Bull thistles are weeds that have pointy spines on the tips of their leaves and prickly hairs on the surface of the leaves. Flowers range from pink to purple. Bull thistles spread only from seed, which makes controlling and killing them much easier than thistles that spread by both seed and underground root stems. Bull thistles spread their seed after flowering, so by treating the weeds before flowering, you not just kill this year's plants but also eliminate next year's.
Wait until late spring or early summer when most of the bull thistles have begun to grow, but before most of them have flowered.
Mow the bull thistles or cut them down with pruning shears. You can also take a hoe and try to dig them up or at least damage the roots below their crowns. Repeat this step in one month for those bull thistles that grow later in the season.
Spray an herbicide after mowing to help kill and eliminate the bull thistles more quickly. Use an herbicide such as one that contains pricloram; 2,4-D; dicamba; or glyphosate. Application rates differ among herbicides and range between ½ to 1½ pounds per acre; therefore, adhere to the directions that are printed on the label.