Here’s an easy way to divide up dense clumps of irises:
Divide and transplant irises in late summer to early fall. Using a spading fork, dig around the perimeter of the clump, rocking the fork back and forth to loosen the rhizomes.
Lift the entire clump out of the soil. Remove excess soil from around the rhizomes and clip off yellowing leaves. Trim each group of leaves into a fan shape; each fan should be about 4 to 6 inches tall. Using a sharp knife, separate the big clump into sections. Make sure each new section has a firm rhizome, roots, and one fan of leaves. Discard any rhizomes that are soft or rotten, or have pest damage.
Work 2 inches of compost into the area where you plan to transplant the irises. Dig a 4-inch-deep hole and create a slight mound of soil in the middle. Position the rhizome over the mound, draping the roots over it (above). Fill the hole with soil. Make sure that the top of the rhizome is even with (not under) the soil line, or it will not bloom. Plant three to five sections together, placing each section about 12 inches apart with the fans running parallel to one another. Water the transplants well.