Bidens pilosa (ASTERACEAE) Cobbler’s pegs

Weeds to Whack

A cosmopolitan weed of tended areas (such as gardens and parks) especially closer to the coast throughout the warmer regions of the world. Native to tropical America. Mostly hairless and green to purplish stems are square in cross-section. Opposite, serrated leaves. At the base of the plant, leaves tend to be simple and more or less oval in shape, higher up the plant leaves are mostly compound (pinnate) with 3-7 ovate leaflets. The uppermost leaves are smaller and simple or with three leaflets. Flower heads are 7-8 mm across with yellow central (tubular) florets arranged in branched inflorescences at the ends of the branches throughout most of the year.

Photo: Robert Whyte

Close-up of Seeds

Seeds are black, flattened, with a row of two to four barbed awns at one end. They attach to clothing and fur. Annoying, but certainly not the worst weed in a site. Mostly harmless and can actually be useful after rain when it will shoot up quickly, protecting local native plants slower to germinate. Difficult, if not impossible, to eradicate. Its weed risk is high mainly due to the amount of sunny sites it can colonise, especially recently weeded ones.
Photo: Mark Crocker