Iris flowers are a hardy plant that will expand rapidly over a few years until they become overcrowded. Transplanting and dividing your irises gives you a chance to space out your irises and spread them to other places.
Before Beginning: When to Transplant
Transplantation can be done any time between when the soil warms in the spring and the first frost of the winter. The most popular time to transplant is 6 to 8 weeks after the blooms fade.
Transplanted irises will usually skip the next year's blooming. If they are transplanted before blooming, they will skip the current year as well as the next. In all situations, you should allow the irises a month or more to reestablish themselves before a frost.
Step 1 – Dig Up Your Irises
For thick clumps, use a trowel or larger shovel. Start at a distance and work inward. Don't worry about cutting roots. The point is to remove the cluster of rhizomes—the large tubers at the base of the leaves—from the soil. Lay the rhizomes out on a sheet of newspaper as you remove them for later inspection.
Step 2 – Separate Your Rhizomes
Rinse off the roots with water until you can see the clumps. There should be a central rhizome with branching rhizomes for other plants. Cut the rhizomes apart with a sharp knife. You can snap them apart, but a sharp cut will heal better.
Each rhizome for planting should be at least be 3 to 4 inches long, have roots at the base and leaves at the top. Cut the leaves down to 6 or 8 inches.
Step 3 – Check Your Rhizomes
Before replanting, dispose of any damaged rhizomes. Soft, mushy rhizomes have rotted. Dispose of them. Rhizomes with blackened circles have been infected with borers. If the infected sections can be cut off and still leave a viable sized rhizome, then cut and replant. If the damage is too expansive then it is better to throw out the damaged rhizome. Do not mulch or compost damaged rhizomes.
Step 4 – Wait Before Planting
The cuts you have made in the rhizomes should have a few weeks to dry. Place your rhizomes in a dry, shady location for 2 weeks. It is possible to wait for several months before replanting as long as the rhizomes are kept dry. If you have more rhizomes than you need, share with friends and neighbors.
Step 5 – Replant Your Irises
Plant your irises in well-draining soil. Space them 18 inches apart so there is plenty of room to expand. Dig a hole for each rhizome and place a small mound of soil at the bottom. Press the root end of the rhizome into the soil, draping the roots around the mound. Plant so that the top of the rhizome is above the soil. Planting high is better than planting too deep.
Fill in the soil around the rhizome, packing lightly to minimize air pockets that encourage rot. Water well to further minimize air pockets. Keep your irises well watered but not saturated until they reestablish themselves and throw up new leaves (1 to 2 months).