Step by Step
1. Carefully dig the clumps with a garden fork or spade, taking care not to chop into the rhizomes more than necessary.
2. Divide the rhizomes by pulling them apart with your hands. In some cases, you may need to use a sharp knife to separate the baby rhizomes from their mothers. If so, dip your knife into a 10-percent bleach/water solution between cuts.
A good rhizome will be about as thick as your thumb, have healthy roots, and have one or two leaf fans. Large, old rhizomes that have no leaf fans can be tossed.
3. Wash the soil off the rhizomes to that you can inspect each one for iris borer (a fat, white worm). If you find a borer, destroy it. Some gardeners like to wash their iris rhizomes in a 10-percent bleach solution to protect against disease, but that won't help plants that are already rotting.
Soft, smelly, or rotting plants should also be destroyed. Discard any that feel lightweight or hollow, and appear dead, like the rhizome shown here.
4. Clip off the leaf blades so that they're 4 to 6 inches long. This reduces the stress that the plant goes through as it concentrates on regrowing new roots instead of trying to maintain long leaves.
5. Replant divisions, setting the rhizome higher in the planting hole than the fine roots, which should be fanned out. A bit of the top surface of the rhizome should be just visible at the soil surface.
6. Space the plants 12 to 18 inches apart (closer for dwarf varieties, farther apart for the largest). For the best display, plant the rhizomes so the fan of leaves face the same direction. Water them well at planting, but do not continue to water unless the weather becomes dry.