Greater beggar's ticks

Bidens subalternans, the greater beggar's ticks, is an annual herb in the Asteraceae family native to tropical and subtropical South America, particularly Argentina, Brazil, Bolivia, Paraguay and Uruguay, and now widely naturalised as a weed in southern Europe, parts of Africa, southern North America, Asia and Australia. It grows in disturbed open ground, fallow fields, soybean and maize crops, roadsides and waste places. The plant produces an erect, branched, glabrous to sparsely hairy, quadrangular stem 30 to 150 centimetres tall, often reddish at the nodes. The opposite leaves are petiolate, deeply pinnately compound or twice-divided into three to seven narrowly lanceolate segments with serrate margins, each segment 2 to 8 centimetres long; upper leaves are smaller and less divided. The flower heads are borne in lax terminal corymbs on slender peduncles; each head has an involucre of unequal phyllaries and consists of yellow tubular disc florets, occasionally with a few short, inconspicuous yellow rays. The fruit is a narrow, linear to slightly flattened, four-angled black achene 8 to 16 millimetres long bearing three to four divergent barbed awns at the apex that catch tenaciously in fur and clothing. The species is recognised as an agricultural weed.