The most troublesome weeds in Wisconsin pastures are thistles which are especially troublesome because in addition to lowering productivity of pastures, their spiny nature prevents livestock from grazing near them. In fact, heavy thistle infestations may cause large areas of pastures to be left ungrazed. Three species of thistles are of greatest concern. The most common thistles in pastures have a two-year life cycle. Bull, musk, and plumeless thistles are biennials and only grow vegetatively during the first year.
Seeds germinate during the spring and summer when soil moisture and temperatures are favorable.
After germination, they form a rosette ranging from 4 to 18 inches in diameter before becoming dormant in the late fall of the first year. In the spring of the second year, bull, musk, and plumeless thistles resume vegetative growth. Bull thistles normally bolt and flower 2 to 3 weeks after musk and plumeless thistles. Each plant can send up several stalks and produce many flower heads, each with many viable seeds. After flowering or with the first frost, biennial thistles die in the second year.