To reach the park, from Interstate 94 take Exit 4 and head north on U.S. Hwy. 12 for about 1.6 miles. When Hwy. 12 turns east, continue straight on County Road U for about 0.3 miles to County Road A, where you’ll continue for another 1.5 miles. The park’s entrance is on the road’s left/west side. Park in the lot for the Group Campground on the entry road’s south side.
Here’s a video of a portion of the prairie along the Knapweed Trail, taken in early May:
Here's an article more fully describing the trail.
Learn more about nearby day hiking trails in my Day Hiking Trails of St. Croix County guidebook.