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 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.
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.
Distribution and habitat
Horseweed originated in North America and Central America and is widespread in its native range. It 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 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. It is an indicator for disturbance because it often colonizes disturbed areas in which it was sparse...
