The star thistles, cornflowers and knapweeds are a group of thistle-like plants in the Genus Centaurea of the Family Asteraceae. Some 350 species of herbaceous flowering plants belong to Centaurea, most native to the Old World. One species in this genus is the Centaurea solstitialis, shown in the picture. Also called the Yellow star thistle, this is an annual that grows to a height of 1 to 2.5 feet. This plant is widely naturalized outside of its native Europe.Photo credit: Peggy Greb