Bidens alba

Bidens alba, the romerillo, shepherd's needles or beggar-ticks, is an erect, branching annual to short-lived perennial herb in the family Asteraceae, often treated as a variety of the closely related Bidens pilosa and native to the warmer parts of the Americas, from the southeastern United States and the Caribbean through Mexico and Central America to South America, now naturalised pantropically. It typically reaches 30 to 150 centimetres in height, with slender, four-angled green stems and opposite, pinnately compound leaves divided into three to five ovate, serrate leaflets 2 to 7 centimetres long. The flower heads are about 1.5 to 2.5 centimetres across and borne in loose terminal cymes, each with five to seven, occasionally up to eight, broad, pure white, conspicuously notched ray florets surrounding a flat yellow disc of tubular florets. The fruit is a slender, black, four-angled achene 7 to 12 millimetres long, tipped with two to three barbed awns that readily attach to clothing and animal fur, accounting for its wide distribution as a roadside and field weed.