Description:
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)

• Mechanical: 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.
• Biological: 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.
• Herbicide: MCPA is recommended for application during the seedling or rosette stage of slenderflower thistle. Applications after rosettes are larger that six inches in diameter may be less effective. Other growth regulator type herbicides (2,4-D, dicamba, or picloram) and Roundup may be effective, but references are not available to substantiate. Refer to the State Noxious Weed Control Board site

For More Information:
Detailed information about Slenderflower Thistle is available at the Washington State Noxious Weed Control Board Web Site.