Botany
Weed of the Month
January, 2010: Mikania micrantha, mile-a-minute
An invasive vine that has recently been found for the first time in the continental United States.Family: Compositae (Asteraceae), the Daisy Family
Distinguishing Characteristics: Perennial twining vine with stems that are green, terete or indistinctly 6-angled, and glabrous or sparsely pubescent; leaves are yellow-green, opposite, with long petioles, 5-7-nerved from the base, mostly glabrous, and 2 – 5 inches long by 1 – 3 inches broad; the leaves are heart-shaped or triangular, with a heart-shaped base and a long, slender, tapering tip, and a coarsely toothed margin; flower heads are made up of only 4 disc florets and 4 involucral bracts (phyllaries), they are white, less than ¼ inch long, and are densely arranged in long-stalked, axillary panicles; achenes (“seeds”) with numerous, fine, hairlike bristles on one end.
Distribution: The species is native throughout tropical America, but it has become widely naturalized in the Old World tropics, particularly in Asia and the Pacific islands. In the United States, it is found only in Puerto Rico, where it is native. A reported find of M. micrantha in the District of Columbia is of dubious validity.
Occurrence in Florida: So far, the plant has only been found in the Redlands area of Miami-Dade County. Surveys are in progress to determine if it has spread further.
Similar Species in Florida: Two species of Mikania are native to Florida. Mikania scandens is common throughout the state, and it is confusingly similar to M. micrantha. The following field characteristics may be useful in distinguishing between them. Mikania micrantha grows in disturbed habitats, exhibits rampant growth, and has pale green or yellow-green leaves with green petioles and white flowers. Mikania scandens grows mostly in moist, natural habitats, exhibits more restrained growth, and has medium green leaves with reddish petioles and pinkish flowers. Mikania cordifolia is more common in the southern part of the state. It is easily distinguishable by its hairy leaves and stems; large flower heads, nearly ½ inch long, about twice as long as in the other two species.
Means of Dispersal: The achenes are crowned with a pappus of hairlike bristles, which act like a parachute, allowing the seeds to be dispersed widely by the wind. In addition, as the plant grows, it produces roots at the nodes, and even small, detached pieces with only a single node can take root and start a new colony.
Comments: Eleven populations of this invasive weed were found in late November 2009, by Keith Bradley of the Institute for Regional Conservation. Most of the populations are small, but a larger one, 100 feet square, has been seen as well. In most cases, the vines were growing rampantly. This plant is an important weed of newly planted plantation crops, such as tea, oil palms, coconuts, cacao, and coffee in tropical Asia, but it can be problematical in mature plantations as well. It covers the crops with a dense mat of foliage, shading them and even causing breakage. It acts the same in disturbed forests. Photographs on the Internet show growth reminiscent of the Old World climbing fern, Lygodium microphyllum. Mikania micrantha is on the Noxious Weed Lists of the USDA and several states, including Florida.
Further Information:
The world’s worst weeds, by L.G. Holm, et al. (1977). Available in the DPI library.
DPI Mikania micrantha Updates
Dr. Richard Weaver, Botanist
Florida Department of Agriculture and Consumer Services
Division of Plant Industry
1911 SW 34 Street
Gainesville, FL 32614
