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.
Description Erigeron canadensis is an annual, herbaceous plant growing to tall, with sparsely hairy stems. The leaves are unstalked, slender, long, and up to wide, with a coarsely toothed margin. They grow in an alternate spiral up the stem, and the lower ones wither early. The flower heads are produced in dense inflorescences and are 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.
Erigeron canadensis can easily be confused with Erigeron sumatrensis, which may grow to a height of , and the more hairy Erigeron bonariensis, which does not exceed . E. canadensis is distinguished by bracts that have a brownish inner surface with no red dot at the tip, and are free (or nearly free) of the hairs found on the bracts of the other species.Conyza sumatrensis, International Environmental Weed Foundation.Flora of North America, Conyza canadensis (Linnaeus) Cronquist, Bull. Torrey Bot. Club. 70: 632. 1943. Vergerette du Canada.
Distribution and habitat Horseweed originated in North America and Central America and is widespread in its native range.Biota of North America Program, 2014 county distribution map, Erigeron canadensis. It has spread to inhabited areas of most of the temperate zone of Asia,Flora of China, Erigeron canadensis Linnaeus, Sp. Pl. 2: 863. 1753. 小蓬草 xiao peng cao Europe,Altervista Flora Italiana, Cespica canadese, avoadinha, Berufkraut, Erigeron canadensis L. includes photos and European distribution map and Australia.Atlas of Living Australia, Conyza canadensis (L.) Cronquist Canadian Fleabane It is found in Britain from northern Scotland to Cornwall, growing as a weed of arable land and man-made environments. It is considered invasive in China.
Horseweed can grow in a range of natural habitats, from sand dunes to old fields, as well as in gardens and other disturbed areas.Weakley, A. S. (2015). Flora of the Southern and Mid-Atlantic States. Chapel Hill, NC, University of North Carolina Herbarium. It is an indicator for disturbance because it often colonizes disturbed areas in which it was sparse or absent prior to the disturbance. Horseweed is a common pest of agricultural fields, and can tolerate a range of acidic to neutral soils (pH 4.8 to 7.2).Tilley, D. 2012. Plant Guide for Canadian horseweed (Conyza canadensis). USDA-Natural Resources Conservation Service, Aberdeen, ID Plant Materials Center. 83210-0296.
Weed status Horseweed is commonly considered a weed, and in Ohio, Oregon, and some other locations, it has been declared a noxious weed. It was the first weed to have developed glyphosate resistance, reported in 2001 from Delaware.
It can be found in fields, meadows, and gardens throughout its native range. Horseweed infestations (specifically of 105 plants per 10 ft2) have reduced soybean yields by as much as 83%, with one estimate claiming it can reduce yields by up to 90%. Severe infestations have reduced sugar beet yields by 64%.
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.
Ecology Horseweed produces a large amount of seeds that are dispersed by wind, allowing it to easily colonize disturbed soils.
The seeds can germinate at any time of year with sufficient moisture and proper temperatures. With fall germination, it overwinters as a rosette and bolts in the spring to flower in the summer. With spring germination, it spends less time as a rosette before bolting to flower. The seeds can persist in the seedbank of disturbed and non-disturbed sites, though it is more prevalent in disturbed sites.
The seeds are capable of persisting in the seedbank after a fire,Creech, M. N., et al. (2012). "Alteration and Recovery of Slash Pile Burn Sites in the Restoration of a Fire-Maintained Ecosystem." Restoration Ecology 20(4): 505-516. and the plant increases in frequency in response to more frequent fire regimes.Burton, J. A. (2009). Fire frequency effects on vegetation of an upland old growth forest in eastern Oklahoma. Environmental Science. Stillwater, Oklahoma, Oklahoma State University. Bachelor: 78.
Erigeron canadensis is insect pollinated and is recorded to have been visited in northern Florida by Ceratina.Ulyshen, M. D., K. Robertson, S. Horn, and C. Dixon. 2024. Characterization of the bee community and pollination network in a southeastern U.S. pine savanna. Frontiers in Ecology and Evolution 12:1403602.
Uses The Zuni people insert the crushed flowers of E. canadensis var. canadensis into the nostrils to cause sneezing, relieving rhinitis.Stevenson, Matilda Coxe 1915 Ethnobotany of the Zuni Indians. SI-BAE Annual Report #30 (p.55). The Seminole people used it for cold and cough medicine, the Iroquois people used it to help combat fevers, and the Navajo and Chippewa peoples used it for stomach pain. Other medicinal applications include use as a nervine, stimulant, and antiplasmodic (anti-parasitic specific to a genus of plasmids - malaria is one) .Rafinesque, C. S. (1828). Medical flora; or Manual of the medical botany of the United States of North America.
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