I am giving up on my iris - after a year of treatment they still have borer damage and the fancy iris have died leaving me with only the plain purple ones. So I am going to dig them up and throw them away. What should I do about the soil with the borers still in it. I won't plant iris for a while and not there again ever. Is there anything that isn't too harsh chemical wise to kill the borers in the dirt? Could I plant lambs ear in this space with no problem? Another bed where I moved the iris to see if they would come back "clean" is where I want to plant raspberries. I did not treat this area and plan to dig up the soil anyway. Is there anything else I should do? Thanks for your help on these three questions.

 

If you see what looks like tunnels in the iris leaves, you have iris borers. A moth lays eggs on the rhizomes in the fall. They are safe beneath fall leaf litter and mulch. In spring the eggs hatch, and the larvae climb up the plant and bore into leaves and work their way down into the rhizomes where they feed. When through feeding they drop out into soil and pupate until mid to late fall when they emerge as a moth and start the cycle all over again.

In mid August, you must dig up the iris that are showing symptoms. Cut leaves back to 3-4 inches in a fan shape. Wash dirt off rhizomes. Inspect. Discard rotten rhizomes. (You may see maggots feeding on decaying organic material.)

Look for holes and cut off affected sections of rhizomes. Or, if you like adventure, you can drop rhizomes with holes into bucket of 10% bleach and water solution. This is easier than trying to cut with a knife. Within the next hour or two, you can see the borers exit and drown. Let rhizomes dry out and then plant after dusting with fungicide or bulb dust.

At this time the borers are pupating in soil and look like reddish, brown, segmented cases. Treating soil with insecticide while pupating will have no effect on iris borers. They are protected by their covering.

It is very important to clean out iris bed in fall. Remove leaves and stalks. You don't want anything in the iris bed which can provide protection to overwintering iris eggs. If you mulch to provide winter protection of iris, wait until after ground freezes or the first snow. Then, remove it in the spring.

In spring when iris fans reach 5-6 inches while borers are in caterpillar stage, spray with systemic insecticide. Systemic insecticides are absorbed by leaves and into rhizomes where it will kill the borers.

Tall bearded iris are most at risk because their rhizomes are not so deep in the ground as other iris varieties and thus more accessible to the borer moth that lays eggs on rhizomes.

By being a strict mother to your bearded iris, you can nurture some healthy children. You must be a good housekeeper and clean all the debris out of the iris bed, including iris leaves and stems before putting iris to bed for winter. If you want to give the babies a blanket of mulch for winter protection, do so after first freeze or first snow. When the little ones awake in the spring and become 4-6" tall, apply systemic insectide to kill the borers that hatched from overwintering eggs. In early to mid August, the babies get a physical. Inspect for symptoms, dig, give bleach bath if there are holes, let dry, powder with fungicide, and return to bed. Before you know it, your bearded babies will be thanking you with a grand show of color.

 

From this site:
http://www.bachmans.com/tipsheets/in...irisborers.cfm
Also from this site:
http://www.bachmans.com/tipsheets/in...irisborers.cfm
I would suggest beneficial nematodes as soon as the soil warms in spring as a precaution, along with a good clean up of any debris from the iris.
http://www.bachmans.com/retail/tipsh...ficialNema.cfm

Newt

 

Nematodes do offer a non-toxic, environmentally safe way to control iris borers. Survival of nematodes is a concern because studies show that chances are better in sandy or sandy-loam soils with adequate moisture, and temperatures between 60 and 70°F. Chances of survival are less in clay soils with excessive moisture, and where there are extremes of high and low temperatures. Too, there are mites, fungi, and other nematodes that prey on juvenile nematodes. Consult with your local Cooperative Extension Agent re: survival of nematodes in your area. He can also help with soil test to determine soil type and make recommendations for any soil amendments that may be needed.

 

Thanks so much for the help. I have never used nematodes so this will be a learning experience.

 

Please report what the Cooperative Extension Agent says. We have had few discussions re: nematodes here. Your information and experience should prove helpful to all.