3. Use a sharp knife to cut the rhizomes into several sections. Keep at least one clump of leaves per section. Discard any rotten sections or those showing iris-borer damage.
4. Be careful not to point all the rhizomes in the same direction. Some leaf clumps should point outward and others inward so that they will spread in all directions.
5. Lay the rhizomes flat and bury them just below the surface. The point where the rhizome meets the leaf clump (collar) should not be buried. Firm the soil around the roots, build up a shallow rim of soil to trap water around each division and water thoroughly. (source: Montreal Botanical Gardens click here.)
Virginia Farm Bureau - In the Garden - Propagating Plants