An Illustrated Guide to ARIZONA WEEDS
The University of Arizona Press
HORSEWEED-Conyza canadensis (L.) Cronq. (Erigeron canadensis L.)
DESCRIPTION-An erect annual or biennial, the stems slender to very coarse, usually branching only in the flower part, 1/4 to 4 or more feet high; reproducing by seeds. The plants are thinly rough-hairy to practically hairless. The dark green leaves are alternate, l to 4 inches long, and 1/4 to 1/2 inch broad, often so crowded as to nearly hide the stem. They are stalkless or the lowermost short stalked, lance or strapshaped, the margins smooth or more often wavy to few-toothed.
The greenish-white flower heads are small, innumerable in a long, many- branched flowering mass (panicle) at the top of the plant, which is 1/2 to 2 feet long. Individually they are inconspicuous, about 1/8 inch high and 1/4 inch across, with many whitish ray flowers ("petals"), too small to be seen without the aid of a lens. The narrow achenes are tan colored, flattened, 1/25 to 1/16 inch long, with a few scattered hairs on the surface and a thin tuft of about a dozen fine colorless hairs at the blunt end, which are deciduous after maturity.
DISTRIBUTION-Horseweed is introduced from the eastern United States, preferring the rich moist soil of cultivated lands or sandy alluvial soil, but growing in any type of disturbed soil. It is widespread throughout the entire state, but is a pest in the agricultural valleys in cultivated fields, along ditchbanks, row ends, and borders. Also common along roadsides, pastures, yards, gardens, waste places, streams, and hillsides; 100 to 7,500 feet elevation; flowering July to October.
The leaves and flowers of horseweed are reported to contain a terpene which may cause dermatitis and throat irritation in susceptible individuals, and irritation in the nostrils of horses. At one time an oil was distilled from this plant which was used medicinally in the treatment of dysentery and diarrhea.
Copyright (c) 1972 The Arizona Board of Regents