Bidens pilosa (weed)

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Bidens pilosa (click on image to enlarge it)
Source: Wikimedia Commons

Bidens pilosa (weed) Linnaeus - (common blackjack)

This is an important, herbaceous weed in tropical and subtropical regions which affects soybean, maize and various other crops. It is believed to be native to South America but is now widely distributed. Yield losses can be substantial, e.g. a density of 10 plants per m2 can cause about 50% yield losses in beans.

A single plant may produce up to 6,000 seeds. These stick to animal fur, feathers of birds or clothing and also disperse by wind and water. They germinate readily, but can also survive in the soil for several years. For control, mowing at intervals of less than 8 weeks is recommended. This should be done before seeds are produced or, alternatively, flowers and seed heads should be carefully removed and burned before mowing. Herbicides are also commonly used.

Vernacular names
• Deutsch: Behaarter Zweizahn
• English: common blackjack
cobbler's pegs
hairy beggarticks
• Español: aceitilla
• Français: herbe à aiguilles
• Português: picão-preto

The plant reaches a height of about 1 m. The stems are square in cross-section. Leaves are typically divided into 3 or 5 leaflets which are pointed and have a toothed margin. The composite flower heads are 5-15 mm wide, with 4-5 white petals. The slender black seeds (about 1 cm long) have 2 or 3 awns (bristles) bearing barbs.