Yes, iris can be planted under trees as long as they are not in full shade. You should dig up and divide your irises every three to four years in the late summer or early fall. Iris prefer well-drained, moist soil. You should cut back irises in late summer after they are done blooming. Irises typically bloom twice a year. To keep irises blooming all summer, deadhead the spent blooms and fertilize regularly. You can divide irises in the winter if the ground is not frozen. Reasons for irises not blooming could be too much shade, not enough water, or too much fertilizer. To divide iris, cut the rhizomes apart and replant. The best place to plant iris bulbs is in full sun with well-drained soil.