Dividing iris is a necessary chore, but luckily, it only needs to be done every three to five years. You’ll know it’s time if your iris plants don’t bloom as vigorously, if the rhizomes become so crowded they push up to the surface of the soil to the point where they’re visible, and your iris form that telltale ring with a bare spot in the center, as though something’s been sleeping in them. The best time to lift and divide iris is after they’ve bloomed, sometime in July or August.
With a spade or garden fork, dig straight down into the soil a few inches out from the base of the plant, and then gently lift. Do this around the perimeter until rhizomes have come free of the soil, doing your best to keep the base of the plant intact. Gently knock, brush, or wash off excess dirt.
Using a sharp, clean knife, cut rhizomes into sections. Each section should be at least three or four inches long, have at least one fan of leaves, and a few good roots. Portions from the center of the rhizome mass will likely not have leaves, so these should be discarded. Any rhizomes that are soft should be discarded as well.
Trim the leaves to 1/3 their original size to make handling the plants easier and to reduce the plant material the rhizomes will have to support while becoming established. Dig a hole in well-draining soil (amend if soil quality is poor) and where there is full sun. Make a small mound in the center and place rhizome on top, spreading out the roots. Fill in so that rhizome is just below soil level and firm gently. Space twelve to twenty-four inches apart, and position so leaves are pointing away from other transplants. Water well and then weekly for the rest of the growing season. Mulch the first winter. You’ll likely have lots of divisions, so consider potting some to share with gardening friends.