There's not much work to do with bearded irises. Just plant them and forget them. Although they’re very forgiving plants as long as they’re not planted too deeply, bearded irises will benefit from occasional dividing. Here’s how.
If your bearded irises have been in the ground for 4-5 years, they may be ready for dividing. Telltale signs: diminished flowering and the bulbous root structures (called rhizomes) becoming crowded and more prominent, as seen above.
To divide, wait till flowering is finished (mid to late summer), then use a spade fork to lift the rhizomes from the ground. Wash off excess soil and discard any rhizomes that are soft and mushy or thick and woody. Separate individual rhizomes with a garden knife, making sure each new division includes a fan of leaves.
Cut off the top portion of fans, leaving 4–6 inches of leaves to help rhizomes adjust to transplanting. Amend the planting bed with compost. Replant rhizomes 6–12 inches apart with fans facing outward. The whitish tops of the rhizomes should be just barely above ground level. Firm the soil at the base and water well.