Bidens pilosa

Bidens pilosa is a species of flowering plant in the aster family, Asteraceae. It is native to the Americas but it is known widely as an introduced species of other regions, including Asia and the Pacific Islands. Its many common names include black-jack, beggar-ticks, cobbler's pegs, and Spanish needle.
This plant is considered a weed in some tropical habitats. However, in some parts of the world it is a source of food or medicine. For example, in sub-Saharan Africa, the tender shoots and young leaves are used fresh or dried as a leaf vegetable, particularly in times of scarcity.

Bidens pilosa is an annual forb of gracile habit, growing up to 1.8 meters tall. It grows aggressively on disturbed land and often becomes weedy. The leaves are oppositely arranged and pinnate in form with three to five dentate, ovate-to-lanceolate leaflets. The petioles are slightly winged.

The plant may flower at any time of the year, but in temperate regions it blooms mainly in summer and autumn. Flowers are borne in small heads on relatively long peduncles. The heads bear about four or five broad white ray florets, surrounding many tubular yellow disc florets.

The fruits are slightly curved, stiff, rough black rods, tetragonal in cross section, about 1 cm long, with typically two to three stiff, heavily barbed awns at their distal ends. The infructescences form stellate spherical burrs about one to two centimeters in diameter. The barbed awns catch onto animals or clothing, and can injure flesh. It is an effective means of seed dispersal by zoochory, as the seeds are transported by animals. This mechanism has helped the plant become a cosmopolitan weed in temperate and tropical regions.