Ageratina riparia (Regel) R. King & H. Robinson
(=Eupatorium riparium Regel)
Family Asteraceae, Tribe Eupatorieae
Fruit an achene, narrowly elliptic to narrowly oblanceolate, straight to slightly curved, sometimes gibbous, 1.5–3 mm long, 0.3–0.7 mm in diameter; cross-section 5-angled due to 5 longitudinal ribs; surface dark brown to black, dull, shallowly wrinkled between ribs, sparsely covered (especially along the ribs) with short hairs angled towards the fruit's apex. Scar basal, a short white cylindrical collar. Pappus a ring of white plumose bristles, 2–4 mm long, deciduous, sometimes absent. Style base white, flanged. Apical collar conspicuous (more so if pappus absent), white, round. Embryo straight, spatulate; endosperm absent.
The sparse pubescence along the ribs of the fruit differentiates this species from the similar species listed below.
Ageratina adenophora (Sprengel) R. King & H. Robinson
Mikania scandens (L.) Willd. (non-FNW)
Mikania micrantha Kunth
Mikania cordata (Burm. f.) Robinson
native to Mexico
a serious weed in parts of southern Africa, tropical Asia, Papua New Guinea, some Pacific islands, Australia, New Zealand, Jamaica, and Hawaii, United States
Misty, upland meadows and mountainous areas with high rainfall. Can invade a variety of natural forest habitats, especially along riverbanks, as well as disturbed areas such as roadsides, pastures and plantations. It is shade tolerant but frost sensitive.
Ageratina riparia is a perennial herb to subshrub measuring up to 1 m tall with numerous ascending and spreading stems. It can propagate by seed or by stem cutting. The fruits are dispersed naturally by wind, water and mud sticking to animals. They also can be spread by machinery, including vehicles, and as contaminants in agricultural produce or road construction fill (e.g., sand, gravel). This species is a prolific seed producer and quick grower, with roots readily forming at the nodes when decumbent stems contact soil.