Bidens pilosa (Blackjack)

Scientific name

Bidens pilosaL.


Bidens leucantha (L.) Willd.; Bidens sundaica (Blume)

Common names

Blackjack, Spanish needle, hairy beggar ticks, farmer's friend, cobbler's pegs, pitchforks, kichoma mguu (Kiswahili), labika (Acholi), muceege (Kikuyu), ononot (Lango)


Asteraceae (Compositae)


Native to tropical America

Naturalised distribution (global)

Bidens pilosa is naturalised throughout the tropics.

Introduced, naturalised or invasive in East Africa

Bidens pilosa is invasive in parts of Kenya, Tanzania and Uganda (Global Invasive Species Database).


Bidens pilosa can invade roadsides, crops, pastures, gardens, disturbed areas, fallow lands and urban open space


Bidens pilosa is an annual herb with an erect habit to 1.5 m in height (Stanley and Ross 1983-1989). It is easily recognised by the elongated fruits that bear hooked bristles (burrs) that embed themselves in people's clothing as they brush past the stems.

Stems are square in cross section (quadrangular), mostly hairless and green to purplish in colour. The leaves are oppositely arranged with leaf stalks (petioles) 1-6.5 cm long. Their margins have forward pointing teeth (serrated). Leaf blades vary in shape. At the base of the plant, leaves tend to be simple and more or less oval in shape, higher up the plant leaves are mostly compound (pinnate) with 3-7 egg-shaped with broad end at base (ovate) leaflets, and the uppermost leaves are smaller and simple or with 3 leaflets (trifoliolate). Leaf or leaflet size varies from 1.5-7 x 0.5-3.5 cm.

Flower-heads are 7-8 mm across with yellow central (tubular) florets. Some plants have flower-heads with white or cream 'petals' (ray florets) up to 1.5 cm long, however these 'petals' may be absent or quite small. Heads are arranged in branched inflorescences at the ends of the branches. Flowering occurs over most of the year. The seeds are black, flattened, linear in shape (0.4-1.6 cm long) with a row of two to four barbed awns (2-4 mm long) at one end.

Reproduction and dispersal

Bidens pilosa normally behaves as an annual weed but at least one form, B. pilosa var. radiata, may behave as a perennial. One isolated plant can produce over 30,000 seeds, which are generally highly viable. Seeds germinate on the soil surface or in shallow soil (to a depth of 1 cm). Seeds at greater depths remain viable in the soil for many years. There is usually a great flush of germination after tillage of the soil. Seeds are widely dispersed through the fruits hook-like bristles that embed themselves in clothing and the fur of mammals and feathers of birds. They are also spread by wind, water and soil.

Economic and other uses

The fresh or dried tender shoots and young leaves are eaten in some cultures, especially in times of food scarcity. It is used as a medicinal plant in many regions of Africa.

Environmental and other impacts

Bidens pilosa is a serious weed in many cropping systems in many countries where it reduces yield because of its fast growth and competitive abilities which include allelopathic properties. One study found that B. pilosa is a serious weed in many cropping systems in many countries where it reduces yield because of its fast growth and competitive abilities which include allelopathic properties. One study found that B. pilosa competition reduced dry bean, Phaseolus vulgaris harvests by 48% in Uganda. Bidens pilosa is also a host and vector to harmful parasites such as root knot nematodes (Meloidogyne species) and tomato spotted wilt virus (GISD 2010). It is also a weed of open areas where dense stands reduce access to roads, trails, and recreational areas, and can damage pavements and walls. Its burrs are a nuisance to people, as well as to sheep and goats. The burrs are also a seed contaminant.

B. pilosa has been included in the Global Invasive Species Database (GISD 2010).


The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of invasive species management is prevention. As Bidens pilosa is so widespread this is unlikely to be possible in many instances.

If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

Bidens pilosa can be controlled by persistent mowing, hoeing and hand pulling in order to prevent seed production. Thorough cultivation discourages growth. Chemical control regimes depend upon the cropping system in which one is working. Details for individual crops can be found in the CABI Invasive Species Compendium. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

Biological control agents against B. pilosa are yet to be developed.


Not listed as a noxious weed by the state or governments in Kenya, Tanzania and Uganda.


CABI invasive species compendium online data sheet. Bidens pilosa (blackjack). CABI Publishing 2011. Accessed March 2011.

GISD (2010). Global Invasive Species Database online data sheet. Bidens pilosa (herb) Accessed March 2011.

Kleinschmidt, H.E., Holland, A. and Simpson, P. (1996). Suburban Weeds. 3rd Edition. Department of primary Industries, Brisbane.

Stanley, T.E. and Ross, E.M. (1983-1989). Flora of South-eastern Queensland. Volume 2. Department of Primary Industries, Brisbane.


Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.


This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).


BioNET-EAFRINET Regional Coordinator: [email protected]