By the time a clump of bearded iris is three to five years old, it has usually become so congested that flowering begins to decrease. At this point it’s time to dig, divide, and replant the clump, an operation best carried out in July or August. Bearded irises are classified according to size. The largest are the tall bearded irises (abbreviated as TB in catalogs) at 28 inches or more, followed by the border bearded (BB; 16–27 in.), intermediate bearded (IB; 16–27 in.), miniature tall bearded (MTB; 16–25 in.), standard dwarf bearded (SDB; 8–15 in.), and miniature dwarf bearded (MDB; less than 8 in.). In general, the smaller the plant, the earlier it blooms. (Although intermediate bearded irises are the same overall height as the chunkier border beardeds, they tend to bloom earlier.) Whatever the size of your irises, the process of dividing them is the same.
1. Dig up the clump Although you can use either a spade or a garden fork a fork will do less damage to the roots. Gently work around the clump, easing it out of the ground.
2. Rinse, divide, and trim With a hose, rinse off the soil that clings to the roots and rhizomes. Then, with a sharp knife, cut through the rhizomes, making sure that each piece has a fan of leaves and firm, healthy roots. Discard any mushy or insect-damaged rhizomes. Trim the foliage on each division to about one-third the original length. (This reduces water loss while the new plant is becoming established.)
3. Replant and water Prepare a planting site in full sun with fertile, well-drained soil. Form a cone-shaped mound of soil, and spread the roots around its sides. Cover the roots with soil and firm. The rhizome should be just below the surface. Water in well. It’s a good idea to mulch the plant its first winter to prevent frost heaving.