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FNA | Family List | FNA Vol. 21 | Asteraceae | Ageratina

11. Ageratina occidentalis (Hooker) R. M. King & H. Robinson, Phytologia. 19: 224. 1970.

Western snakeroot

Eupatorium occidentale Hooker, Fl. Bor.-Amer. 1: 305. 1833

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.


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