Bidens pilosa (blackjack).
Bidens pilosa is a widespread weed of tropical, subtropical and warm temperate regions of the world. This species has high reproductive potential and fast-growing rates, which are traits enabling it to rapidly spread and colonize new areas. A single plant may produce up to 6000 seeds per year and its seeds can easily be dispersed attached to animals, birds, human clothes or by wind and water. Seeds may remain viable for 5-6 years. This species is also adapted to grow in a wide range of habitats and soil types. It benefits from disturbances and quickly invades after fire and soil tillage. It has the potential to grow rapidly forming dense stands that outcompete and eliminate crops and native vegetation. The leaf and the root contain allelopathic substances that suppress the germination and establishment of seedlings of native plant species. The dense thickets can also affect roads, rails and recreation areas and are nuisance to travellers and tourists. Its burs irritate people and livestock and the roots, leaves, and flowers are strongly phytotoxic and poisonous. Currently it is listed as an agricultural and environmental weed in more than 40 countries.