When clumps get large and crowded and flower less, which tends to happen every 3 to 5 years, irises are easy to divide. Just lift the cluster of rhizomes—swollen underground stems—with a garden fork, gently tease it into sections, and replant them. Or give some away. The only serious pest problem is iris borers, insect larvae that tunnel through the rhizomes. If you spot them, eradicate the infested plant, and plant different irises somewhere else in the garden.