Ageratina occidentalis

(Hooker) R. M. King & H. Robinson

Phytologia 19: 224. 1970

Common names: Western snakeroot
Basionyms: Eupatorium occidentale Hooker
Treatment appears in FNA Volume 21. Treatment on page 552. Mentioned on page 547, 548.
Click plate for higher resolution version.
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.

Phenology: Flowering (Jun–)Jul–Sep.
Habitat: Crevices, outcrops, rocky slopes, ridges, talus, gravelly and sandy stream bars, mixed hardwood-conifer woodlands, aspen, open and brushy vegetation
Elevation: (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.



Lower Taxa

No lower taxa listed.