Ageratina riparia, commonly known as mistflower, is a species of flowering plant in the family Asteraceae, native to Mexico.

It has also been introduced as an ornamental plant and naturalized in a variety of regions, including parts of Hawaii, South Africa, Southeast Asia, Macaronesia, Oceania, Peru, and the Indian subcontinent.

[5][6] In tropical climates, A. riparia is highly invasive and a variety of control methods have been developed to reduce its spread.

[4][9] It was given its current classification as member of the genus Ageratina by Robert Merrill King and Harold E. Robinson in a 1970 edition of Phytologia.

[5][11][12] The name Ageratina is derived from Greek meaning "un-aging", in reference to the way the plant's flowers keep their color for an extended period of time.

From Linden's specimen, mistflower was introduced to many parts of the globe as an ornamental plant during the late 19th and early 20th centuries.

It has become naturalized in many regions, including parts of South Africa, tropical Asia, Oceania, Macronesia, Madagascar, the Mascarenes, Hawaii, and Peru.

The branches of mistflower plants may intertwine with each other, creating a blanket effect which out-competes native flora and prevents indigenous species from propagating because of the lack of sunlight.

[21][23][24] In Sri Lanka, for example, it chokes out native plants in the country's mountain and cloud forests, and in pastures, roadsides, barren areas and bushlands of other places where it has been introduced with often disastrous effects.

[25] There is evidence that mistflower's decaying plant matter further inhibits the germination of native flora as it produces allelopathic compounds in decomposition that limit soil nutrients.

The species grows in either sun or shade, but prefers higher levels of light found in marginal habitats: at the edge of forests and on streambanks.

These include, the gall fly Procecidochares alani, the plume moth Hellinsia beneficus, and the smut fungus Entyloma ageratinae.

After a number of host range studies, proved that the pathogen only posed a threat to mistflower plants, the fungus was released at three sites on Oahu in 1975.

One of these sites saw the area previously covered by mistflower drop from ~75% to less than 1% within 9 months, allowing for indigenous plants to reemerge.

[23] Hellinsia beneficus, a plume moth from Mexico, was collected for the control of A. riparia because its larvae feed on the species' leaves.

The gall fly population did grow exponentially at release sites, but was comparatively slow to spread across New Zealand.

[23] The gall fly, however, has been proven to be more effective at targeting mistflower plants in dry areas, while the fungus E. ageratinae prefers moist habitats.

Research into the antifungal aspects of mistflower suggest that its components may be exploited to create a fungicide able to control Colletotruchum musae, the cause of banana anthracnose disease.