Horseweed

Conyza canadensis

Conyza canadensis (sometimes called Erigeron canadensis L.) 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.
Conyza 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.
Conyza canadensis can easily be confused with C. sumatrensis, which may grow to a height of 2 m, and the more hairy C. bonariensis which does not exceed 1 m (40 inches). Conyza 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.

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