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.