Irises are hardy perennial herbs that grow from rhizomes and quickly spread as the plant ages. The flowers generally bloom on tall stalks, although there are dwarf varieties available. In mass plantings, irises provide an attractive background for spring and summer annuals and are good for cut flower arrangements. Iris is a low-maintenance plant, asking only for well-drained soil and a clean garden bed. Plant the iris rhizomes in July, August or September if you live in northern regions of the country or in September and October in the southern latitudes.
Decide where you would like to plant the irises. Keep in mind that this plant needs sunshine for half the day, so an area with morning sun and afternoon shade is ideal. If you live in the northern part of the country, you can get away with a full day of sun. Do not plant the irises in depressions in the landscape where water can sit and rot the rhizome.
Perform a soil pH test. Although you can do this yourself with a home testing kit, it's a good idea to take a soil sample to the county cooperative extension office for analysis. The agents there can recommend suitable amendments and give suggestions on pesticide control in your region. Irises thrive in a slightly acidic soil (6.8) but tolerate more alkalinity as well.
Aerate the soil in the planting bed by tilling it to a depth of 12 inches. If you don't own a tiller, a long-handled gardening fork will work well for this task. Dig into the soil, turning it and breaking up lumps until it is light and airy.
Add a 3-inch layer of compost and any amendments suggested by the results of the soil pH test. Use the gardening fork to mix them into the soil to a depth of 6 inches.
Use a bulb planter or a gardening trowel to plant the rhizomes. They should be planted with the roots down and the tops just barely exposed above the soil. Plant the rhizomes 12 to 24 inches apart, depending upon the effect you wish to achieve in the garden. If the weather is particularly hot at planting time, cover the tip of the rhizome with 1 inch of soil. Press the soil firmly around each rhizome, and water the area just enough to settle the soil. It should be just slightly moist.
Place a 2-inch layer of mulch around the iris plants when they begin sprouting. Spread the mulch completely around the plant. Mulch helps to conserve moisture in the soil and discourages weed growth.
Water the new iris bed to keep it moist. Rather than frequent small applications of water, it is best to provide long, slow irrigation, such as with a soaker hose. Once the plants are established, they seldom need water, unless you live in a dry area.
Fertilize the irises with a 6-10-10 formula, at half the strength recommended on the package, in early spring and again one month after the plants bloom.