An Illustrated Guide to ARIZONA WEEDS

The University of Arizona Press




MALTA STARTHISTLE-Centaurea melitensis L.

DESCRIPTION-Grayish green annual from a taproot, with winged stems, 1 to 2 feet high, branched mostly above the base, reproducing by seed. The stem leaves and alternate, narrow, and unlobed, with the bases prolonged down the ridged stems as narrow green wings. The basal rosette leaves are 2 to 5 inches long, and cut into several pairs of lobes, with the end lobe much larger.

Each flower head, bearing small tubular yellow flowers, is about 1/2 inch high, and woolly in bud. Each head is enclosed by stiff bracts which bear slender spines l/8 to 3/8 inch long, usually purplish at the base. The largest spine of each of the middle bracts is usually branched below, with smaller spines at the base. The achene is smooth, grayish to dark brown, somewhat longitudinally striped, and about 1/8 inch long with a hooklike notch on one side just above the base. At the top it bears about 3 rows of whitish bristles of unequal length.

DISTRIBUTION-Malta starthistle, originally introduced into the United States from Europe, is occasional in alfalfa fields, grain fields, pastures, the banks of irrigation ditches roadsides, and waste places, in Apache, Yavapai, Maricopa, Pinal, Graham, Pima, and Cochise counties; mostly below 4,000 feet, but sometimes to 7,000 feet elevation; flowering May to July or spasmodically to frost. Not abundant enough in any state (except Oregon) to be so serious as to be declared a noxious weed.

YELLOW STARTHISTLE-Centaurea solstitialis L.

DESCRIPTION-Annual with yellow flowers and winged stems resembling Malta starthistle, but differing in that the spines on the middle bracts enclosing the heads are stout, bright yellow, unbranched, and the longest is 1/2 to 1 inch long. The achenes are also notched on one side just above the base, but the notch is shallow and not clearly hooklike. On the outermost achenes, the bristles at the top are lacking.

Yellow starthistle, another European introduction, is not declared a noxious weed in Arizona, but it is in seven other western states where it is particularly troublesome in small-grain and alfalfa fields. Uncommon in Arizona; occasional in Coconino, Mohave, Yuma, and Pima counties, and probably elsewhere in Arizona.

POISONOUS PROPERTIES-In California, horses on ranges where yellow starthistle is common and green forage is scarce may develop a nervous disorder known as "chewing disease." It is unknown in Arizona.

Copyright (c) 1972 The Arizona Board of Regents

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