Erigeron canadensis

Erigeron canadensis is an annual plant growing to 1.5 m (60 in) tall, with sparsely hairy stems. The leaves are unstalked, slender, 2–10 centimetres (0.79–3.94 in) 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.[4]

E. 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 (40 in). E. 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.[5][6][7]

Horseweed originated in North America and is very widespread there,[2] but has spread to inhabited areas of most of the temperate zone of Asia,[8] Europe,[4][9] and Australia.[10] It is found in Britain from northern Scotland to Cornwall, growing as a weed of arable land and man-made environments. It considered invasive in China.[11]

Horseweed is commonly considered a weed, and in Ohio, it has been declared a noxious weed.[12] It can be found in fields, meadows, and gardens throughout its native range. Horseweed infestations have reduced soybean yields by as much as 83%. It is an especially problematic weed in no-till agriculture, as it is often resistant to glyphosate[3] and other herbicides.[13] Farmers are advised to include 2,4-D or dicamba in a burndown application prior to planting to control horseweed.

The Zuni people insert the crushed flowers of E. canadensis var. canadensis into the nostrils to cause sneezing, relieving rhinitis.[14] Other Native Americans used a preparation of the plant's leaves to treat sore throat and dysentery.[15] 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.[4]