Horseweed
Conyza canadensis
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Photographed at Phoenix Tres Rios wetland, part of a wastewater treatment fcility in Maricopa Co., Arizona. September 2009.
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ANNUAL: Herbaceous annual germinating from seed by mid spring and
persisting through summer, depending on available moisture. Generally with a
few upright stems occasionally exceeding 60 cm
tall.
FLOWERS: Numerous small flower heads remain mostly closed up with just a
bit of white bristles appearing above the bracts.
LEAVES: Leaves and stems are set with hairs giving the plant a
silvery appearance. Leaves are elongate to linear, have usu. a few teeth on the
margins, and are evenly distributed along stems.
RANGE: Frequent weed in moist urban and agricultural areas in the
Sonoran Desert in Arizona - rare in desert areas lacking irrigation. Otherwise
this plant has found its way to most of North America and Eurasia, favoring
disturbed, man-altered habitats. A closely related species that is more likely
in drier situations such as xeriscape gardens and bermuda lawns is C.
bonariensis.
FRUIT: Heads produce abundant, single-seeded achenes topped by fine
bristles that serve to distribute seeds via wind.
UNARMED. Without thorns.
Two insects are sometimes abundant on this plant. Seed Bugs feed on developing seed heads and lace bugs are sometimes so abundant as to defoliate plants.