Perennials or subshrubs, 15-70 cm (caudices woody, rhizomatous). Stems (green or purple) erect or ascending, puberulent. Leaves opposite proximally, alternate on distal ¹/4-¹/2 of stems; petioles 5-12 mm; blades triangular to ovate, 2.5-5 × 1.7-4 cm, bases truncate to cuneate, margins serrate, apices acute, abaxial faces gland-dotted. Heads clustered. Peduncles 2-5 mm, minutely puberulent. Involucres 3-3.5(-4) mm. Phyllaries: apices acute, abaxial faces viscid-puberulent and/or sessile-glandular. Corollas pink, bluish, or white tinged with purple, lobes glabrous or glabrate. Cypselae sessile-glandular. 2n = 34.

Flowering (Jun-)Jul-Sep. Crevices, outcrops, rocky slopes, ridges, talus, gravelly and sandy stream bars, mixed hardwood-conifer woodlands, aspen, open and brushy vegetation; (40-50)900-2800(-3200) m; Calif., Idaho, Mont., Nev., Oreg., Utah, Wash.

Ageratina occidentalis is the only species of the genus in the flora area with sessile-glandular cypselae; peduncles also may be sessile-glandular. Its identity also can be confirmed among flora area species by its relatively long (5-6 mm) corollas.