Individual gardening style partly dictates how irises are cut back. While it is generally agreed that it is wise to remove flower stalks after blossoms fade in order to direct more energy to plant growth, there is disagreement about trimming foliage. Some gardeners leave the foliage alone as long as it is green so it can produce food for plant growth. But trimming foliage with a sharp pair of garden shears can be done as soon as 6 to 8 weeks after bloom to make a flower bed look tidier or prepare for division and replanting of the kinds of iris that grow from tuberous roots called rhizomes. A sharp pair of sharp garden shears is the tool of choice.
While some iris species are rhizomatic, others grow from bulbs. Trimming foliage excessively or too early can limit rhizome and bulb growth for the next season. Rhizomatic species include the tall bearded and beardless irises with long, strap-like leaves that gradually go limp after flowers die. In contrast, bulb species such as Japanese iris, are slighter, have slender leaves and aren't divided for replanting until fall dormancy.
Although it isn't necessary to cut back iris in years when the plants aren't being divided, gardeners sometimes can't tolerate the look of floppy foliage. Cutting it back to a height of 6 to 8 inches still promotes rhizome or bulb growth. University of Arkansas Cooperative Extension Service says there is a good reason why some gardeners trim their bearded and Siberian iris in the shape of a fan. In rhizomatic iris beds containing hundreds of plants the fan shape allows better air circulation and sun penetration to prevent diseases. Also, foliage and flowering of rhizomatic iris will be more vigorous if the tubers are not completely covered with soil when planted. They need the warm kiss of sunlight to thrive.