Iris, cannas and rhizomes all look rather like sweet potatoes. They are long and firm with multiple growing points over the surface. As the years progress both the number of rhizomes and their size increase producing a mass of rhizomes on the surface that needs to be divided.
Most division is done in spring as the new buds are starting to grow. For those plants that are hardy through the winter, dig the whole clump up as they break dormancy. In areas where the plants are not winter hardy, wait until the top growth has been killed by frost, remove the dead stems and lift the rhizomes for storage over the winter where they can be left until spring to divide.
Before you start, rinse and inspect the rhizomes and remove any soft or decaying areas, then identify the growth points, just as you can see eyes on potato tubers. With both iris and cannas you can see the growth points easily and there are multiple ‘eyes’ on each rhizome. For very large groups the rhizomes will fall apart when you dig them making that first division for you. If the smaller groups fit your landscape needs they can be replanted directly. Often larger groups require a knife to help break apart the rhizomes. You can also cut the individual rhizome into smaller pieces if you want to.
Iris are particularly prone to disease, so it is important that you disinfect the knife both before you start dividing the group as well as before working on another group of iris. A solution of one tablespoon of bleach in a gallon of water should be strong enough to cleanse the knife or blade. Rinse the bleach off the blade before cutting the rhizome.
When you have your new divisions ready, they are planted just below the surface. Do not plant the rhizome too deeply as they prefer to almost sit on top of the ground.