Extension News

Dr. Grow-It-All Talks Iris

6/18/2007

By James Romer
Horticulturist
Iowa State University Extension

Dear Dr. Grow-It-All;
A friend and neighbor grows some of the most beautiful iris I have ever seen! The rest of the residents in the area and I have dubbed her the Iris Lady. 

Dr. Grow-It-All, I just found out that she is moving to Arizona in late July and has sold her house. I don’t have to tell you that Arizona and iris are not made for each other. I’m sad to see her go, but the exciting news is that she is giving me first dibs on her iris. Now doesn’t seem like the best time of year to be transplanting any plant. Dr. Grow-It-All, I need your help.

April in Paris

Dear April,
Wow!  Did someone write a song about you? Anyway, bearded irises are popular, widely grown perennials.While bearded irises are beautiful, they do require more maintenance than many perennials. One important task is to divide bearded irises every three to five years. If not divided on a regular basis, the plants become overcrowded and flower production often decreases. Crowded plants are also more susceptible to disease problems. The best time to dig, divide and transplant bearded irises is in July and August.

Bearded irises grow from underground stems called rhizomes. Carefully dig up the iris clumps with a shovel. Cut the leaves back to a third of their original height. Wash the soil from the rhizomes with water and cut the rhizomes apart with a sharp knife. Each division should have a fan of leaves, a healthy rhizome, and several roots. Discard the old, leafless rhizomes in the center of each clump. Also, discard any diseased or damaged rhizomes.

Bearded irises perform best in fertile, well-drained soils in full sun. While they tolerate light shade, maximum flower production occurs in full sun. Bacterial soft rot is often a problem in wet, poorly drained sites. These sites often can be improved by incorporating organic matter, such as peat or compost, into the soil prior to planting. Raised beds are another option for gardeners with poorly drained soils.

When planting bearded irises, dig a hole large enough to accommodate the rhizome and roots. Construct a mound in the center of the hole. Place a rhizome on top of the mound, spread the roots in the surrounding trench and then cover with soil. When planted, the rhizome should be just below the soil surface. Finally, water each newly planted division thoroughly.

To obtain a good flower display, plant three or more rhizomes of one variety in a group. Space the rhizomes about 12 to 24 inches apart. Point each fan of leaves away from the other irises in the group.

Newly planted bearded irises are susceptible to injury their first winter. Repeated freezing and thawing of the soil during the winter months may lift the rhizomes out of the soil and damage or destroy them.

To prevent damage, cover the plants with several inches of straw or pine needles in late fall. Remove the mulch in early spring. The transplanted irises will bloom sparsely the first spring. The plants should be in full bloom in their second and third years.

The most serious bearded iris pest is the iris borer. It overwinters in the egg stage attached to garden debris. The eggs begin to hatch in late April. The tiny caterpillars crawl up iris leaves and begin chewing and mining their way down within the leaf fold, eventually reaching the rhizome.

Once in the rhizome, they continue to feed, sometimes hollowing out the rhizome. Leaves which yellow and are easily detached indicate damage to the rhizome. About mid-August the fully-grown borer crawls out of the rhizome and pupates in the soil. Pupation is completed in about one month when the drab, gray-brown moth emerges (mid-September to October).

The moth lays eggs on dry plant material and other garden debris, and then dies. Eggs overwinter until the following spring, when they hatch and the cycle repeats.

Here are two ideas for borer control.

  • Eliminate the eggs by removing and destroying debris in and around the iris planting in late fall or early spring.
  • Spray the bearded iris with an insecticide in spring. Treat when new growth is 6 to 9 inches in height.

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Contacts :

James Romer, Horticulture, (515) 294-2336, jromer@iastate.edu

Jean McGuire, Extension Communications and Marketing, (515) 294-7033, jmcguire@iastate.edu