Erigeron bonariensis is a species in the family Asteraceae, found throughout the tropics and subtropics as a pioneer plant; its precise origin is unknown, but most likely it stems from Central America or South America. It has become naturalized in many other regions, including North America, Europe and Australia.

Common names of E. bonariensis include flax-leaf fleabane, wavy-leaf fleabane, Argentine fleabane, hairy horseweed, asthma weed and hairy fleabane.

 Erigeron bonariensis grows up to 75 cm in height and its leaves are covered with stiff hairs, including long hairs near the apex of the bracts. Its flower heads have white ray florets and yellow disc florets. It can easily be confused with Erigeron canadensis, which grows taller, and E. sumatrensis.

It flowers in August and continues fruiting until the first frosts. It is instantly recognisable by its blue-green foliage, very narrow, undulate stem-leaves, and purple-tipped involucral bracts. It reproduces only by seed, which are easily blown and spread by wind.

Distribution and habitat: Erigeron bonariensis is found throughout the tropics and subtropics as a pioneer plant; its precise origin is unknown, but most likely it stems from Central America or South America. It has become naturalized in many other regions, including North America, Europe and Australia.

E. bonariensis is a rare alien in southeastern England, found along walls and in cracks in pavements and concrete driveways. It is widespread throughout Australia, where it thrives on roadsides, fallows, pastures, gardens, lawns, footpaths, parks, riparian vegetation, forest and wetland perimeters, waste dumps and disturbed grounds.

Dry summer form: 

• The following are bright dry summer first-year forms; in contrast to shady damp winter forms that follow.

Winter, damp, shady, biennial forms: 

• Showing diffusely branched or strangled appearances with small flower heads