Erigeron canadensis L.

Common Horseweed; Mare's Tail; Canadian Horseweed

Herb

Annual

Vascular

Common Horseweed is a native herbaceous annual in the Sunflower family (Asteraceae). It can be found throughout Alabama, and is now a common weed world-wide. Common Horseweed occurs on disturbed sites such as roadsides, along paths, in clear cuts, in vacant lots, and in gardens and fields. It is an annual with a taproot. The stems are erect, green in color, 1-6 feet in height, usually pubescent, and branched towards the top. The leaves are petiolate or sessile, oblanceolate to linear in outline, with toothed or entire margins. The leaves are glabrate on the surface of the blade and ciliolate along the margins. Flowers are produced in heads. The heads are arranged in panicles or corymbs. Each head has 20-40 glabrous greenish to straw colored lanceolate to linear phyllaries. There are 25-45 small white pistillate ray florets and 12-25 perfect yellowish disc florets. The fruit is a tan, sparsely pubescent achene with white pappus. This species has sometimes been treated as a member of the genus Conyza. However, Conyza as traditionally circumscribed is nested within Erigeron and para- and/or polyphyletic. Common Horseweed is resistant to glyphosate, the common ingredient in many herbicides. It is a serious weed in no-till agriculture. The leaves of Common Horseweed contain herpene which is an irritant to the nostrils and can cause skin irritations in some people. It is also an alternate host to several plant pathogens such as aster yellows, tobacco mosaic virus, and tobacco ring spot virus.—A. Diamond.

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Native FACU (NWPL)

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