Erigeron canadensis
Conservation status
Secure  (NatureServe)
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Asterids
Order:	Asterales
Family:	Asteraceae
Genus:	Erigeron
Species:	E. canadensis
Binomial name
Erigeron canadensis L.[1]
Synonyms[1]
Alphabetical list Aster canadensis (L.) E.H.L.Krause Caenotus canadensis (L.) Raf. Caenotus pusillus Raf. Conyza canadensis (L.) Cronquist Conyza parva Cronquist Conyzella canadensis (L.) Rupr. Erigeron myriocephalus Rech.f. & Edelb. Erigeron paniculatus Lam. Erigeron pusillus Nutt. Erigeron ruderalis Salisb. Erigeron strictus DC. Inula canadensis Bernh. Leptilon canadense (L.) Britton Leptilon pusillum (Nutt.) Britton Marsea canadensis (L.) V.M.Badillo Senecio ciliatus Walter Tessenia canadensis (L.) Bubani Trimorpha canadensis (L.) Lindm.

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, herbaceous plant growing to 1.5 metres (5 feet) tall, with sparsely hairy stems. The leaves are unstalked, slender, 2–10 centimetres (0.8–3.9 inches) long, and up to 1 cm (0.4 in) 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 1 cm (0.4 in) 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.^[2]

Erigeron canadensis can easily be confused with Erigeron sumatrensis, which may grow to a height of 2 m (6.6 ft), and the more hairy Erigeron bonariensis, which does not exceed 1 m (3.3 ft). 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.^[3][4][5]

Horseweed originated in North America and Central America and is widespread in its native range.^[6] It has spread to inhabited areas of most of the temperate zone of Asia,^[7] Europe,^[2][8] and Australia.^[9] 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.^[10]

Horseweed is commonly considered a weed, and in Ohio, Oregon, and some other locations,^[11] it has been declared a noxious weed.^[12][13] It was the first weed to have developed glyphosate resistance, reported in 2001 from Delaware.^[14]

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%,^[15] with one estimate claiming it can reduce yields by up to 90%.^[16] Severe infestations have reduced sugar beet yields by 64%.^[15]

It is an especially problematic weed in no-till agriculture, as it is often resistant to glyphosate^[14] and other herbicides.^[17] Farmers are advised to include 2,4-D or dicamba in a burndown application prior to planting to control horseweed.^[18][19]

The Zuni people insert the crushed flowers of E. canadensis var. canadensis into the nostrils to cause sneezing, relieving rhinitis.^[20] Other^[who?] Native Americans have used a preparation of the plant's leaves to treat sore throat and dysentery.^[21] A tincture can be made from the dried flowering tops of the plants.^{[citation needed]}

Horseweed is a preferable material for use in the hand drill-method of making friction fire.^{[2][failed verification]}

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  General form, may be light green, inflorescence fairly tight
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  Stemmy form
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  Flower head, hairless or mostly so, ray petals small but conspicuous, base of head not widely bulging
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  Flower head, angled
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  Flower head face on
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  Individual flowers in the head
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  Hair profile of leaf, projecting at base usually, directed forward at tip
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  Leaf tip with forward hairs not curving inward
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  Leaf surfaces not hairy
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  Leaf bases near stem showing projecting hairs
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  May become large

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  Biennial growing in sand by a river from a surviving stem
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  Biennial with ad-hoc branching
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  Form with broken biennial stem top
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  Unusual spike-like form growing in winter
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  seeds
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Media related to Erigeron canadensis at Wikimedia Commons