Erigeron canadensis (weed)

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Erigeron canadensis flower head (click on image to enlarge it)
Author(s): Wohlert Wohlers
Source: Open Media

Erigeron canadensis (weed) L. - (horseweed)

This weed is a winter or summer annual and believed to be native to North America. It now has a world-wide distribution and is common in temperate regions, where it grows in orchards, vineyards, roadsides, and fields. It can form dense stands and various field crops can be affected (e.g. soybean, maize, wheat), especially when tillage has been reduced. Yield reductions of more than 25% have been reported for several crops during infestations. One plant can produce 100,000 and more seeds which disperse by wind. Herbicides are commonly used for management, but the weed has developed resistance to various herbicides. Tillage is also effective as the seeds do not survive in the soil.

Vernacular names
• Deutsch: Kanadisches Berufkraut
Katzenschweif
• English: horseweed
mare's-tail
Canada fleabane
Canadian horseweed
• Español: erígero del Canadá
hierba carnicera
• Français: érigéron du Canada
vergerette du Canada

E. canadensis grows to a height of up to 1 m and has a deep tap root, almost equally long. The stem is unbranched and the leaves are long (up to 8 cm) and narrow with a ciliate-serrate margin. During flowering time, it develops narrow branches near the top with a small white flower head at the end of each branch. A plant has 20-40 flower heads, each only about ½ cm wide The fruits are elongated, 1-1½ mm long, with a whitish pappus, at least twice as long as the seed.

Synonyms:
Conyza canadensis

For a review see Weaver, 2001