The ideal time to transplant an iris is between late July and September. The dry hot weather at this time of year helps the plant recover by preventing bacterial soft rot.
To transplant an iris first, cut back the leaves. Use a spade to dig up the clump, and cut apart the rhizomes with a sharp knife, discarding any old or unhealthy rhizomes. Dig a hole about 5 inches deep in a sunny well-drained spot, and mound the soil in the center of the hole. Place a rhizome in the center of the mound, letting the roots drape over it. Cover it with soil, but leave the tip of the rhizome visible.