Bearded iris can be divided any time after the last spring frost. However, many iris growers believe the optimal time is 6-8 weeks after blooming. August is a good time to begin. This gives plants time to root well before winter. Extremely hot days (90 +) should be avoided when transplanting. Plant division every 3 to 4 years is beneficial for maximum bloom. With a garden fork lift the entire plant clump and shake or rinse excess soil from roots. …