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Gardening: Iris Borers

I’ve had irises in my garden as long as I can remember gardening, I received some of my iris tubers/rhizomes from my mother, who received them from a neighbor.  When we moved into Corcoran 7 years ago I happily transplanted  these irises into our front yard where they got a lot of afternoon sun. They did quite well in the well drained sandy and gravely soil the city had put there after some pipe work.
For the first couple of years they looked amazing.

Blooming irises, these where taken before they started becoming infected with borers.

Then, suddenly one year, in early July ( after flowering ) they just didn’t.
The leaves where turning brown and shriveling up. They seemed to be rotting, which could mean they were being over watered, but it was unlikely in my dry front yard. I dug up a couple of plants to check the rhizome, some of which had turned to mush. Luckily, in one I found a strange white caterpillar. After some googling, including some stomach turning image searches, it turned out this unwelcome guest was an iris borer.
Iris borers are the caterpillar stage of a moth that lays it’s eggs at the base of the plants in fall and after hatching in the spring. They work their way up the plants where they then bore into the leaves and begin to eat their way down to the roots.
I’d always found irises to be an easy perennial to care for, until I had to start dealing with these guys. I’m guessing they have found enough plants in this area to return yearly to wreak havoc on me and my neighbors gardens. Because irises are tough and the borers don’t always kill the whole plant, you might not know you even have them.

Eww, I Don’t Want Those or Prevention: How to avoid even getting borers

In spring clean up plant matter around the base of your plants. This will remove any eggs laid the previous fall. This is a good idea anyway, as too much rotting leaves and plant matter can cause the rhizomes to rot too.
Keep an eye on your plants as they are beginning to flower, and especially as they are finishing. Look for brown spots in the leaves where the caterpillar may have entered. If you aren’t sure gently spread the leaves apart and look for damage specifically on the inner leaves. You can also give the center leaf  a little tug, if it comes out easily and is brown, mushy or chewed on looking on the bottom end, it’s probably borer damage.
I start cutting plants that are showing damage, down to 3 or so inches. If you are lucky you’ll catch the caterpillars before they’ve gotten too close to the root.  Anything that you cut off, should be thrown away or burned. Do not put in your compost or somewhere else in the yard.

A tiny iris borer
This is a young iris borer. They’ll usually be found still in the leaves and not in the rhizome yet.
Inner leaves of iris showing sign of a borer eating them and traveling down to the root.



Come fall, after the first real frost and after the moths are done laying eggs, again remove extra plant matter from around the plants. This will, hopefully, remove any new eggs and it may also be too late in the year to get more. If you’d like to put down something to protect your plants through winter, try fresh straw and remember to clean up again in the spring.

Well Darn It, I think I have them: How to treat your Plants

If you find evidence borers have moved into the lower plant, it’s time to get dirty. Go ahead and dig the whole tuber out.
The U of M Gardening page suggests just throwing them away. You can find that information here:

However, if you are stubborn like me, you can do this: Cut the leaves down to 3 or so inches. Again, throw away these cuttings.  Now take the infected rhizomes and submerge them in a bucket of water, the deeper the better. This will cause the caterpillars to flee the rhizomes and if the water is deep enough they’ll drown. (I always feel awful about this part so I usually walk away and don’t watch.).  Leave the rhizomes to soak for about 24 hours, but not too long as they might begin to rot.
After removing your plants, let them dry and then check them for damage. Cut away any thing mushy and dead and check to see if the caterpillar is still inside the tuber. You can again soak him out, poke or shake him out. I often check for the number of dead caterpillars against the number of rhizomes I had soaked. Usually there is only one in each plant. If there aren’t many caterpillars,  I know to very closely re-check the rhizomes.

More mature caterpillar. probably found in the lower leaves or inside the rhizome.
A mature caterpillar that has been flushed out of the rhizome.

I usually will let the rhizomes dry anywhere from a day to as long as a week or two. If it’s going to be on the longer side, keep the plants in a dry and ideally cool spot.

Once the rhizomes are clear of infection, go ahead and replant them. If your garden bed is a bit crowded, it is a good chance to spread the irises out to give the roots more space or start a new bed. When you dump  the bucket, make sure to do it into a fire pit or the alley where any surviving caterpillars won’t come back.

I have read things about treating your soil to kill the caterpillars, but personally, I prefer to avoid chemicals as I have pets. With the heat, drought and my own forgetfulness to clean out the garden this spring, my plants got hit early and pretty hard. It’s best to just check your plants regularly. If more of us in Corcoran treat our irises, the area will hopefully become less inviting to the borers.


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