Canadian Horseweed, Conyza canadensis
Other names: Erigeron canadensis, horseweed, marestail, Canada fleabane, butterweed, Canadian fleabane, fireweed, verguette du Canada.
Origin and Distribution
Canadian horseweed is native from North America and is found throughout the United States in agronomic crops, pastures, orchards, fallow fields, waste areas, and roadsides.
Roots are fibrous from taproot.
Seedlings and Shoots
Cotyledons are oval in shape and are 2-3 mm long, lack evident veins, and are smooth. The first leaves develop into a rosette. Young leaves are egg-shaped with toothed margins and are hairy.
Stems are erect, solid, hairy reaching 6 1/2 ft tall in height.
The mature plant has leaves that are entirely without petioles (sessile). Leaves are 4 inches long, 10 mm wide, alternate, linear, entire or more often toothed, crowded along the stem, and hairy. Leaves become progressively smaller up the stem.
Many small inconspicuous flower heads occur at the top of the central stem. Individual flowers are 5 mm in diameter with white or slightly pink ray flowers and yellow disk flowers.
Fruits and seeds
It is a prolific seed producer with approximately 700,000 seeds per pound. The fruit is a 1 mm long achene, tapered from base to the apex with many small white bristles that help in wind dispersal.
Canadian horseweed is easier to control when plants are young (less than 2 inches tall). Control efforts should be concentrated in late fall and early spring when plants are young.
Canadian horseweed increases under reduced tillage and no-till situations. Seed production and dissemination in disturbed areas can be reduced with mowing to reduce spread. Small occurrences can be controlled by pulling out the weed.
The most effective treatment for controlling Canadian horseweed is a 2,4-D ester or a combination of 2,4-D ester plus glyphosate. Herbicide treatments with residual activity to control later emerging horseweed are also recommended.