Bearded irises are just one of more than 200 iris species. They have underground root structures called rhizomes that grow horizontally and self-multiply. In about three to four years, a garden can become overcrowded with bearded irises. Overcrowding not only looks overgrown and crowded, but the bearded irises bloom less than they did in years past. To control the growth of bearded irises and to keep a well tended, neat and thriving garden bed, you must dig up, divide and replant the rhizomes every three to four years.
Cut down the foliage to only one-third of the usual height. Use clean, sharp pruning shears. Wait until mid-summer or early fall to start this process.
Dig up the clumps with a fork or spade. The rhizomes should be just below the soil's surface. Then, remove the excess soil with your hands and examine the roots.
Dip a sharp knife, such as utility knife, in a water-and-bleach mixture that is made of 10 percent chlorine bleach. Do this after each cut so you do not inadvertently spread disease.
Cut off and discard old, large and rotted rhizomes, which are usually at the center of the clump. Cut off new, smaller rhizomes to replant. Leave the remaining foliage on.
Replant in a sunny location and only cover the rhizomes slightly. Fan out the foliage and make sure the leaves are all facing the same direction. Space rhizomes 18 to 24 inches apart to give them plenty of space to grow before you have to divide them again.