Iris are among the few perennials that should be divided and transplanted during the warmest months of the year. This should be done in mid- to late summer so that the plants have plenty of time to re-establish themselves well before winter. If your iris blooms have begun to decrease in size and number, or the clump appears to be overcrowded, it’s time to divide. Otherwise, your plants will be deprived of nutrition, and the rhizomes may even begin to die. For best results, plan on dividing your iris rhizomes about every 3-4 years.
Start about 6 inches away from iris stalks. Use a spading fork to loosen the soil all the way around the clump to be divided. Do your best not to cut any of the rhizomes situated around the outer edges. Slowly lift the whole clump straight upward. Shake as much soil as you can from the roots and rhizomes.
Use a clean, sharp knife to cut the rhizomes apart. Make sure each new rhizome is about 3-4 inches long, has a fan of healthy leaves, and some roots. Cut the leaves back to about 6-8 inches tall. Toss the old center rhizome onto the compost heap. Also discard any rhizomes that feel soft, or that show signs of disease or decay.
Carefully examine the rhizomes for the presence of any caterpillars or borers. If you see any, pick them off with your fingers and squash them.
Soak the rhizomes in a 10 percent household bleach solution for 30-45 minutes to help prevent any possibility of infection Set them aside in the shade so that the cuts can heal and the rhizomes can dry for a few hours.
Dig a shallow hole in a sunny, well-draining location. Pile a little mound of soil in the center for the rhizome to sit on, and spread the roots downward over the mound and outward into the hole. Position the rhizome so that it will be planted about an inch below the surface of the soil. Firm the soil carefully around the roots and rhizome. Space the plants about 18-24 inches apart.
Water the planting site so that the soil is uniformly moist, but not wet or soggy. Feed a half strength water soluble fertilizer.