Ageratina adenophora

Name and classification

Ageratina adenophora (Sprengel) R. King & H. Robinson
(=Eupatorium adenophorum Sprengel)
Family Asteraceae, Tribe Eupatorieae

Common names

crofton weed

Disseminule

fruit

Description

Fruit an achene, narrowly elliptic to narrowly oblanceolate, often gibbous, 1.5–2 mm long, 0.3–0.5 mm in diameter; with 5 prominent ribs, cross section 5 sided, sides +/– concave. Surface glistening, dark brown to black. Scar basal, a short, light brown, cylindrical collar. Pappus one row of 5–10 +/– flexible white plumose bristles, 2.5–4 mm long. Pappus easily deciduous, often absent. Style base straw-colored, +/– cylindrical, closed, usually flanged, surrounded by honey-brown ring. Apical collar conspicuous (more so if pappus absent), straw colored, horizontal, round, +/– flanged or rounded. Embryo straight, spatulate; endosperm absent.

Identification considerations

Similar species

Ageratina riparia (Regel) R. King & H. Robinson

Mikania scandens (L.) Willd. (non-FNW)

Mikania micrantha Kunth

Mikania cordata (Burm. f.) Robinson

Distribution

Africa: Zimbabwe; Asia: India, Philippines, Thailand; Europe: France, Greece, Portugal, Spain; Oceania: Australia, New Guinea, New Zealand, Tasmania; North America: Mexico, Trinidad and Tobago, United States.

native to Mexico

Habitat

humid subtropics, primarily in creek beds and forest clearings; coastal areas, roadsides, overgrazed pastures

General information

Ageratina adenophora is a perennial herb, up to 2 m tall. It was introduced to Hawaii and Australia as an ornamental, but has now become naturalized, invading pastures, reducing the carrying capacity of grazing lands, and restricting movement of animals and machines. Cattle find it unpalatable, and horses are poisoned by it. The plant spreads primarily by seed; its achenes are dispersed by wind and water, and in mud sticking to animals and equipment. Light is required for germination, so A. adenophora does not invade dense, well managed pastures.

achenes lacking pappus

achenes lacking pappus

achenes with pappus

achenes with pappus

achene, detail

achene, detail

A, achene; B, outline showing entire pappus; C, longitudinal section showing embryo; D, transection; drawing by Lynda E. Chandler

A, achene; B, outline showing entire pappus; C, longitudinal section showing embryo; D, transection; drawing by Lynda E. Chandler

apex of achene

apex of achene