Most Thistles Are Noxious Weeds
The introduced thistles arrived as weed seeds in the United States 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. If you live in a state where these thistles are considered noxious weeds, you may be obligated to eliminate them from your property. 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.