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Yard and Garden: All about Irises

AMES, Iowa — Available in nearly every color, bearded iris is a beautiful spring flower. Gardeners with questions about irises and other garden plants may contact horticulturists with Iowa State University Extension and Outreach at Hortline, hortline@iastate.edu or 515-294-3108.

My bearded irises are no longer blooming well. Why?

The bearded irises may need to be divided. Bearded irises should be divided every three to five years, as the plants quickly become overcrowded and don’t bloom well. July or August is the best time to dig, divide and transplant bearded irises.

Lack of sunlight could be another possibility. Bearded irises bloom best in full sun (plants need at least six hours of direct sun per day for best flowering). Plants in partial shade may not bloom well and should be transplanted to a sunny site.

When and how do you divide bearded irises?

While bearded irises are easy-to-grow perennials, they need to be divided every three to five years. If not divided, plants become overcrowded and flower production decreases. Crowded plants are also more prone to disease problems. In Iowa, July or August is the best time to dig, divide and transplant bearded irises.

Bearded irises grow from thick, underground stems called rhizomes. Carefully dig up the iris clumps with a spade. Cut back the leaves to one-third their original height. Wash the soil from the rhizomes and roots with a steady stream of water. Then cut the rhizomes apart with a sharp knife. Each division should have a fan of leaves, a healthy rhizome and several large roots. Discard all diseased or insect damaged rhizomes.

Bearded irises perform best in fertile, well-drained soils and full sun. In clay soils, incorporate compost, sphagnum peat moss or well-rotted barnyard manure into the soil prior to planting. When planting bearded irises, dig a hole large enough to accommodate the rhizome and roots. Build a mound in the center of the hole. Place a rhizome on top of the mound and spread the roots in the surrounding trench. Then cover with soil. When planted, the rhizome should be just below the soil surface. Finally, water each plant thoroughly.

To obtain a good flower display, plant at least three rhizomes of one variety in a group. Space the rhizomes 12 to 24 inches apart.

How can I control the iris borer?

The iris borer is a serious pest of bearded irises. The mature stage of the iris borer is a grayish moth. Female moths lay eggs on iris foliage and other nearby plants in late summer/early fall. The eggs hatch the following spring. The small larvae (caterpillars) bore their way into the iris foliage and feed on leaf tissue. Over time, the larvae tunnel down through the leaves and into the rhizomes. The caterpillars continue to feed inside the rhizomes and eventually destroy much of it. When fully grown, the larvae move into the soil and pupate. Adults (moths) emerge in late summer.

Bacterial soft rot often invades rhizomes damaged by iris borers. Rhizomes infected with bacterial soft rot become soft and foul-smelling.

Iris borers can be controlled by sanitation and the timely application of insecticides. Remove and destroy dead iris foliage in late fall or early spring. This will eliminate many of the iris borer eggs. An insecticide should be applied in spring when the new shoots are 4 to 6 inches tall. An application of an insecticide at this time should destroy small iris borer larvae before they have the opportunity to bore into the iris foliage. Products that contain carbaryl, permethrin, acephate or spinosad should be effective. As always, carefully read and follow label directions when using pesticides.

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