Erigeron canadensis (
synonymConyza 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.[2]
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.[3][4][5]
Distribution and habitat
Horseweed originated in
North America and
Central America and is widespread in its native range.[6] It has spread to inhabited areas of most of the temperate zone of Asia,[7] Europe,[2][8] and
Australia.[9] 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.[10]
Weed status
Horseweed is commonly considered a weed, and in
Ohio,
Oregon, and some other locations,[11] it has been declared a
noxious weed.[12][13] It was the first weed to have developed
glyphosate resistance, reported in 2001 from
Delaware.[14]
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%,[15] with one estimate claiming it can reduce yields by up to 90%.[16] Severe infestations have reduced sugar beet yields by 64%.[15]
The
Zuni people insert the crushed flowers of E. canadensis var. canadensis into the
nostrils to cause
sneezing, relieving
rhinitis.[20] Other[who?]Native Americans have used a preparation of the plant's leaves to treat sore throat and dysentery.[21] A tincture can be made from the dried flowering tops of the plants.[citation needed]
^Wu, Bingde; Zhang, Huanshi; Jiang, Kun; Zhou, Jiawei; Wang, Congyan (2019). "Erigeron canadensis affects the taxonomic and functional diversity of plant communities in two climate zones in the North of China". Ecological Research. 34 (4): 535–547.
Bibcode:
2019EcoR...34..535W.
doi:
10.1111/1440-1703.12024.
S2CID198244222.
^Kruger, Greg R.; Davis, Vince M.; Weller, Stephen C.; Johnson, William G. (2010). "Growth and Seed Production of Horseweed (Conyza canadensis) Populations after Exposure to Postemergence 2,4-D". Weed Science. 58 (4): 413–419.
doi:
10.1614/WS-D-10-00022.1.
S2CID55366555.
^Stevenson, Matilda Coxe 1915 Ethnobotany of the Zuni Indians. SI-BAE Annual Report #30 (p.55).
^Niering, William A.; Olmstead, Nancy C. (1985) [1979]. The Audubon Society Field Guide to North American Wildflowers, Eastern Region. Knopf. p. 377.
ISBN0-394-50432-1.