Irises come in two types, with roots that are rhizomatous or bulbous, both of which must be split every few years for healthy, blooming plants. Rhizomes grow horizontally and look much like the ginger root you see in the grocery store. Bulbs are more familiarly round and, well, bulbous, and multiply rather than grow outward. When irises become too crowded, the plant stops producing flowers, though they will still grow leaves each spring. To ensure plenty of beautiful flowers, the rhizomes or bulbs must be dug up, split and replanted.
When to Split
Rhizomatous irises like the warm, dry weather, so late summer—from July to September—is the best time to divide them. Bearded, Siberian and Japanese iris are all rhizomatous. Bulbous irises, which include Dutch and reticulate irises, can be divided as late as October. Wait until the blooms are gone, and give the plant time to prepare for winter before digging them up. Spring and early summer would be a very bad time to divide irises. Winter, also, would be a bad time, even if the ground doesn’t freeze where you live, for the roots need to recover from cutting.
How to Split
The procedure for splitting is pretty much the same for both rhizomatous and bulbous irises. Using a fairly large shovel or spade, dig the plant from the ground. If your irises are extremely crowded, you may have to dig up more than one plant at a time and separate them by hand. Clean the dirt from the roots and bulbs. Cut the leaves to about 6 inches for both rhizomatous and bulbous irises. If many bulbs or rhizomes are in a clump, separate them with a disinfected knife. To disinfect, dip the knife in a 10-percent bleach solution. Bulbs are self-evident units, and rhizomes should be about 3 inches long when they are divided. A good place to divide rhizomes is where they fork.
Be sure to check your plants for borers and soft rot, and cut away any affected areas. If your iris are infested, when you are dividing, you'll find a pink caterpillar inside the rhizome. Sanitation is the best method of borer control. After the first hard frost, remove and destroy the old leaves to get rid of the eggs. Destroying the caterpillars in late spring is also recommended.
Replant your irises in holes about 2 to 3 inches deep, with the front of the leaves facing the sun.