In cold-winter areas, divide bearded irises in mid- to late summer, so the new plants will have plenty of time to become established before freezing weather arrives. Gardeners in milder climates can safely delay this project until September or even October, which will allow the divisions to settle in more comfortably after the heat of summer.
1. DIG UP THE CLUMP If the soil is dry, water the bed thoroughly a day or so before digging. When you have more than one variety scheduled for division, it is wise to deal with each in turn and to label each clump to avoid mix-ups. Use a spading fork to loosen the soil around and under the clump, taking care not to cut into the rhizomes growing near the edge. Lift the entire clump out of the ground and shake off or wash away any soil clinging to the rhizomes and roots.
2. MAKE DIVISIONS The clump will consist of older, spongy rhizomes with lighter-colored young ones growing from their sides. Cut the young rhizomes away from the older segments with a sharp knife. Discard the older pieces and any parts that are undersize or diseased.
In some areas of the country, bearded irises may be infested with borers, pinkish-colored larvae (or grubs) with brown heads. These creatures tunnel into and devour rhizome tissues, leaving a wound open to infection by bacterial soft rot. Extract and kill any borers you find, and cut away all damaged tissue.
To reduce moisture loss, trim the leaves to about a third of their original height. Each division should consist of a vigorous, firm rhizome and a fan of healthy leaves.
To help prevent infection, soak the rhizomes for about half an hour in a 10 percent solution of household bleach, followed by a dusting of powdered sulfur. Then lay the trimmed plants in a shady place for several hours to allow the cut ends to dry and heal.