Slenderflower Thistle (Carduus
tenuiflorus W. Curtis) FamilyFamily: Compositae Description and Variation: Slenderflower thistle is a winter annual broadleaf weed that is similar in appearance to Italian thistle, C. pycnocephalus. Slenderflower thistle plants grow up to 100 cm tall with triangular shaped stem wings that are up to one centimeter tall and are tipped with a spine. The leaves are oblanceolate to lanceolate with six to eight pairs of spine-tipped lobes.The underside of leaves and stems are covered with cobwebby hairs. Five to fifteen pinkish, cylindrical flowers are borne in terminal clusters. The flowers are up to two centimeters in length and the bracts are not hairy as in Italian thistle (C. pycnocephalus). Economic Importance: Slenderflower thistle has no known beneficial economic value, but it has invaded range and pastures. On grazed lands, slenderflower thistle, like other thistles, can reduce productivity by physically interfering with grazing and by displacing desired grasses. Native vegetation is threatened by this aggressive weed that tends to form dense stands. Geographical Distribution: Slenderflower thistle is reported to be scattered over about 100 acres (ten acres in total is infested if the area is consolidated) in southeastern Thurston county (T 16 N, R 2 E, sect. 19, 20, 29, and 30). It has been introduced to Oregon, California, Texas, Australia, New Zealand, and Argentina and its native range is western and southern Europe. Habitat: Slenderflower thistle prefers dry, open areas such as pastures, rangelands, and right-of-ways. History: Thurston County discovered and had slenderflower thistle identified in 1991. Growth and Development: Typically slenderflower thistle germinates in the fall, over winters as a rosette, and flowers in late spring. The seeds have no after-ripening requirements and germinate over temperatures ranging from two to 30 degrees Centigrade. Reproduction: Slenderflower thistle reproduces by seeds only. Response to Herbicides: Response to Cultural Methods: As an annual broadleaf, mechanical control such as tilling or digging will kill slenderflower thistle. Deferring autumn grazing of sheep has been effective in reducing stand density because the thistles grow etiolated and less spiny when competing with ungrazed grasses. Then sheep will graze the thistle along with the grass. Biocontrol Potentials: Two fungi have been evaluated as agents on slenderflower thistle. An Alternaria sp., which killed C. pycnocephalus in the cotyledon stage, was also pathogenic to slenderflower thistle. A Puccinia sp., isolated and active on C. pycnocephalus, was not active on slenderflower thistle. Rationale for Listing: Slenderflower thistle is another aggressive exotic thistle that currently has an extremely limited distribution in Thurston County, Washington. This plant has been introduced into other countries and states, where it has established, shown its aggressive nature, and become a problem weed in pastures and rangeland. Targeting this weed for eradication will protect Washington's range and natural resources from infestation, damage, and costly control measures. References: *Andersen, G.L. and S.E. Lindow. 1985. Biological control of Carduus pycnocephalus with Alternaria sp. Proc. VI Int. Symp. Biol. Cont. Weeds. pp. 593-600. *Bendall, G.M. 1973. the control of slender thistle, Carduus pycnocephalus L. and C. tenuiflorus Curt. (Compositae), in pasture by grazing management. Aust. J. Agric. Res. 24:327-332. *Groves, R.H. and P.E. Kaye. 1989. Germination and phenology of seven introduced thistle species in southern Australia. Aust. J. Bot. 37:351-359. *Olivieri, I. 1985. Comparative electrophoretic studies of Carduus pycnocephalus L., C. tenuiflorus Curt. (Asteraceae), and their hybrids. Amer. J. Bot. 72:715-718. *Oliveri, I. 1984. Effect of Puccinia cardui-pycnocephali on slender thistles (Carduus pycnocephalus and C. tenuiflorus). Weed Sci. 32:508-510. *Robbins. W.W., M.K. Bellue, and W.S. Ball. Weeds of California. pp. 433. *Tutin, T.G., V.H. Heywood, N.A. Burges, D.M. Moore, D.H. Valentine, S.M. Walters, and D.A. Webb. 1976. Flora Europaea. Cambridge Univ. Press. Vol 4:231. See also references under Italian thistle, Carduus pycnocephalus [Return to Class A Noxious Weeds]
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