Horseweed (marestail)

Encyclopedia Article

Horseweed/marestail          Conyza Canadensis  (L.) Cronq.

Family:  Asteraceae (Composite family)
Life cycle:   Winter (primarily) and summer annual
Native status: Native to North America
Habitat:  Disturbed areas, no-till fields

General description:  Produces a basal rosette of hairy, irregularly toothed leaves 2-3 inches long.  Stem reaches heights of 6 ft, terminal portion is a large panicle of inconspicuous flowers producing numerous wind-dispersed seed.  Pubescent stem covered with alternately arranged, hairy leaves 3- 4 inches long

Key ID traits:  Hairy, lanceolate or linear leaves.  Lower leaves irregularly toothed.

Miscellaneous:  Horseweed is commonly called marestail in Iowa. In typical years, about ¾ of the population germinates in late summer/fall, the remainder the following spring.  It was the first weed to develop resistance to glyphosate in glyphosate resistant crops.  Resistance spread rapidly due to wind-dispersed seed.


Seedlings form a rosette of pubescent, club-shaped leaves with irregularly toothed margins.


Dense stands of seedlings are produced due to prolific seed production.

 


Elongating stems densely crowded with lanceolate leaves.


Unbranched stems are topped with a conical inflorescence that produces small, wind-dispersed seed.


Horseweed is capable of forming dense monocultures in bare areas with no competing plants.

Category: 
Weeds
Tags: 
horseweed
marestail
weed identification
winter annuals
Authors: 

Bob Hartzler

Meaghan Anderson Field Agronomist in Central Iowa

Meaghan Anderson is a field agronomist in central Iowa and an extension field specialist at Iowa State University Extension and Outreach. Educational programming is available for farmers, agribusinesses, pesticide applicators, certified crop advisors, and other individuals interested in...