Irises are among the easiest of perennials to grow, and they give an abundance of beauty with minimum care. The iris has a thick fleshy root called a "rhizome" (pronounced rye-zome) about like a tough potato in texture. When you buy a new iris, you will probably receive a rhizome with clipped roots and leaves. It can remain out of the ground for a week or two without serious harm, but the sooner it is planted, the better.
To plant your irises, choose a sunny spot in well drained soil. Prepare the soil well, by spading or turning over the soil with a garden fork to a depth of at least 10 inches. Spread fertilizer and work it into the top of the soil. If possible, this should be done 2 to 3 weeks before you are ready to plant. A well prepared bed will result in better growth and more bloom. Donít starve your irises or make them compete with nearby grass or weeds for food and water. Many gardeners, iris and otherwise, have soil analyses made of their garden soil, then add the fertilizer of the kind and quantity the tests show the soil needs.
The soil should be light. If it is clay soil, add very coarse sand and humus. Bone meal and a good garden fertilizer, low in nitrogen, are good for irises, but manure should be used only after it has aged for about a year. Otherwise, it may cause rot. The roots must be buried firmly to hold the plant in place, but the rhizome should be near the surface. An easy way to achieve this is to dig two trenches with a ridge between them, place the rhizome on the ridge and spread the roots carefully in the trenches. Be sure to firm the soil tightly and allow enough for settling to keep the rhizome above any possible standing water. Then fill the trenches with soil, letting the top surface of the rhizome be just barely beneath the surface of the soil.
If you have several plants, plant them at least a foot and a half apart, "facing" the same way. The rhizomes will then increase in the same direction, without crowding each other too soon.
From the new parts of the rhizome, new bloom stalks will come up in later years and the flowers will be exactly as the original flower. This is called "vegetative propagation".
In about 2 or 3 years, the new rhizomes will begin to crowd each other and you will want to divide the plant. After digging the clump, divide into separate rhizomes with a knife. You will have plenty to replant as well as to give to all your friends!
Courtesy of the American Iris Society