Kingdom: Plantae
(Unranked): Angiosperms
(Unranked): Eudicots
(Unranked): Asterids
Order: Asterales
Family: Asteraceae
Genus: Ageratina
Species: A. riparia
Binomial name: Ageratina riparia
Synonyms: Eupatorium riparium 
Common names: Mistflower, Mist flower, Creeping Croftonweed.

Ageratina riparia is native to Mexico, Cuba and Jamaica. It is a low growing, sprawling perennial herb that grows up to 40x60 cm high. The serrated leaves on average reach 7.5 cm long and 2.5 cm wide and taper at each end. 
The flowers are white with dense corymbs 4 to 6 mm wide at the ends of the branches. The bracts around heads are hairy. It flowers mainly during winter and spring. The seeds are black, slender, angular, 2 mm long, with fine white hairs at the tip. They are mostly dispersed by wind, water and in mud sticking to clothing, machinery and animals etc. Existing plants spread vegetatively by producing roots where the stems touch the ground.

Ageratina riparia was introduced to New Zealand as an ornamental plant and now is considered an invasive species in several regions of New Zealand. Where it has escaped it has smothered native plant communities. It is a major weed of stream banks, causing sediment build up, flooding and instability of steep gullies and streams.
Ageratina riparia grows in a wide range of habitats, preferring warm, wet, semi-shaded areas. 
Mistflower is maybe poisonous for mammals to eat and it has been reported to have caused lung lesions in horses.

Two agents have been released to attack the mistflower. The Mistflower fungi (Entyloma ageratinae) and the mistflower gall fly (Procecidiocharis alani). These two agents have been successful in controlling the Mistflower in Hawaii. Both have established well and have stopped the plant from spreading and have reduced its abundance to much lower levels.
Mistflower was once harvested to use as a tanning agent.

1-Ageratina riparia Mistflower.JPG

Ageratina riparia Mist flower-001.JPG

Ageratina riparia starting to smother a bank of ferns.
Mist flower Ageratina riparia.JPG

Mist flower Ageratina riparia-002.JPG

Mist flower Ageratina riparia-005.JPG

The reddish stems.
Ageratina riparia Mist flower-004.JPG

Upper surface of the leafAgeratina riparia top surface.JPG

Underneath surface of a leaf
Ageratina riparia under side of a leaf..JPG

Thanks to Wikipedia for text and information: