False Indigo (Baptisia australis) is a big perennial that's tough as nails, false indigo kicks off summer with spires of pure blue (cultivars come in a variety of other colors). Its rounded form makes a great contrast to the verticality of prairie grasses. False indigo develops deep taproots, so site it in a spot for the long term. It self-seeds modestly, and it's safe to move seedlings when small. Grows up to 4 feet high and wide in Zones 3 to 9.