Flora of North America species comparison

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Ageratina thyrsiflora

congested snakeroot
Habit 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.
Peduncles

3–8 mm, puberulent.
Involucres

2.5–3 mm.
Corollas

white (orange-veined), glabrous.
Phyllaries

apices acute to obtuse (dark orange-veined), abaxial faces.
Heads

clustered (in dense, terminal aggregates).
Cypselae

hispid.

Ageratina thyrsiflora
Phenology Flowering Sep–Nov.
Habitat Rocky sites, oak woodland
Elevation 1000–2200 m (3300–7200 ft)
Distribution
from FNA
AZ; Mexico (Chihuahua, Durango, Jalisco, Sinaloa, Sonora)
[BONAP county map]
Discussion

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).

(Discussion copyrighted by Flora of North America; reprinted with permission.)
Source FNA vol. 21, p. 553.
Parent taxa Asteraceae > tribe Eupatorieae > Ageratina
Sibling taxa
A. adenophora, A. altissima, A. aromatica, A. havanensis, A. herbacea, A. jucunda, A. lemmonii, A. luciae-brauniae, A. occidentalis, A. paupercula, A. rothrockii, A. shastensis, A. wrightii
Synonyms Kyrstenia thyrsiflora
Name authority (Greene) R. M. King & H. Robinson: Phytologia 19: 227. (1970)
Web links 
Unless otherwise noted, all data comes from the online version of the Flora of North America, who have made most of their content available via a Creative Commons license (CC BY 4.0).



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