Ageratina thyrsiflora

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

Phytologia 19: 227. 1970.

Common names: Congested snakeroot
Basionym: Kyrstenia thyrsiflora Greene Leafl. Bot. Observ. Crit. 1: 9. 1903
Treatment appears in FNA Volume 21. Treatment on page 553. Mentioned on page 548.

Perennials or subshrubs, 50–100 cm. Stems erect (usually forming compact ‘bushlets’), finely and evenly puberulent (hairs minute, bent). Leaves usually alternate, sometimes subopposite (densely overlapping internodes); petioles 3–20 mm; blades ovate-lanceolate to triangular, 2.5–6.5 × 1–2.5(–3) cm, bases obtuse to cuneate, margins shallowly and coarsely crenate to serrate to subentire, apices rounded-obtuse, abaxial faces hirtellous. Heads clustered (in dense, terminal aggregates). Peduncles 3–8 mm, puberulent. Involucres 2.5–3 mm. Phyllaries: apices acute to obtuse (dark orange-veined), abaxial faces. Corollas white (orange-veined), glabrous. Cypselae hispid.

Phenology: Flowering Sep–Nov.
Habitat: Rocky sites, oak woodland
Elevation: 1000–2200 m



Ariz., Mexico (Chihuahua, Durango, Jalisco, Sinaloa, Sonora).


Ageratina thyrsiflora is known from the United States from a single collection dated 1929 from “near Nogales” in Santa Cruz County. It is recognized by its strict, unbranched or few-branched stems with alternate, densely arranged leaves, relatively small heads densely clustered in terminal aggregates, orange-veined phyllaries and corollas, and closely puberulent stems and petioles (hairs minute, sharply upwardly bent).

Selected References


Lower Taxa