Check bearded Irises for borer problems

Monitor Irises for problems in mid-summer/ photo by Rose Field

Bearded Iris are a long-time garden favorite in our area, but they can develop troublesome problems. In mid summer gardeners need to be alert to the symptoms of a pair of maladies that travel together to attack Irises.

To understand what's developing, notice how bearded Irises grow from a rhizome, a thick underground stem. The process starts with the Iris borer, an insect that lays eggs on old Iris leaves in the fall. During this period of its life cycle, the borer is a dull looking and unremarkable brown moth, active at night and not easily noticed.

After spending the winter in the old leaves, the borers hatch in spring as small caterpillars. They live inside the Iris leaves, and as the leaves grow, the borers rise up with them. As they get larger, they eat a path of destruction down the leaves, leaving long vertical slits in the leaves. Some browning might occur on the leaf tips and some leaves may die.

In July, the voracious caterpillars arrive at the rhizomes and begin to tunnel around. By now they are pinkish tan and measure one to two inches long. The second part of the trouble strikes at this point, when bacterial soft rot begins. Think of a damaged potato in your pantry: an injury to the tuber's flesh would start to rot before long. If you've ever experienced the yucky discovery of a reeking bad potato, you know how bacterial soft rot can cause a nasty odor. If you haven't noticed the brown markings on the Iris leaves, you may catch a whiff of something amiss when near the Iris plant. Sometimes the leaves will look mushy or watery when damaged by bacterial soft rot and the whole plant may collapse.

The solution is not the most pleasant garden task, but if you wish to save the Irises, it's got to be done. I've spoken to many gardeners who have Irises grown from their grandparent's garden, and they don't want to lose them. Dig up the plant and shake off the soil from the roots. Back to the potato analogy, cut out any damaged tissue as you would remove spots on a potato. Cut the leaves back to a length of 4”-5 ”. Unfortunately, you might come across the nasty little borer itself.

Soak the rhizome in a solution of 10% laundry bleach and 90% water for about an hour to make certain to eliminate the bacteria, as well as any hidden caterpillars. Dry the rhizomes and dust with garden sulfur. Replant the same day. Any time you plant Irises, leave part of the rhizome above the soil.

Insecticides only work during a narrow window of this cycle, when the leaves measure about 5”-6” in height, and I would never advise using them. Take nature-friendly precautions, such as cleaning up the dead leaves in the fall so that the borer has no place to overwinter. Many gardeners opt to plant Japanese or Siberian Iris instead, as they aren't very susceptible.

Beneficial nematodes are a natural predator of the Iris borer. They are sold as a dust or on a moist sponge from organic gardening supply stores, and these microscopic critters can be applied to the Irises in two applications before flowering. The harmless nematodes multiply in moist soil and build up a population which attacks the borers.

I decided to switch to Siberian Irises years ago after a particularly messy borer/bacterial rot attack, but I still miss the longer bloom time and gorgeous colors of the bearded Iris.