Dividing irises
To alleviate overcrowding and invigorate blooming, lift and divide irises every three to five years. July and August are the best months for this task. Unearth entire clumps with a garden fork, shovel or sharp spade and shake off excess dirt. Cut leaves back by one-half to one-third. Using a sharp knife, cut the rhizomes (rootstock) into sections, with each new transplant including a “fan” of leaves, a few inches of firm, plump rhizome and several roots. Discard outer sections of rhizome and any small pieces. (Be on the lookout for any holes in the rhizomes. These are signs of borer caterpillars, a common iris pest that tunnels through rhizomes. Remove caterpillars with a knife, and discard heavily infested rhizomes.) To replant irises, dig a shallow hole, mound the soil in the center and spread roots downward around it. Plant so that the top of the rhizome is visible at the soil surface. Don’t fuss over the process of division. Like daylilies, irises are as tough as oxen.