AUGUST 2008

Washington State University
WHITMAN COUNTY EXTENSION

STEVE'S

Horseweed
Also Known As: mare’s tail, Canadian horseweed, Canadian fleabane, colt’s tail, butterweed

Horseweed (Conyza Canadensis (L.) Cronq.) is a winter or summer annual, native to North America. The plant grows upright, tall and narrow, and is unbranched at the base unless damaged. Horseweed can reach heights exceeding 6 feet. Leaves are alternate, dark green, crowded on the stem, hairy, and lanceolate. Leaves grow progressively smaller up the stem. A dense inflorescence is borne at the end of branched stems, with small white ray and yellowish disk florets. The leaves and flowers contain a terpene, which may cause skin and mucosal irritation in humans and animals and may inhibit grazing. Each horseweed plant can produce hundreds of thousands of seeds. The seeds are small, flattened and have a bristly pappus that aids winddispersal. Horseweed seed germinates readily as soon as falling off mature plants.

Horseweed is a strong competitor for water and grows rapidly. It is a common weed of fields and meadows, and prefers disturbed open places, such as roadsides, construction sites, pastures, empty lots and other waste areas. It has become problematic and difficult to control in glyphosate-resistant crop production systems, especially in eastern United States. Horseweed is often susceptible to common tillage practices of conventional-tillage cropping systems; conversely, it can thrive in minimum or no-tillage systems.

Control Methods

Mechanical: Mowing infested areas when the plants are in the bud stage will prevent seed production, but will not control horseweed. Tillage effectively deters the weed.

Chemical: Glyphosate has been effectively used for horseweed control, although resistance has become a problem in the eastern states. Horseweed has reported cases of resistance to ALS, triazine, and methyl viologen dichloride herbicides. In the case of glyphosate-resistant horseweed, dicamba may be used for horseweed control.

Relying strictly on post-emergence products can make horseweed very problematic, so effective soil-residual herbicides may need to be considered.

Controlling horseweed in the seedling or rosette stage is the most effective, since small plants are easily controlled and residual herbicides applied at these stages can provide control through early June. Emerged plants should be controlled by 2,4-D ester.

More information can be found in the
PNW Weed Management Handbook.

Refer to the label for all herbicides.
Always follow label directions prior to use.

Biological: No known biological control agents exist. Livestock do not graze horseweed and grazing may cause irritation.

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Questions: contact Steve Van Vleet or phone (509) 397 - 6290