|Photo taken in Netherlands in 2004|
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. 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.
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.
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.
Distribution and habitat
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 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.
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 E. canadensis var. canadensis into the nostrils to cause sneezing, relieving rhinitis. Other Native Americans used a preparation of the plant's leaves to treat sore throat and dysentary. A tincture can be made from the dried flowering tops of the plants.
- The Plant List, Erigeron canadensis L.
- Biota of North America Program, 2014 county distribution map, Erigeron canadensis
- Van Gessel, MJ (2001). "Confirming glyphosate-resistant horseweed (Conyza canadensis) in Delaware". Weed Sci. 49: 703–712. doi:10.1614/0043-1745.
- "Canadian Fleabane: Conyza canadensis". NatureGate. Retrieved 2013-12-31.
- Conyza sumatrensis, International Environmental Weed Foundation
- Green, Deane. "Horseweed, Marestail". Retrieved 2014-08-09.
- Flora of North America, Conyza canadensis (Linnaeus) Cronquist, Bull. Torrey Bot. Club. 70: 632. 1943. Vergerette du Canada
- Flora of China, Erigeron canadensis Linnaeus, Sp. Pl. 2: 863. 1753. 小蓬草 xiao peng cao
- Altervista Flora Italiana, Cespica canadese, avoadinha, Berufkraut, Erigeron canadensis L. includes photos and European distribution map
- Atlas of Living Australia, Conyza canadensis (L.) Cronquist Canadian Fleabane
- "Conyza canadensis". Germplasm Resources Information Network (GRIN). Agricultural Research Service (ARS), United States Department of Agriculture (USDA). Retrieved 2017-12-18.
- 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.
- Stevenson, Matilda Coxe 1915 Ethnobotany of the Zuni Indians. SI-BAE Annual Report #30 (p.55).
- Niering, William A.; Olmstead, Nancy C. (1985) . The Audubon Society Field Guide to North American Wildflowers, Eastern Region. Knopf. p. 377. ISBN 0-394-50432-1.
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