Mark Renz UW Madison Associate Professor and Extension Specialist, Chelsea Zegler UW Madison Associate Research Specialist
We wanted to emphasize Wisconsin’s common weedy biennial thistles in this weed identification series installment. While Wisconsin has dozens of different thistles, we have three relatively common biennial thistles that are considered weeds. Recall that biennial plants germinate and grow leaves only in the first year and then after overwintering produce a stem and flowers. Once flowering is complete the plant dies. It is important to distinguish these thistle from our perennial thistle (Canada thistle) which has plant parts that can last more than two years and result in dense patches that persist and spread.
Plumeless thistle is by far the most common of the biennial thistles in Wisconsin as it can be found throughout the state. In contrast musk thistle is more common in southern Wisconsin, but populations have been spreading north. Bull thistle, while common throughout Wisconsin, rarely forms large populations, but none the less can be problematic.
These three species are common to pastures and right of way areas, but have become more common in no-till fields over the past decade. These species can be difficult to differentiate from one another, but close examination of leaves and flowers will find unique characteristics that can help. To aid in identification we have provided a summary table along with side-by-side pictures.
Click here to download. There are 4 files.