Erigeron canadensis

Erigeron canadensis is an annual, herbaceous plant growing to tall, with sparsely hairy stems. The leaves are unstalked, slender, long, and up to 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 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 , and the more hairy Erigeron bonariensis, which does not exceed . 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 or absent prior to the disturbance. Horseweed is a common pest of agricultural fields, and can tolerate a range of acidic to neutral soils (pH 4.8 to 7.2). ==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%. Farmers are advised to include 2,4-D or dicamba in a burndown application prior to planting to control horseweed. ==Ecology==

Horseweed produces a large amount of seeds that are dispersed by wind, allowing it to easily colonize disturbed soils. The seeds are capable of persisting in the seedbank after a fire, and the plant increases in frequency in response to more frequent fire regimes. ==Uses==

The Zuni people insert the crushed flowers of E. canadensis var. canadensis into the nostrils to cause sneezing, relieving rhinitis. The Seminole people used it for cold and cough medicine, the Iroquois people used it to help combat fevers, and the Navajo and Chippewa peoples used it for stomach pain. ==Gallery==

Standard form Erigeron canadensis from Antalya by D Merrick 08.jpg|General form, may be light green, inflorescence fairly tight Erigeron canadensis from Britain by D Merrick 15.jpg|Stemmy form Erigeron canadensis from Antalya by D Merrick 05.jpg|Flower head, hairless or mostly so, ray petals small but conspicuous, base of head not widely bulging Erigeron canadensis from Antalya by D Merrick 06.jpg|Flower head, angled Erigeron canadensis from Antalya by D Merrick 02.jpg|Flower head face on Erigeron canadensis from Antalya by D Merrick 07.jpg|Individual flowers in the head Erigeron canadensis from Britain by D Merrick 13.jpg|Hair profile of leaf, projecting at base usually, directed forward at tip Erigeron canadensis from Antalya by D Merrick 03.jpg|Leaf tip with forward hairs not curving inward Erigeron canadensis from Antalya by D Merrick 04.jpg|Leaf surfaces not hairy Erigeron canadensis from Antalya by D Merrick 01.jpg|Leaf bases near stem showing projecting hairs Erigeron canadensis from Antalya by D Merrick 09.jpg|May become large Other forms Erigeron canadensis from Antalya by D Merrick 107.jpg|Biennial growing in sand by a river from a surviving stem Erigeron canadensis Kurşunlu in Antalya by D Merrick 02.jpg|Biennial with ad-hoc branching Erigeron canadensis Kurşunlu in Antalya by D Merrick 01.jpg|Form with broken biennial stem top Erigeron canadensis from Antalya by D Merrick 105.jpg|Unusual spike-like form growing in winter Conyza-canadensis-seeds.jpg|Seeds Canadese fijnstraal plant Conyza canadensis.jpg| ==References==