Non-native thistles arrived as weed seeds in the U.S. accidentally either in shipments of crop seeds or in ballast in the holds of ships. They have become major problems in agricultural landscapes and 22 states have designated them as noxious weeds. Thistles do attract pollinators and birds, but the spiny leaves and stems keep grazers away. Some thistles release chemicals into the soil that inhibit the growth of other plants.
Thistles tend to take hold on disturbed soils. It is an opportunist that can grow in field and road edges, wet and dry sites, pastures and your backyard. To keep thistles from establishing, minimize soil disturbance and cover bare soils with mulch. Because of their tough roots, thistles can be difficult to control. Thistle seedlings have bristles or spines on their leaves, so as soon as you notice a seedling, pull it out before the roots are well established. Large thistles can be controlled with herbicide most effectively by spraying rosettes in late summer or early fall.