“Like a cloud of butterflies,” says the description on the website of the Historic Iris Preservation Society. It is a succinct picture of Iris sibirica ‘Summer Sky,’ a diploid with pale blue falls, yellow hafts, stunning white style arms — easy to see why it was named ‘Summer Sky.’ Introduced in 1935, the variety was hybridized by Mrs. Frances E. Cleveland, who developed a number of Siberian iris cultivars. Siberian iris are one of the easiest flowers to grow, especially in Maine. They are mostly carefree, with swordlike foliage topped by elegant flowers in late spring and early summer. ‘Summer Sky’ should reach about 36 inches in height. It is hardy to Zone 4 and tolerant of full sun to partial shade and still thrive. Give them well-drained but moist soil and the plants will spread fairly quickly over time.