Reducing borers on bearded irises

By George McVey, Ph.D.
Contributing Writer
Union County Post

The following article was taken from The Ohio State University Plants Facts Web site.
Q. I grow bearded irises, but am frustrated with the borers that get into the rhizomes. How can I reduce or eliminate the problem?
A. Iris borer is the most serious pest of bearded iris. It overwinters in the egg stage attached to leaves. 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 reducing the rhizome to a hollow shell. Yellow leaves that easily detach 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 in mid-September to October. The moth lays eggs on dry plant material and other garden debris, then dies. Eggs overwinter until the following spring, when they hatch and the cycle repeats.
Ideas for borer control include eliminating the eggs by removing and destroying debris in and around the iris planting both in spring and late fall; digging rhizomes, and physically removing the borer, then replanting (usually done in late July or August); and treating leaves in spring with a labeled insecticide. Treat when new growth is 6 to 9 inches in height.
Q. What can I do to keep my perennial gardens looking nice all summer?
A. Keeping up with good maintenance practices may help improve the garden's appearance through hot, dry weather. Try to keep the garden irrigated with one inch of water a week, if no rainfall occurs. Apply water at the soil line, rather than irrigating overhead, which may lead to disease problems. If it isn't possible to water the entire garden, prioritize watering those plants that were installed this spring, either as new plants or divisions. In hot and dry weather, these plants will struggle to establish an adequate root system.
Most gardeners are aware of deadheading perennials (removing spent blooms) to encourage rebloom and reduce self-sowing. "Dead-leafing" is another maintenance chore that will help improve the garden's appearance. Foliage of many perennials suffers in the heat, and scorched, browned leaves are quite apparent on plants that can't tolerate the heat or may be exposed to a bit too much sun. Scorch often appears on many hostas, Alchemilla (lady's mantle), Heuchera (coralbells), some of the hardy geraniums, Brunnera, astilbes, Astrantia (masterwort), and others. Generally, only a leaf here and there needs to be removed, but this greatly improves the appearance of the plant.
A few perennials look ragged after flowering, including the daylilies, monarda (beebalm), and Shasta daisies. Often, the foliage of these plants declines and needs to be cut back. After daylilies finish blooming, dead-leaf the plants by pulling out the dead foliage at the base. If most of the foliage has declined, cut the plant back, and water to encourage new growth to fill in. Monarda also declines after blooming. Cut back stems to the base, where new foliage emerges. This new growth may only reach a few inches in height for the remainder of the season. Shasta daisy does the same thing; new foliage develops at the base after flowering is finished. Cut back old stems to this new growth.
The Master Gardener offers horticulture advice and tips, focusing on current issues, by volunteers who are with The Ohio State University Extension, Union County Master Gardener Program. For answers to gardening questions, call 937.644.8117 or e-mail mcvey.22@cfaes.osu.edu. Hours are 8 a.m. to 4:30 p.m. Monday through Friday.