Iron Butterfly Ironweed. This native perennial from Arkansas has fine foliage similar Amsonia. It has brilliant small purple aster-like flowers in late summer which are superb butterfly attractors. It needs full sun on a well drained soil for best performance. In its native haunts, this species grows on rocky riverbanks where it might be inundated by rising water for some time. But don't take this as an indication that it likes water. It will reach a height of 30-36".