Bidens frondosa

Bidens frondosa is an annual herb, usually growing to tall, but it may reach . The stems are square in cross-section and may branch near the top. The leaves are pinnate, divided into a few toothed triangular or lance-shaped leaflets usually long, exceptionally up to . The inflorescence is often a solitary flower head, but there may be pairs or arrays of several heads. The head contains many orange disc florets. Most flower heads lack ray florets but some may have a few small yellow rays. The fruit is a flat black or brown barbed cypsela up to a centimeter long which has two obvious hornlike pappi at one end. The barbed pappi on the fruit help it stick to animals, facilitating seed dispersal. ==Distribution and habitat==

Bidens frondosa is native to northern North America. It is recorded as naturally occurring in the whole of the contiguous United States and Alaska, but not Montana, as well as all of Canada except for the three Arctic territories and Labrador. It does not naturally occur south to Mexico, but it is recorded by CABI as introduced there. Bidens frondosa has been widely introduced outside of North America, mostly in regions with temperate climates. Most of these introductions have been intentional, because of the purported medicinal, herbal, and decorative properties of the species. In Italy, it was first introduced there in the 1700s as an ornamental plant, and was first reported as a naturalised weed in 1849. It is widespread across Europe, where it has been reported from almost every country except for Finland, Sweden, and Greece. In Asia, it has been reported from the Middle East (Iran, Lebanon, Syria, Turkey), the Caucasus, Central Asia and Western Siberia, China, Korea, Japan, and the Russian Far East. It has also been introduced to New Zealand and Morocco. ==Ecology==

The defoliating caterpillar of Hadjina chinensis, which is limited to Bidens species, has been observed on this plant. Invasiveness This plant is invasive in some parts of the world. It is also weedy in its native range, occurring in pastures and fields and along roadsides. Across Eurasia, this species is a problematic environmental and agricultural weed that threatens native species of Bidens through competition and genetic pollution. Studies comparing B. frondosa with B. tripartita and B. radiata in the Middle Urals show that B. frondosa plants have larger sizes, higher seed production, and greater biomass, indicating their high competitive ability. In Italy, where it is a rapidly spreading agricultural weed in the Po Valley, studies of the phenology of B. frondosa with B. tripartita suggest that its invasive capacity could be related to the length of its vegetative phase, allowing it to be more adaptable and permitting greater growth by delaying reproductive development. ==References==