horseweed

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.

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).

Horseweed produces a large amount of seeds that are dispersed by wind, allowing it to easily colonize disturbed soils. The seeds can germinate at any time of year with sufficient moisture and proper temperatures. With fall germination, it overwinters as a rosette and bolts in the spring to flower in the summer. With spring germination, it spends less time as a rosette before bolting to flower. The seeds can persist in the seedbank of disturbed and non-disturbed sites, though it is more prevalent in disturbed sites. 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. Erigeron canadensis is insect pollinated and is recorded to have been visited in northern Florida by Ceratina.

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. Other medicinal applications include use as a nervine, stimulant, and antiplasmodic (anti-parasitic specific to a genus of plasmids - malaria is one) .