Meadow anemone

Meadow anemone (Anemone canadensis), also known as Canadian anemone, windflower, or round-headed thimbleweed, is a perennial wildflower of the Ranunculaceae (buttercup) family, native to the northern United States east of the Rockies, often found on river or pond banks. It tolerates varying sun and shade conditions and in favorable conditions will form dense cover a foot or two tall, sometimes aggressively. Small, usually-five-petaled white flowers with yellow centers appear in spring in mature plants and can last through summer.
For more about meadow anemone and native plants for the Midwest, especially those that do well in shade/moist areas, see the "book" on this website, Gardening with Native Plants in Shady Evanston.