Conyza canadensis

Erigeron canadensis
photo taken in Netherlands in 1984
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Asterids
Order:	Asterales
Family:	Asteraceae
Tribe:	Astereae
Genus:	Erigeron
Species:	E. canadensis
Binomial name
Erigeron canadensis (L.) Cronquist
Synonyms
Synonymy Aster canadensis (L.) E.H.L.Krause 1905 not (L.) Kuntze 1891 Caenotus canadensis (L.) Raf. Caenotus pusillus (Nutt.) Raf. Conyza canadensis (L.) Cronquist Conyzella canadensis (L.) Rupr. Erigeron myriocephalus Rech.f. & Edelb. Erigeron pusillus Nutt. Erigeron setiferus Post ex Boiss. Leptilon canadense (L.) Britton & Brown Leptilon canadense (L.) Britton & A.Br. Leptilon pusillum (Nutt.) Britton Marsea canadensis (L.) V.M.Badillo Senecio ciliatus Walter 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 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.

Erigeron 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 inches). Erigeron 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 found in Britain from northern Scotland to Cornwall growing as a weed of arable land. It is not invasive of any natural or semi-natural habitats.

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Wikipedia