Conyza canadensis (formerly Erigeron canadensis L.) is an annual plant native throughout most of North America and Central America. Common names include Horseweed, Canadian Horseweed, Canadian Fleabane, Coltstail, Marestail and Butterweed.
It is an annual plant growing to 1.5 m tall, with sparsely hairy stems. The leaves are slender, 2-10 cm long and up to 1 cm broad, with a coarsely toothed margin. The flowers are produced in dense inflorescences 1 cm in diameter, with a ring of white or pale purple ray florets and a centre of yellow disc florets.
Horseweed is a common weed of fields, meadows, and gardens throughout its native range. Although little information is available, Horseweed infestations have reduced soybean yields by as much as 83%. Little information is available on methods of control.
This is much the commonest of the alien Conyza species in Britain, and is found from northern Scotland to Cornwall. It is the only one of the British Conyza species that grows as a weed of arable land: the others are casuals of waste and disturbed ground in towns and by roads and railways. It is not invasive of any natural or semi-natural habitats.