Sleuthing in the garden-Pacific Coast irises

In 2019, I was very excited to finally find some yellow Pacific Coast iris hybrids. I had been wanting to add the native yellowleaf iris (Iris chrysophylla) to my garden for years, but I had never seen them for sale. There are plenty of our native purple Oregon iris (Iris tenax) in the Douglas-fir plantations around our property and these have always thrived when transplanted into our garden.

Native Oregon iris in a Douglas-fir plantation
Purple Iris tenax
Pale blue selection of Oregon iris
Pale blue Iris tenax

So, I jumped at the chance when I found some beautiful hybrids of Iris chrysophylla in shades of white, yellow, and rusty orange. Yes please!

A yellow and white Pacific Coast iris hybrid
Healthy yellow and white Pacific Coast iris hybrid

However, I was soon disappointed with the performance of the iris from one of the nurseries. The plants limped along during the first year and one died.

Too many yellow and brown leaves on this Pacific Coast iris hybrid
Brown and yellow leaves on new Pacific Coast iris hybrid

In 2020, they started out strong and looked healthy. I thought…Good! It was just transplant shock all along…  I remembered that several people had told me that Pacific Coast irises don’t transplant well, which always puzzled me, because I have never had any problems with Oregon iris that I have literally ripped out of a hard-packed logging road and transplanted into my garden, regardless of whether it was May or Mid-August. Yet, here was the proof, right? Or, maybe Iris chrysophylla hybrids are more sensitive to transplanting than Iris tenax? Who knows.

Pacific Coast iris hybrid with brown and yellow leaves
Pacific Coast iris hybrid looking sad

However, after some wet weather in late May, they were back to looking like crap again. I was starting to think that my plants weren’t suffering from transplant shock at all (especially when it was more than a year after transplanting), but perhaps it was something more sinister. My first thought was that they had Phytophthora root rot. I hoped that the symptoms would go away with the onset of drier summer weather. The symptoms did go away eventually and the plants looked much better by late summer 2020.

This spring, the plants still looked good. They were pushing new growth and thriving. I was elated and thought perhaps that they had outgrown their problem. But no, sure enough, starting in early May I began to see this.

And this. See those yellow and brown leaves in the center of each plant? Those are brand new leaves. They should be green and healthy, not brown and dying. Usually it is the older leaves that die each year. New growth should never die unless something is seriously wrong.

I decided to tug on one of the new leaves that were turning brown and it easily slipped right out. Closer investigation at the bottom of the leaf revealed some holes and notches. Phytophthora root rot doesn’t do that.

I dug one up and looked even closer. All the new growth was shriveling up and dying!

And, see that sawdust looking stuff? That’s frass – dry, crumbly insect poop that is typical of borers. At this point, I was starting to suspect iris borer (Macronoctua onusta), but I didn’t find anything online that says it occurs in Oregon.

And then, I found this crawling in the soil. Definitely a larva. It didn’t look like our usual cutworms, but I didn’t pull it directly from the iris either, so I wasn’t sure this was the culprit.

Stupid iris borer with a reddish-orange head capsule
Iris borer (Macronoctua onusta)?

Next, I poked around in the iris leaves and rhizomes. I found very clear evidence of tunneling and then, this popped out! The same darn larva. A quick search online showed some pictures of iris borer that looked like this, but other pictures that look completely different (large, fat, pink larvae). Maybe I just found younger larvae that haven’t turned pink yet? However, the description of the damage caused by iris borer is the same as what I observed. Any entomologists out there care to venture a guess on its identity?

At this point, I was freaking out a little. I like iris. I have them in my yard. And I am starting to think…Oh No! have I accidentally introduced this pest into our area and will they spread out into the forest and decimate our local native iris populations?… I immediately dug up all of the irises that I had purchased from that nursery, including a purple hybrid from a few years earlier that had never thrived. All of them had the same symptoms with new leaves that were dying, the same borer tunnels, and the same larvae.

Could I save them? I shook the soil into a bucket, picked off all of the damaged tissues, and then put all this into a plastic bag for the dumpster. Everything that was left that looked salvageable went into a bucket of water. I hoped this would drown the larvae and then I could rehabilitate the iris by growing them for a year or two in my greenhouse. There, I can keep a close eye on them for any new symptoms of borer damage. I am also going to be monitoring the other iris in my yard very closely.

I did find a few larvae floating in the water after soaking the plants overnight.

I can only imagine how many other infested plants might have already been sold and are being planted out into people’s yards right now. At some point, I wonder about the potential for this pest to affect our native iris. They obviously have no problem eating the hybrids, so I suspect they would also enjoy our native I. tenax and I. chrysophylla.

I think this just goes to show that if a plant is not doing well in your yard, it might not be your fault. So many gardeners assume that if a plant looks sick or dies that they are somehow to blame. However, the problem can sometimes come in on the plants themselves. Another possibility is that this borer was already in my yard and then spread to my new iris plants. However, the fact that none of the native iris that I transplanted into my garden, nor any of the other Pacific Coast iris hybrids that I purchased from other nurseries have borer damage suggests that the problem came in specifically from this nursery. This highlights the importance of monitoring any new plants that you purchase for a period of time to see if you may have inadvertently brought something else home that you don’t want (like borers and scale insects!).

8/8/2021 update

Well, I can now say rehabilitation has not been very successful. Of the eight plants that I potted up and put in the greenhouse, five died and only three remain. They are obviously not happy. It’s a little heartbreaking. I really liked these iris, and I don’t know if I will be able to find another purple Pacific Coast iris hybrid like the one below. It was an unnamed hybrid when I bought it about 8 years ago and I don’t remember where I got it from. Hopefully, I won’t find any more borers. Only time will tell.

Casualty of the iris borer war
Casualty of the iris borer war

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