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Dividing Irises

Q. Please advise on the correct time and procedure for dividing iris.

A. If you want to divide bearded iris, do so after it flowers, but before August. With a sharp, sterilized knife, cut the rhizome into pieces containing a set of leaves and healthy roots. Plant the pieces 1 inch deep, and cut the foliage down to a 6-inch fan that faces away from the planting circle. Siberian iris (I. sibirica) can be divided in July by digging up the entire clump with a sharp shovel and then separating the clump into as many sections as you want. For an established clump, this might require a pruning saw. Replant the sections in full sun at the same planting depth as the original clump. Early or spring-flowering little irises (I. danfordiae, I. reticulata) can be lifted, separated and replanted in fall, although this is often not necessary since they tend to naturalize by themselves.