Conyza canadensis (sometimes called Erigeron canadensis L.) is an annual plant native throughout most of North America and Central America. It is also widely naturalized in Eurasia and Australia. Common names include horseweed, Canadian horseweed, Canadian fleabane, coltstail, marestail and butterweed. It was the first weed to have developed glyphosate resistance, reported in 2001 from Delaware.
Conyza canadensis is an annual plant growing to 1.5 m (60 inches) tall, with sparsely hairy stems. The leaves are unstalked, slender, 2-10 cm long and up to 1 cm (0.4 inches) across, with a coarsely toothed margin. They grow in an alternate spiral up the stem and the lower ones wither early. The flowers are produced in dense inflorescences 1 cm in diameter. Each individual flower has a ring of white or pale purple ray florets and a centre of yellow disc florets. The fruit is a cypsela tipped with dirty white down.
Conyza canadensis can easily be confused with C. sumatrensis, which may grow to a height of 2 m, and the more hairy C. bonariensis which does not exceed 1 m (40 inches). Conyza canadensis is distinguished by bracts that have a brownish inner surface and no red dot at the tip, and are free (or nearly free) of the hairs found on the bracts of the other species.
Horseweed originated in North America and is very widespread there, but has spread to inhabited areas of most of the temperate zone of Asia, Europe, and Australia. It is much the most common 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.
Horseweed is commonly considered a weed, and in Ohio it has been declared a noxious weed. It can be found in fields, meadows, and gardens throughout its native range. Horseweed infestations have reduced soybean yields by as much as 83%. It is an especially problematic weed in no-till agriculture, as it is often resistant to glyphosate and other herbicides. Farmers are advised to include 2,4-D or dicamba in a burndown application prior to planting to control horseweed.
The Zuni people insert the crushed flowers of C. canadensis var. canadensis into the nostrils to cause sneezing, relieving rhinitis. A tincture can be made from the dried flowering tops of the plants.
Horseweed is a preferable material for use in the hand drill method of making friction fire.