Last year, I lost most of my irises. The leaves turned brown and the rhizomes were soft and mushy. What caused this and how can I keep it from happening again?
—Whynona Fenske, Wausau, Wisconsin
Melinda: The iris borer is your culprit. To control it, it's best to understand its life cycle. This day-flying moth lays its eggs in the dried iris leaves each fall. The eggs don't hatch until spring, when the new leaves are 4 to 6 inches tall. The borer then enters the leaf and eats its way to the rhizome, where it feeds in early summer. The borer forms a pupa in the soil, and emerges as a moth in fall to start the process all over again. So the best place to start is by cleaning up the old iris leaves in fall—with any luck, this should send the moths looking for a better place to lay their eggs. Also, whenever you dig and divide your irises, remove and destroy any borers you find. Then cut out and discard damaged portions of the rhizome, replanting the healthy part for future blooms.