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 m tall, with sparsely hairy stems. The leaves are unstalked, slender, 2–10 cm long, and up to 1 cm 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 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, and the more hairy Erigeron bonariensis, which does not exceed 1 m. 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.

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.

Uses

The Zuni people insert the crushed flowers of E. canadensis var. canadensis into the nostrils to cause sneezing, relieving rhinitis. Other Native Americans have used a preparation of the plant's leaves to treat sore throat and dysentery. A tincture can be made from the dried flowering tops of the plants. Horseweed is a preferable material for use in the hand drill-method of making friction fire.

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