Ageratina riparia, commonly known as mistflower, is a species of flowering plant in the family Asteraceae, native to Mexico. The species is widely adventive and has spread to Cuba, Jamaica, and other parts of the Caribbean. 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. In tropical climates, A. riparia is highly invasive and a variety of control methods have been developed to reduce its spread.
Taxonomy
The species was identified by Eduard Regel as Eupatorium riparium in the 1866 edition of Gartenflora. 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. The species has several heterotypic synonyms, including: Ageratina repens, Ageratina ventillana, Eupatorium harrisii, Eupatorium ventillanum, and Fleischmannia repens.
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. This name was used by Dioscorides for a number of different plants. The name riparia is derived from the Latin word ripa meaning "riverbank"' or "streambank". The suffix -aris denotes that it "belongs to" the bank. Thus, riparia means 'growing on the banks'.
A wide variety of common names exist for the plant. In English it has been referred to as creeping croftonweed, mistflower, river-eupatorium, spreading snakeroot, and William Taylor. In KwaZulu-Natal, it may be called mistblom. In Hawaii, the plant is commonly referred to as hāmākua pāmakani. The term pāmakani means "windblown" in the Hawaiian language, referring to the way its seeds are naturally dispersed.
Description
Mistflower is a low growing, sprawling perennial herb that may grow up to 1 meter in height in some climates. The species has a fibrous rootstock and its stems are often woody and covered in purple hairs. The stems may produce additional roots if they contact the ground.
The serrated leaves on average reach 7.5 cm long and 2.5 cm wide and taper at each end. The leaves form in opposing pairs which are coarsely serrated except near the base. The species is related to and closely resembles Ageratina adenophora. However, its distinctive serrated leaves may be used to distinguish mistflower from A. adenophora. The white flowers are also similar in appearance to A. adenphora. The flowers are white with dense heads at the ends of the branches. Mistflower buds from July until August, and may flower from August through March in some climates. Its seeds are black, slender, angular, 2 mm long, with fine white hairs at the tip. They are dispersed by wind and flowing water. Mature plants can produce around 10,000 to 100,000 seeds per year.
Distribution and invasiveness
Mistflower is native to western Mexico. In its natural habitat the flower occurs sparsely and may even be considered endangered. Specimens were originally collected from the canyon of El Mirador, Mexico in 1857 for use as an ornamental plant. It was then brought back to Germany by Jean Jules Linden. 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.
Outside of its natural habitat, the species spread quickly. It has become naturalized in many regions, including parts of South Africa, tropical Asia, Oceania, Macronesia, Madagascar, the Mascarenes, Hawaii, and Peru. Mistflower has become especially invasive in tropical and warm temperate climates. In these regions, the species can form dense underbrush. 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. 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. 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. It also adversely affects grazing pastures, as it can be toxic to horses and is often avoided by grazing stock.