Divide the plants anytime after blooming is completed. For best results divide in late summer and early fall. Iris grow from enlarged underground stems called rhizomes that grow just below the soil surface. The rhizome is the plant part that is vegetatively propagated to give new plants of the same type. Iris multiply rapidly and require dividing every two to five years. For most irises a peak of bloom will be about three to four years after the original planting. Before dividing iris, cut the leaves to about one third their full height. Dig up the entire clump of rhizomes. Carefully cut the clump apart, saving the vigorous rhizomes and discarding the inner, leafless ones. Cut the rhizomes into pieces with each one containing one or two terminal leaf fans.
Smaller divisions do not need dividing again for three to five years, but are slower to produce good flowers. Larger divisions produce flowers more rapidly, but require division in two to three years. When separated from the clump, each division is ready to plant.
Iris are semi-dormant during the late summer (mid-July to early September). This is the best time to plant the rhizomes to allow for adequate root growth and establishment before winter. During late summer the buds for next year's bloom are formed so plant the iris early enough to avoid cold damage to these buds. Plant iris rhizomes just below the soil surface in a good soil. In heavier soil, plant with the rhizomes exposed slightly to prevent rotting. Roots should be buried to provide good anchorage.
Plant rhizomes 18 inches apart with fans facing the same direction. Since the flower stem will emerge from the fan, this allows adequate space for blooms. For a larger display of individual colors, plant three or more rhizomes of a single variety in a group. To keep iris looking their best and producing good, large blooms divide and replant every two to five years. While they are being divided, be sure that all rhizomes are carefully labeled.