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a genus of plants of the family Compositae. The members of the genus are perennial (less frequently, biennial) herbs. The flowers are tubular, gathered in calathide heads with a fairly prickly sheath. The achenes have pappi of featherlike hairs. There are more than 200 species in the northern hemisphere; in the USSR there are 111 species. Some species of Cirsium are pernicious weeds, some are melliferous, and some are decorative. The term Cirsium is sometimes also used to describe other plants of the family Compositae that have prickly leaves and calathide heads.
The creeping thistle or Canada thistle (C. arvense) occurs throughout the USSR except the polar and mountain regions and the arid regions of Middle Asia. It is a perennial soboliferous weed and is difficult to eradicate. The plant has many forms (creeping thistle, bristly thistle, gray-felt thistle, and pale-scaled thistle), which cross-pollinate and yield numerous hybrids. The stem is 50–130 cm tall, branched on top, with an underground part of 10–30 cm. The leaves are sessile, constricted at the base, dentate or lobed, and prickly. The flowers are tubular, lilac, and unisexual; the inflorescence is an anthodium (calathide head). The plant propagates by means of seeds and also vegetatively. The achenes are small, naked, and slightly curved, with a pappus that falls away easily. The root system consists of a vertical root reaching 2–3 m (sometimes 5–9 m) into the soil and layers of lateral roots of the first and subsequent orders of propagation leading away from it. The roots are capable of giving off shoots or suckers. Even small pieces broken off the roots and grown will yield new plants. The weeds choke out crops, sharply reducing the harvest and lowering the quality. Means of control include correct crop rotation, early deep autumn plowing with preliminary fertilization, interrow treatment of tilled crops, the application of herbicides, and the destruction of the weed along roads and field boundaries; and in a fallow field, systematic cultivation according to the amount of creeping thistle growth.
I. G. DEIANOV