Erigeron canadensis, commonly known as Horseweed or Canadian Fleabane, is a hardy annual plant native to North America and Central America. This versatile plant has also made itself at home in Eurasia and Australia. Notably, it gained notoriety as the first weed to develop glyphosate resistance, a trait identified in 2001 in Delaware.
Horseweed is characterized by its annual herbaceous nature, reaching heights of up to 1.5 meters (5 feet). Its stems bear sparse hairs, while its slender, untoothed leaves measure 2–10 centimeters (0.8–3.9 inches) in length and up to 1 centimeter (0.4 inches) in width. The leaves grow alternately along the stem, with lower leaves typically withering early. When in bloom, the plant produces dense inflorescences with small flower heads, each featuring white or pale purple ray florets and yellow disc florets. Its fruit takes the form of a cypsela topped with a covering of dirty white down.