Erigeron canadensis

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Erigeron canadensis
Photo taken in Netherlands in 2004
Scientific classification
E. canadensis
Binomial name
Erigeron canadensis
Canadian fleabane (Erigeron canadensis) essential oil in a clear glass vial

Erigeron canadensis (synonym Conyza canadensis) is an annual plant native throughout most of North America and Central America. It is also widely naturalized in Eurasia and Australia.[2] 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.[3]


Erigeron canadensis is an annual plant growing to 1.5 m (60 in) tall, with sparsely hairy stems. The leaves are unstalked, slender, 2–10 centimetres (0.79–3.94 in) 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.[4]

E. canadensis can easily be confused with Conyza sumatrensis, which may grow to a height of 2 m, and the more hairy Erigeron bonariensis, which does not exceed 1 m (40 in). E. 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.[5][6][7]

Distribution and habitat[edit]

Horseweed originated in North America and is very widespread there,[2] but has spread to inhabited areas of most of the temperate zone of Asia,[8] Europe,[4][9] and Australia.[10] It is found in Britain from northern Scotland to Cornwall, growing as a weed of arable land and the built environment. It is not invasive of any natural or seminatural habitats.[citation needed]

Weed status[edit]

Horseweed is commonly considered a weed, and in Ohio, it has been declared a noxious weed.[11] It can be found in fields, meadows, and gardens throughout its native range. Horseweed infestations have reduced soybean yields by as much as 83%.[citation needed] It is an especially problematic weed in no-till agriculture, as it is often resistant to glyphosate[3] and other herbicides.[12] Farmers are advised to include 2,4-D or dicamba in a burndown application prior to planting to control horseweed.[citation needed]


The Zuni people insert the crushed flowers of E. canadensis var. canadensis into the nostrils to cause sneezing, relieving rhinitis.[13] Other Native Americans used a preparation of the plant's leaves to treat sore throat and dysentary.[14] 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.[4]


  1. ^ a b The Plant List, Erigeron canadensis L.
  2. ^ a b Biota of North America Program, 2014 county distribution map, Erigeron canadensis
  3. ^ a b Van Gessel, MJ (2001). "Confirming glyphosate-resistant horseweed (Conyza canadensis) in Delaware". Weed Sci. 49: 703–712. doi:10.1614/0043-1745.
  4. ^ a b c "Canadian Fleabane: Conyza canadensis". NatureGate. Retrieved 2013-12-31.
  5. ^ Conyza sumatrensis, International Environmental Weed Foundation
  6. ^ Green, Deane. "Horseweed, Marestail". Retrieved 2014-08-09.
  7. ^ Flora of North America, Conyza canadensis (Linnaeus) Cronquist, Bull. Torrey Bot. Club. 70: 632. 1943. Vergerette du Canada
  8. ^ Flora of China, Erigeron canadensis Linnaeus, Sp. Pl. 2: 863. 1753. 小蓬草 xiao peng cao
  9. ^ Altervista Flora Italiana, Cespica canadese, avoadinha, Berufkraut, Erigeron canadensis L. includes photos and European distribution map
  10. ^ Atlas of Living Australia, Conyza canadensis (L.) Cronquist Canadian Fleabane
  11. ^ "Conyza canadensis". Germplasm Resources Information Network (GRIN). Agricultural Research Service (ARS), United States Department of Agriculture (USDA). Retrieved 2017-12-18.
  12. ^ Kruger, Greg R.; Davis, Vince M.; Weller, Stephen C.; Johnson, William G. (2010). "Growth and Seed Production of Horseweed (Conyza canadensis) Populations after Exposure to Postemergence 2,4-D". Weed Science. 58 (4): 413–419. doi:10.1614/WS-D-10-00022.1.
  13. ^ Stevenson, Matilda Coxe 1915 Ethnobotany of the Zuni Indians. SI-BAE Annual Report #30 (p.55).
  14. ^ Niering, William A.; Olmstead, Nancy C. (1985) [1979]. The Audubon Society Field Guide to North American Wildflowers, Eastern Region. Knopf. p. 377. ISBN 0-394-50432-1.

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