Dividing irises every three to five years rejuvenates them and increases flowering, Ward Upham of K-State says. Late July through early August is the best time to do it. It’s pretty easy to dig the fairly shallow clumps; use a sharp knife to cut the rhizomes apart so that you have a fan of leaves and a section of rhizome in each division. If a rhizome has borer damage that’s not too severe, you can remove the borers and replant. If a rhizome has a mild case of soft rot, you can scrape out the bad tissue and leave the rhizome to dry in the sun before dipping it in a 10 percent solution of bleach. Rinse the rhizome with water and allow it to dry again before replanting.
To replant divisions, cut the leaves back by two-thirds. Remove weeds from the planting area and fertilize according to soil test recommendations or with a complete fertilizer, such as a 10-10-10, assuming the area has not been heavily fertilized in the past. Apply the fertilizer at the rate of 1 pound per 100 square feet and mix it in 6 inches, Upham says.