Perennials: You can divide German bearded iris now through the end of September.
Cut the leaves back about halfway so the plants are easier to handle.
Yellowing foliage can be a result of infestation by iris borer larvae, and you may see small, roundish white pellets that are actually frass (larval feces) at the base of the leaves.
If you see that, dig up the rhizomes and check them for iris borers. The borers are mature at this time of year; they are pinkish and about 1½ inches long.
A factsheet on iris borer is available at insectlab.russell.wisc.edu/ under the “Resources” tab. Look under the “Factsheets” dropdown menu for the title “Iris borer” under the Flower Insect Pests subsection.
You will likely note a fair amount of feeding damage if you find iris borers. Remove them from the rhizome and dispose of them.
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Unfortunately, bacterial soft rot may have set in following borer damage. That further deteriorates the rhizome, leaving it mushy and foul-smelling. If there is too much damage, it is best to discard the rhizome as well.
However, if there is unaffected tissue remaining, cut off the infected material and wipe off the cut end of the rhizome with rubbing alcohol or hand sanitizer. Also disinfect the cutting tool with rubbing alcohol.
After the rhizome is cut back to healthy tissue, let it dry and callus off indoors for a couple of days. Then, replant it, making sure the rhizome is horizontal in the soil and about half of it is above and half below the soil grade.
Firm it in well so it doesn’t fall over and watch to be sure that squirrels don’t dig it up, as they are attracted to areas with freshly disturbed soil, especially at this time of year.
Some folks temporarily use landscape staples (also called sod staples), which are narrow U-shaped pieces of wire about 6 inches long, to hold the rhizomes in place until they re-root. They are available in most home improvement stores and garden centers. You could make your own out of wire coat hangers.
— Lisa Johnson, Dane County UW-Extension horticulture educator
— Lisa Johnson, Dane County UW-Extension horticulture educator
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