Flowering starts in early May and continues into June. Irises are generally easy to grow. Most of them grow best when they get full sun six to eight hours a day. Some shade in the afternoon will help keep flower colors from fading in the heat. Excellent drainage will help in preventing bacterial soft rot. Places with good air circulation will also help prevent other disease problems.
As a general rule, irises should be divided every three to five years. Iris not only flowers poorly when crowded, but is more susceptible to disease and attack by iris borers.
One of the best times to divide them is immediately after the blooming season is completed. When clumps are divided right after blooming, the new plants will be well established before winter and ready to produce a fine crop of flowers next spring.
When preparing to replant an iris, cut the leaves back to about 6 inches. Then, with a pointed shovel, dig around all sides of the plant and lift the entire clump out of the ground. Wash away the soil. With a sharp knife, cut off the short side branches that have a cluster of leaves at the end. Discard the old center portion of the clump.
Inspect each rhizome for signs of insect feeding or decay. The most serious insect pest of the iris, the iris borer, is often found at this time. Check the rhizomes for the pinkish, 1¼-inch larvae with a row of black spots down the sides, and check the soil for the inch-long borer pupae. Destroy any that are found. Rhizomes infested with borers may also be infested with bacterial soft rot. Remove diseased areas with a knife. If extensive rot is present, discard the entire rhizome.
Replant the new divisions after cutting the feeder roots back to about two inches. If feeder roots are broken, new roots will develop quickly. Set rhizomes 12 to 15 inches apart. Plant in well-drained soil, and cover about one inch deep. In heavy or poorly drained soils, the top of the rhizomes should be exposed to reduce the possibility of rot.
After you complete the planting operation, apply a side dressing of 5-10-15 fertilizer (or an organic equivalent) at the rate of about one pound per 100 square feet. Scratch the fertilizer into the top inch of soil and water thoroughly. Then cover with a light mulch to conserve soil moisture.
REACH SID MULLIS, THE DIRECTOR OF THE UNIVERSITY OF GEORGIA EXTENSION SERVICE OFFICE FOR RICHMOND COUNTY, AT (706) 821-2349 OR SMULLIS@UGA.EDU.