Yard and Garden: Iris

June 3, 2011, 8:31 am | Richard Jauron, Willy Klein

Bearded iris is a beautiful spring flower, available in nearly every color. Each flower is made up of three upright petals (standards) and three drooping petals (falls). Each fall has a fuzzy, beard-like growth at its base. 

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 crowded and don’t bloom well. Mid-summer (July and August) is the best time to dig, divide and transplant bearded irises. 
 Iris Flower
Lack of sunlight could be another possibility. Bearded irises bloom best in full sun. Planting sites should receive at least six hours of direct sun each day. Plants in partial shade may not bloom well and should be transplanted to a sunny site. 

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, the best time to dig, divide and replant bearded irises is in July and August. 

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 forceful stream of water from the garden hose. 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. The planting site should receive at least six hours of direct sun per day. 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 do 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, eventually devouring much of the rhizome. 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 fall or very early spring. This will eliminate many of the iris borer eggs. Also, apply an insecticide in spring when the new shoots are 4 to 6 inches long. An application of an insecticide at this time should destroy small iris borer larvae before they have the opportunity to tunnel into the iris foliage. Products that contain acephate, permethrin or spinosad should be effective. As always, carefully read and follow label directions when using pesticides. 

There are small, brown spots on the foliage of my bearded irises. What is the problem? 

The small, brown spots are likely due to iris leaf spot. Iris leaf spot is a common disease of bearded irises. The disease is caused by the fungus Mycosphaerella macrospora. The first symptom of iris leaf spot is the appearance of small yellow-brown spots on leaves that are surrounded by water-soaked borders. The spots eventually turn grayish brown, develop reddish-brown borders and grow in size. As the spots grow, several spots may coalesce and destroy entire leaves. Iris leaf spot is most severe when the weather in spring is cool and wet. 

Good cultural practices will reduce the severity of iris leaf spot. Remove all diseased leaves and flower stalks in fall or early spring. (The causal fungus survives the winter on infected plant parts.) Plant bearded irises in full sun, space plants adequately, control weeds, and avoid wetting plant foliage when watering. 

Fungicides will likely be necessary to control iris leaf spot on plants that were severely infected the previous year. The first application should be made when the leaves first appear in spring. Continue to spray as directed on the product label. Effective fungicide products include those that contain chlorothalonil and myclobutanil. Since bearded iris leaves are waxy, add a spreader-sticker to the fungicide to make sure the fungicide adheres to the iris foliage. 

Irises from Iowa State University Extension.

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