Robusta is a natural cross between ( I. virginica x I. versicolor) . This very water tolerant iris does well in moist soil to water 4'' deep. Grows 3 - 4' tall and clumping. Zone 4 'Gerald Darby' is one example and a real dandy with three season interest. New purple fans in spring with bright blue flowers that sit on dark purple stems in early summer. Very strong vertical accent the rest of the growing season.