An Illustrated Guide to ARIZONA WEEDS
The University of Arizona Press
RUSSIAN KNAPWEED,turkestan thistle
RUSSIAN KNAPWEED-Centaurea repens L. (C. picris Pall.)
DESCRIPTION-A prohibited noxious weed in Arizona, Russian knapweed bushy, many-branched perennial, 1 to 3 feet high, from creeping horizontal and vertical underground stems, 2 to 4 feet deep, the older ones dark brown to black. Reproducing by leafy shoots from the underground stems and by seeds. The wingless, very leafy stem is covered with soft gray hairs when young. The leaves are of different shapes, the basal deeply lobed, 2 to 4 inches long, ,1/2 to 1 inch broad, and form a quickly-withering rosette on the ground. The lower stem leaves are smaller, lobed or sharply toothed, and the upper leaves, 1/2 to 1 1/2 inches long, are narrowly oblong, the tip sharp pointed and the margins smooth or slightly toothed. The small, coneshaped flower heads, 1/4 to 1/2 inch diameter, and solitary at the tip of leafy branchlets, are composed of about 16 deeply 5-lobed, tubular, lilac to bluish to rose pink flowers. Surrounding the flower heads are many pearly bracts in overlapping series, the outer series broad with rounded papery tips, the inner series with very hairy long taillike tips. The seedlike achenes are oblong, plump, about 1/8 to 1/4 inch long, and grayish or ivory colored, without a notch near the base. These bear numerous whitish bristles on the top, which drop off by achene maturity.
DISTRIBUTION-Russian knapweed is a noxious native of eastern Europe and Asia. Grows in disturbed soil, forming colonies in cultivated fields, orchards, ditches, pastures, roadsides, and waste places. Sometimes abundant locally, and a pest in sorghum, alfalfa, and small grains. Scattered from Navajo to Yavapai counties. Southward to Pima and Cochise counties; from about 1,000 to 7,100 feet elevation; flowering May to October.
Russian knapweed should be considered a very serious weed, and its spread viewed with alarm. Because of its efficient underground system, this weed, once established, is almost impossible to eradicate. This plant is avoided by all classes of livestock because of its very bitter taste. Infestations have steadily increased in the western states.
Copyright (c) 1972 The Arizona Board of Regents