Irises (Iris spp.) flower in early to midsummer, producing multiple blooms on each stalk. They grow in U.S. Department of Agriculture plant hardiness zones 3 through 9, depending on the variety. Irises usually bloom with no help from you, but problems with the growing site or overcrowding can reduce flowering.
Plant Them Right
Irises need full, all-day sun to grow and produce blooms. Too much shade can prevent flowering, so make sure you choose a bed that isn't shaded by buildings or trees. An area with well-drained, slightly sandy soil with a southern exposure usually provides the best area for successfully growing irises. Heavy, wet soils cause iris roots to rot, so avoid those. Plants suffering from the beginning stages of root rot may fail to flower or the buds may rot without opening. Eventually the roots and the entire plant die.
The Right Fertilizer
Irises don't generally require any fertilizer after they are planted, but you can fertilize lightly after flowering if they bloomed poorly. Too much nitrogen-rich fertilizer results in abundant and healthy leaf growth but few flowers. Avoid high-nitrogen blends if your iris is flowering poorly. A low-nitrogen, high-phosphorus fertilizer, such as a 5-10-5 blend, is a better choice for blooms. Sprinkle 1/2 cup of the 5-10-5 fertilizer around each iris clump, about 6 inches from the base of the plant, and water it in immediately after application.
Thin Them Out
Overcrowded iris plants compete for moisture, nutrients and sun, which often results in poor or weak flowering. Get the irises to bloom by digging up the roots and dividing them. Irises need to be divided every three or four years. Dig up the rhizomes after flowering and rinse them off with water. Dispose of any damaged or rotten rhizomes, and then cut the remaining ones apart into 4- to 6-inch pieces, so each piece has a fan of leaves. Trim the leaves down to 4 inches tall and replant the rhizomes so the top of the root is just above the soil surface. Space the rhizomes 18 to 24 inches apart to prevent overcrowding.
Irises sometimes flower a second time in late summer if you cut off the earlier flowers before they set seeds. Leaving spent flowers on an iris can also weaken the plants and reduce flowering the next year, because the irises expend energy fueling seed production instead of storing energy and nutrients for future flower production. Wipe a pair of shears with a cloth soaked in rubbing alcohol to disinfect them, and then cut back the flower stalk to its base after it finished blooming to help the iris flower better next time.