Common Blackjack

Bidens pilosa

Scientific name

Bidens pilosa is an annual forb of gracile (thin) 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 (leaves opposite each other along the stem) in form with three to five dentate (tooth-like or serrated)), ovate-to-lanceolate leaflets. The petioles (stalks attached to leaves) 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 (a stem that attaches a single flower to the inflorescence. In the absence of a pedicel, the flowers are described as sessile. Pedicel is also applied to the stem of the infructescence). 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 (arranged in a radiating pattern like that of a star) spherical burrs about one to two centimeters in diameter. The barbed awns catch onto fur 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 noxious weed in temperate and tropical regions.

Almost all parts of the plant are used medicinally for the treatment of a wide range of issues from diabetes to hepatitis and worms. It is used as an indigenous leafy vegetable in southern Africa, as a beverage base and a pesticide. See references below for more.

One isolated plant can produce over 30,000 seeds, which are generally highly viable and seeds can remain viable for years when buried below the soil surface, and once established it competes with other plants for resources.

Propagation instructions

By seed

Persistent mowing, hoeing and hand-pulling is effective in preventing seed from further spread. Covering the soil with clear plastic sheeting for 15 - 30 days decreases the amount of weeds that come up. Flooding for more than a day is also effective where water is abundant. Herbicides help but It shows regional specific resistance to certain herbicides.

References and further reading

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