Bidens pilosa is a cosmopolitan, annual herb which originates from tropical and Central America. Its hardiness, explosive reproductive potential, and ability to thrive in almost any environment have enabled it to establish throughout the world. Generally introduced unintentionally through agriculture or sometimes intentionally for ornamental purposes, B. pilosa is a major crop weed, threat to native fauna, and a physical nuisance.
Bidens pilosa is an erect, annual herb which stands from 0.3-2 m high and bears opposite, pinnately compound, broadly ovate, (3-)5-9-lobed leaves 3-20 cm long and 2.5-12 cm wide. Leaf segments ovate to lanceolate lobed or bilobed at the base with margins crenate-serrate and apices acute. Stems are reddish tinged; 4-angled, simple, or branched. Heads solitary or in lax paniculate cymes at the ends of the main stem and lateral branches, usually radiate, 5 – 12 mm broad. Heads with 2 rows of involucral bracts, outer ones 7-10, spathulate, reflexed at anthesis, 3-4 mm long, inner ones ovate lanceolate; ray flowers absent or 4-8, sterile, corolla 7-15 mm long, white to yellow or pinkish, disk flowers with 3.5 – 5 mm long, yellow corolla. Achenes are black, 4-8 ribbed, linear, 6-16 mm long, with 2-3(-5) retrorsely barbed bristles of 2-4 mm long.
Agricultural areas, natural forests, planted forests, range/grasslands, riparian zones, ruderal/disturbed, scrub/shrublands, urban areas, wetlands.
Bidens pilosa is a hardy weed capable of invading a vast range of habitats ranging from moist soil, sand, limerock, or dry, infertile soil and low to high altitudes of up to 3,600 m. It thrives in disturbed areas, high sunlight, and moderately dry soils, but is known to invade grassland, heathland, forest clearings, wetlands, plantations, streamlines, roadsides, pasture, coastal areas, and agriculture areas. B. pilosa is capable of surviving severe droughts with a required annual rainfall range is 500-3500 mm. It is tolerant to a pH range of 4-9 and high salinities of up to 100 mM NaCl. It prefers temperatures above 15°C and below 45°C but is tolerant to frosts with roots capable of withstanding and regenerating after temperatures as low as -15°C. B. pilosa is not fire tolerant but is known to quickly invade burnt areas.
Bidens pilosa is a problematic species for many reasons throughout its range. A troublesome weed to at least 30 crops in over 40 countries, B. pilosa is known to significantly reduce crop yields. One study found that dry bean, Phaseolus vulgaris, harvests were reduced by 48% in Uganda and 18-48% in Peru due to impacts by B. pilosa. It forms dense stands that can out compete, out grow, and eliminate crop and native vegetation, specifically the lower vegetative strata, over large areas. B. pilosa prevents the regeneration of these plants as well, given its allelopathic properties. Leaf and root extracts are known to significantly suppress germination and seedling growth of many plants and are believed to remain active throughout decomposition. Furthermore, B. pilosa grows three times faster than similar plant species. All of these properties render it a quite formidable competitor.
Its thick stands impede access to roads, trails, and recreational areas, are a nuisance to travellers and tourists, and inflict damage to pavements and walls. Its burrs are a nuisance to people, as well as, sheep and other fleece producing livestock. The burrs are also a troublesome seed contaminant as they are difficult to separate. Bidens pilosa is also a host and vector to harmful parasites such as Root knot nematodes (Meloidogyne sp.) and Tomato spotted wilt virus (Schlerotinia sclerotiorum).
Native range: Antigua and Barbuda, Argentina, Australia, Barbados, Belize, Bolivia, Brazil, Chile, Coasta Rica, Columbia, Cuba, Dominica, Dominican Republic, Ecuador, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Jamaica, Martinique, Mexico, Monteserrat, Nicaragua, Panama, Peru, Puerto Rico, St. Kitts, St. Lucia, St. Vincent and Grenadines, Suriname, Uruguay, Venezuela, Virgin Islands (USA)
Known introduced range: American Somoa, Austria, Azores, Belgium, Benin, Botswana, British Indian Ocean Territory (BIOT), Cambodia, Cameroon, Canada, Canarias, China, Christmas Island (Indian Ocean), Cook Islands, Congo, Cote d’ Ivoire, Cyprus, Czech Republic, England, Estonia, Federated States of Micronesia (FSM), Fiji, France, French Polynesia, Gaum, Greece, Indonesia, Israel, Italy, Japan, Kenya, Kiribati, Liberia, Madeira, Mauritius, Malawi, Malaysia, Marshall Islands, Mzoambique, New Caledonia (Nouvelle Calédonie), New Zealand, Nigeria, Niue, Norfolk Island, Northern Mariana Islands, Palau, Papua New Guinea, Philippines, Pitcairn, Portugal, Samoa, Scotland, Seychelles, Sierra Leone, Slovenia, Soloman Islands, Taiwan, Tanzania, Thailand, Tonga, Uganda, United Kingdom (UK), United States (USA), United States Minor Outlying Islands, Vanuatu, Viet Nam (Vietnam), Wallis and Futuna, Zambia, Zimbabwe
Physical: Bidens pilosa is susceptible to hand weeding. Germination may be prevented by mulches if they are thick enough.
Chemical: B. pilosa is susceptable to several types of herbicides. Residual herbicides: diuron, bromacil, atrazine, simazine, ropazine, hexazinone, oryzalin, and ametryn; translocated herbicides: 2,4-D, glyphosate, amitrole, metribuzin, and dicamba; and contact herbicides bentazone, diquat, and paraquat have all been evaluated as effective means of controlling B. pilosa when applied at standard rates. B. pilosa is thought susceptible to the majority of broad-leafed plant herbicides.