The following spring, new leaves grow from the crown bud in the rosette. The bolting stage begins when the seed stalk begins to grow and continues until the first flower appears. Flowering begins in July and continues through August in most areas. Bull thistle is not a serious problem in crops requiring a spring seedbed preparation. Tillage eradicates rosettes that established in the preceding summer or fall.
Thistles can be a problem in fall planted crops, such as grains, alfalfa and clover.
Introducing a specific natural enemy can aid in regulating the reproduction and spread of bull thistle. This weevil feeds on the crown center of the rosette. Native to Europe, this weevil was extensively studied by the USDA to insure that it would not damage other desirable plants in the U.S. Figure 1. The adult rosette weevil. In mid-October, adult weevils awaken from summer aestivation and begin to feed and mate on thistle rosettes.