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.^[2]

Erigeron bonariensis grows up to 75 cm (29.5 in) 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.^[3]

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.^{[citation needed]}

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.^[4][5][6][7]

E. bonariensis is a rare alien in southeastern England, found along walls and in cracks in pavements and concrete driveways.^{[citation needed]} 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.^[8][9]

• The following are bright dry summer 1st-year forms; in contrast to shady damp winter forms that follow.

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  General view
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  Top-heavy greyish gangly look with initial central inflorescence subsequently overtopped by some side ones
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  Broad heads (may be narrower especially in winter) usually with reddened tips
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  Head close up
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  Leaf narrow showing hairs
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  Leaf surface hairs (similar underside), sometimes poorly hairy
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  Hairs at leaf tip
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  Leaf base hairs (a few long ones usually present) and underside hairs
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  Stem leaves of robust stems multi-lengthed, grey (sometimes green)

• Showing diffusely branched or strangled appearances with small flower heads

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  Winter form near water in the open
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  Strangled winter form in the open
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  Winter form in damp shade
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  From a biennial form
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  May have particularly reddened heads

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  Erigeron bonariensis growing as a weed on a building in New Delhi, India
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  Stiff hairs cover the plant.
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  Basal rosette leaves, before plant matures and flowers, Maui, Hawaii
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  Foliage becomes grey-green in dry-summer regions, such as Israel
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  White ray flowers and yellow disc flowers in fully "open" flower heads, Maui, Hawaii
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  Flower heads are followed by seeds, which are easily carried by the wind.