By Janelle Rietz-Kamenar
Irises need to be divided every 2 to 5 years in order to maintain full, healthy blooms and avoid insects such as the iris borer or diseases such as soft rot. August is the best time to divide bearded iris when it is dormant! The good news is that it is relatively easy to do!
The night before, water the iris to insure moist soil when digging them up. And decide where you are going to put the extra bulbs in your garden after you do divide them. Remember that iris prefer well drained soil and full sun.
Use a shovel/pitch fork to dig around the iris being careful to lift clumps while maintaining roots attached to the rhizomes. Gently remove soil from the rhizomes. You can use a garden hose if necessary.
Divide the iris rhizomes with a pruning shears or a sharp knife using natural divisions. Make sure that you include part of the rhizome, some roots, and a fan of leaves.
Cut the foliage back approximately 6 inches. If the foliage is yellow or you see dark streaks, inspect for iris borer and either discard those rhizomes with the borer or if limited damage, eliminate the borers and save the rhizomes being careful to cut out any damaged parts. Disinfect the cutting tools between cuts to prevent the spread of disease. Remove any older spongy growth.
To prevent infection, the rhizome can be soaked for about half an hour in a 10% bleach solution, if desired. They can also be treated with sulfur dust or an insecticide/fungicide if pest problems are severe. These steps are usually not needed. Soaked rhizomes, however, would need to dry in a shady place prior to re-planting.
It is also recommended that you allow the cut rhizomes to cure for a few hours before replanting in a cool place. When replanting, give the rhizomes space to grow by planting 12 to 18 inches apart. Make sure the rhizome is planted shallowly on a mound and just cover the rhizome. Avoid planting too deeply. Iris are often planted in groups of three arranged in a triangle, with each fan of leaves pointing away from the other irises in the group.