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Statewide IPM Program, University of California

Horseweed  (Conyza canadensis)

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Life stages of Horseweed seedling young plant mature plant young plant (rosettes) flower and fruit heads

Horseweed, also called mare's tail, is a summer annual or biennial broadleaf plant. It is found throughout California to about 6600 feet (2000 m) and inhabits agricultural land, disturbed managed sites, and landscaped areas. It establishes in areas where the natural vegetation has been disturbed. During wet years it may be found on hillsides of coastal sage scrub. It is a strong competitor for water and grows rapidly.


Ditch and canal backs, urban sites, agronomic crop fields, landscaped areas, orchards, vineyards, roadsides and other disturbed, unmanaged places.


Cotyledons (seed leaves) are oval to egg shaped, hairless to densely covered with short stiff hairs, and up to 1/10 of an inch (about 3 mm) long. Leaves attach alternate to one another along the stem. The first leaf is egg shaped to football shaped and is on a hairy stalk. Later leaves are more-or-less football shaped with slightly toothed edges.

Young plant

Leaves grow as a rosette around a crown.

Mature plant

Mature plants grow erect, 6-1/2 to 10 feet (2–3 m) tall. Usually a single main stem branches out from the upper half of the plant. In hairy fleabane , the main stem usually branches near the base. However, cutting the main stem of horseweed can lead to the growth of several basal branches. An abundance of dark green leaves are alternate to one another along the stem. Shorter, mowed plants may look similar to hairy fleabane, but horseweed leaves are dark green while hairy fleabane leaves are gray-green.


Flowers bloom from June through September. Many flower heads grow at the ends of branched stems. Initially the flower head has an urn-shaped green base with a very small daiseylike flower (head) at the top. Eventually the flower heads mature and expand into seed heads, which look somewhat similar to dandelion puffs.


Fruits are small, roughly 1/16 of an inch (1.5 mm) long, narrow, elliptical or oblong, slightly hairy, and attached to soft, dirty-white bristles.


Horseweed seed bristles are mostly dirty white compared to those of hairy fleabane, which are brownish-white and often redden with age.


Reproduces by seed.

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