villous corolla lobes, the involucre 5-7 mm high, and the lanceolate to lance-ovate leaves. Differentiate this species from A. herbacea by the larger involucre and the strongly hairy corolla lobes. Ethnobotany: Unknown Etymology: Ageratina is a dimunitive of Ageratum, which is from Greek ageratons for not growing old, while rothrockii is named for Dr. Joseph Trimble Rothrock (1839-1922), surgeon on the Wheeler expedition of 1873-1875. Synonyms: Eupatorium rothrockii Editor: LCrumbacher 2011