During the later part of the wildflower season, conditions become dry enough in Upper Bidwell Park that several non-native thistles bloom then share their seeds with the world. These beautiful flowers with a bite include by common names, Venus Thistle, Star Thistle, Milk Thistle, Sow Thistle, and Italian Thistle.

These plants from afar were originally transported to the park as a seed on the bottom of a shoe, or a box. Their vibrant blooms and sharp prickly leaves and seed heads can be spotted along the main park road from just before the Bear Hole swimming area all they way to the end of the road.