Irises are perennial flowers that grow from a swollen bulb called a rhizome. They need to be divided every three to four years in order to continue producing lots of flowers in late spring to early summer. Irises come in colors ranging from white to yellow to shades of blue and purple, including multicolored varieties. Plant them in perennial borders along with other late spring-blooming flowers, such as roses, peonies, daisies and poppies.
Remove the flower stems by cutting them off at ground level. Cut the leaves back so they are 1/3 of their former height.
Use a garden spade or digging fork to dig up your iris rhizomes. Start a few inches from the base of the leaves and loosen the soil by rocking the spade or digging fork back and forth until the rhizome is loose. Remove it from the soil with your hands.
Mix 1 cup of chlorine bleach with 2 1/2 quarts of water.
Cut the rhizomes apart with a knife, dipping the knife into the bleach solution between each cut to prevent spreading disease or pests. Separate the rhizomes so that each section has a piece of root and a fan of leaves. Discard the woody center of the clump of rhizomes and keep the newer ones that have grown around it.
Store the cut rhizomes in the shade while you improve the soil in their planting bed.
Spread a 1- to 2-inch layer of compost on the iris bed. Sprinkle with all-purpose granulated fertilizer, applied following the manufacturer's recommended quantities. Dig this into the soil using a spade or garden fork. Rake the area smooth.
Dig a 5-inch hole. Add some soil to build a small mound that tops off just below the surface. Set the rhizome on top of the mound and spread the roots out. Cover the rhizome with soil so it is just at the surface of the soil, or slightly below. Do not plant iris bulbs too deep or they will rot.
Plant the iris bulbs 18 to 24 inches apart in groups of three.
Water well after the entire bed has been replanted.