Deep root stocks make this weed difficult to control. It is especially fond of marshland areas and if not controlled will take over. Perennial Pepperweed can found on the shores of several different ponds at Sawhill. Scotch Thistle grows up to 13 feet tall making it the giant of all thistles. It's a biennial that produces large spiny leaves covered in fine dense hair and a flower that is purple in color.
Scotch thistle can produce stands so dense they are impenetrable to humans, cattle, and wildlife.
The biggest stand in the park can be found at the southwest end of Sawhill Ponds. Teasel is a stout, tap-rooted biennial which grows up to 6 feet tall. Flowers are purple and surrounded by large spike-like bracts. Teasel is found in areas that are poorly drained where they are typically competing with cattails. In the park, Teasel is found in many places around the perimeter of Boulder Reservoir.