An Illustrated Guide to ARIZONA WEEDS

The University of Arizona Press




CANADA THISTLE-Cirsium arvenseM (L.) Scop. var. mite Wimm. & Grab.

DESCRIPTION-A prohibited noxious weed in Arizona, Canada thistle is a slender spiny-leaved perennial with rigid stems, 1 to 4 feet high. Reproducing by seed, and by its deep underground vertical and extensive horizontal roots which enable it to spread rapidly over large areas. These creeping roots are greatly branched and may extend long distances, frequently giving rise to leafy shoots. The many alternate leaves are stalkless, oblong or lanceshaped, and divided into very irregular lobes with many small yellow spines.

The small flaskshaped flower heads are about 1/2 to 3/4 inch in diameter, each containing many small rose-purple tubular flowers. The many, overlapping bracts enclosing each head are spineless. Canada thistle differs from other species of the true thistle in that there are male and female flower heads, and these are on separate plants. Male plants, and thus possible entire colonies, would, therefore, produce no achenes.

The bone or tan colored achenes, about 1/8 inch long, are oblong and smooth, with a tuft of feathery hairs at the top. These hairs fall off after maturity, showing the rounded apex with a little tubercle in the center.

DISTRIBUTION-Canada thistle is an European introduction, and is not from Canada. It now covers the northern half of the United States from the Atlantic to the Pacific, but is only occasional in the southern part. It is one of the most dreaded weeds known to agriculture, as it grows in all cultivated crops, fields, and waste places. Fortunately, it is rare in Arizona. The first established record in Arizona is from Flagstaff in 1920. It is now occasional in Coconino County, and in the Prescott area, Yavapai County.

Copyright (c) 1972 The Arizona Board of Regents

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