Dealing with Iris Pest Problems

The familiar bearded Iris is a much loved garden plant, but it comes with some nasty problems that appear in mid to late Summer. The most common trouble is a one-two punch delivered by first an insect, and then a subsequent bacterial infection. Here's some ways to spot the infestation and deal with it.

Bearded Iris grow from a fleshy underground stem known as a rhizome. Unfortunately, this is quite tasty to the Iris borer, an insect which begins the cycle of destruction.

The female Iris borer lays its eggs in old Iris leaves in August and September. At this time in its life cycle, it's an ordinary looking brown moth which is nocturnal and not usually seen.

The eggs overwinter in the Iris debris and hatch in the Spring. As the new, growing leaves go up, so do the little caterpillars inside. As they mature, they begin to eat their way down the leaf. You might see long slits in the leaves caused by these worms. It looks as if a cat scratched the leaves. After a few weeks, the leaf tips look brown and some leaves die.

Around July, the Iris borers have worked their way down to the rhizome and by this time they are about two inches long and pinkish tan. This is only the first half of the Iris dilemma, because at this stage, bacterial soft rot attacks. Just like with humans and animals, an injured plant is prone to infections which compound the difficulty.

Bacterial soft rot causes the leaves to look water soaked and mushy, Often they collapse and become a stinky mess. The rotten smell is a tell-tale sign of bacterial soft rot in Irises. To deal with this disease and the borers both at once, dig the entire Iris plant up and shake the dirt off the rhizomes. Examine the rhizome for visible rotten spots and cut them off. Small tunnels may be visible.

Soak the whole rhizome is a container of 10% laundry bleach to 90% water. Let them sit in this solution for about an hour. This should drive out the caterpillars and kill the bacteria. Cut the leaves back to 4"-5" in length. Let the rhizome dry, and for insurance, dust it with garden sulfur. Replant them, keeping the rhizome high in well drained soil to guard against bacterial soft rot in the future.

Yes, this sounds like a lot of work and trouble, but sooner or later, most bearded Iris growers have to contend with Iris borers. I would rarely recommend dangerous insecticides, and here they could really be misused. They're only effective during a very narrow window of the caterpillar's life cycle, in early Spring, when the leaves are about 5"-6" tall. Application at any other stage is simply a waste of money and a needless, ineffective use of poisons.

There are solutions to the Iris borer problems. Many gardeners have switched to planting Siberian or Japanese Iris, which are far less susceptible. Careful Fall clean up of all old leaves destroys the next season's crop of Iris borers. The female will be done laying eggs by the first hard frost.

Organic garden supply sources sell beneficial nematodes, tiny microscopic worms, which are the natural enemy of Iris borers. Don't worry, they're not creepy to use; they simply look like a dust, or sometimes are sold on a moist sponge. They must be applied in two applications, a week apart in early Spring before the Iris flowers. The nematodes need moist conditions to survive and will quickly build up a population to attack the caterpillars.