The Iris (Iridacae) is a rhizomatous perennial that flowers in late spring and early summer. The plant produces delicate flowers atop long stalks that bloom from buds located along the flower stem, often blooming in succession. Starting new plants from seed means waiting three years or longer for blooms, and the seeds from the numerous cultivars and hybrids won't produce the same flower. Dividing the bulbs, called rhizomes, produces identical new plants, often flowering the following year.
Lift the rhizome clumps after the plant is done flowering by carefully prying them from the soil with a garden fork. Shake most of the dirt off the clumps, but don't break them apart.
Select plump, young rhizomes for dividing. Discard any older rhizomes that don't have stems or leaves growing from them.
Cut the rhizomes into sections with a sharp, sturdy knife. Each section must have a set of leaves at one end and be 2 to 4 inches long.
Trim the leaves back to 6 or 8 inches. This prevents the wind from uprooting the divisions and reduces moisture loss. Trim the small roots that hang off the rhizome to half their length.
Replant the divisions with all the leaf ends facing the same way. The rhizome divisions should be planted to about half their depth. The top half of the rhizome should be exposed.
Keep the soil evenly moist throughout the remainder of the growing season.