Iris rhizomes should be divided and transplanted in late July or August so they have a chance to develop a good root system before cold weather sets in. It is a simple task, and if you want to grow irises, it is vital to learn. Irises must be divided and transplanted every three to four years because of the way the rhizomes grow and crowd each other underground.
The rhizome is a thick underground stem that grows horizontally. New rhizomes grow and branch out from the old ones. That’s why an iris patch spreads and becomes larger over time. By dividing and transplanting the rhizomes, you can start new patches elsewhere on your property.
To dig up your iris, use a garden fork and start working on the outside clump of the iris patch. Gently insert your fork underneath the clump and work around it, taking care not to damage the shallow root system. The clump should ease upward out of the soil so you can begin dividing them.
Shake most of the soil off the roots. The rhizomes are usually easy to pull apart with hands, but if they are stubborn, you can cut them apart with a sharp knife. The young rhizomes are white with green leaves attached. The older ones have flower stalks attached, even though blooms have long since gone. Each rhizome produces flower stalks for only one year, but don’t discard the old ones if they look like they are still producing new rhizomes. You can tell whether they are by looking for buds on along the sides of the rhizome stem. If there are buds, you will want to replant the old rhizome, too, so it can continue to produce new rhizomes.
The rhizomes should all be firm. If they are soft, discard them because they are either diseased or infested with pests. Trim the roots growing out of the bottom of each rhizome so they are only about two inches long. Trim the green leaves growing out of the top so they are about six inches high. This will direct the plant’s energy into establishing healthy roots.
Transplant them right away, if possible. If you can’t do it right after digging them up, transplant them within two to three weeks. Until then, they should be kept in a paper bag or cardboard box in a dry, cool place. They should not be stored in plastic.
For transplanting, pick a spot that gets at least six hours of sun every day. The soil should be well-drained with a neutral pH with compost content. Plant each rhizome 18 to 24 inches apart. Dig a large hole about three inches deep with a ridge in the center. Rest the rhizome on the ridge and direct the roots into the holes on either side of the ridge. Cover the roots with soil and place enough soil over the rhizome just to cover it. Water them well immediately after transplanting. Keep the soil moist but not wet for the next few weeks until they have become established.