The Q&A Archives: Preventing Iris Borers

Question: I live in zone 7 and am thinking about getting several beautiful African Irises (Moraea iridioides) for a large, shady patio pot.
I'm an experienced perennial gardener, but have never grown any Irises before. After hearing about the moth larva that can seriously
damage the plant, I would like to know what species are less commonly infested by the pest. If the African Iris is prone to Iris borers, could you please give me some information protecting them, so I can lower the chance of infestation?
Thank you! Any advice you could give would really help!

Answer: African Iris (Moraea iridioides, sometimes sold as Dietes iridionides) are generally not bothered by iris borers, so I wouldn't let the horror stories you've heard keep you from planting and enjoying them. Just to give you a little information on iris borers, the larvae feed on the iris rhizomes in mid to late summer, then leave the rhizome and live in the soil for the pupa stage. The adult moth will emerge in a few weeks and lay eggs on the iris. The eggs overwinter on the plants and hatch in April or May. Then they start feeding on the leaves, working their way back to the rhizomes again. At this point, lifting the rhizomes and cleaning them is probably going to stop the current year's damage. It's important to remove and/or destroy any borers you find. (Some gardeners pierce them with a piece of wire such as an unfolded paperclip.) For added protection, you might also sift the top few inches of soil and remove any that have already moved to that stage. Since bacterial rot may infect damaged rhizomes, any damaged parts should be cut away and the healthy pieces should be dipped in a bleach and water solution prior to replanting. (Use 1 part of household chlorine bleach and 10 parts of water.) The next most important thing to do is to remove and destroy all the iris debris during the fall garden clean up and again in the spring. By doing this you will remove most if not all of the eggs before they can hatch and will eliminate many of next year's generation of borers. While some chemical controls are available, they are applied in the spring while the larvae are in the leaves. Any spring larvae can also be controlled by simply squeezing the leaf between your fingers where you see feeding injury occurrring -- watch for this in May and June. Hope this answers any questions you have. Good luck with your African Iris!

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