Horseweed

Conyza canadensis

Alternate names
Marestail

Weed Description
An erect winter or summer annual reaching 6 1/2 ft in height. Seedlings develop a basal rosette and mature plants have leaves that are entirely without petioles. Found throughout the United States in agronomic crops, pastures, orchards, fallow fields, waste areas, and roadsides.

Seedlings
Cotyledons oval, 2-3 mm long. Young leaves egg-shaped with toothed margins and becoming hairy.

Leaves
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.

Stems
Erect, solid, hairy, reaching 6 1/2 ft in height.

Flowers
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.

Fruit
A 1 mm long achene, tapered from the base to the apex with many small white bristles that help in wind dispersal.

Roots
A short taproot with a secondary fibrous root system.

Identifying Characteristics
Erect plants with mature leaves that are entirely without petioles. When mature, this weed is easily identifiable. However, in the rosette stage of growth, horseweed might resemble other weeds that have this rosette habit, such as Shepherds-Purse (Capsella bursa-pastoris) or Virginia Pepperweed (Lepidium virginicum).

Office of Programs

University of Missouri

Horseweed

Conyza canadensis