Bur oak is the iconic tree of the Midwestern prairies and oak savannas. It survived frequent prairie fires because its thick, corky bark protected it. A sun-loving tree, it attained its greatest size in sunny, open savannas. It is adaptable to sandy soils, heavy clay soils, and even wet soils, and for this reason it is also a hardy urban street tree. Its acorns are enclosed in fringed cups.