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Iris Borers & What To Do About Them Right Now
by Art Drysdale
by Art Drysdale


Art Drysdale, a life-long resident of Toronto and a horticulturist well known all across Canada, is now a resident of Parksville, British Columbia on Vancouver Island, just north of Nanaimo. He has reno-vated an old home and has a new garden there. His radio gardening vignettes are heard in south-western Ontario over radio station Easy 101 FM out of Tillsonburg at 2 PM weekdays.

Art also has his own website at

July 1, 2012

Above, three shots of Siberian Irises: some light blue ones near our street-side fence; a close-up of a veined cultivar growing at the edge of and in our large pond; and a group of light blue cultivars growing on the island within the large pond. Below, a nice Japanese Iris growing at the edge of a friend’s pond; a close-up of Iris ‘Red Revival’ a tall bearded cultivar; and the Dwarf Iris ‘Cyan’ growing in Keith Squires’ scree garden in Campbellville, Ontario. Author photos..

Through my friend and Clematis expert, Peter Keeping of the Bowmanville area Ontario, a few months ago I became a member of the Facebook group “Over the Fence with Peterborough Master Gardeners”. I regularly see questions and comments from the various members. Generally I do not respond to questions because I usually see other members have already responded. However, a month ago or so I noted Steve Paul asking for identification of a shrub I knew well, and so wrote a response. Now, just on Thursday this week, Steve Paul posted another question which I have dealt with previously, both in Toronto, and here on Vancouver Island British Columbia. First, here is Steve’s question.

“Ok I have really had enough. I have been battling Iris borers for about five years now. Every year I dig up every single clump, remove all the borers by hand, dry out the rhizomes and put them back in the ground. But every year some seem to survive to lay eggs and start the cycle over. And I must have five or six varieties in about 20 different spots. Has anyone used anything natural or chemical based that actually works? I am willing to go the chemical route now - I would prefer to avoid systemic pesticides that could harm something that would land on them. Your advice is appreciated.”

With regard to the similar pest—white grubs in lawns—there has been a strong push probably now for over two decades to use expensive nematodes for the grub control. The common suggestion now, in order to avoid chemicals, still is nematodes. Various companies, large and small over the years, have marketed these for control of grubs in lawns and I have never seen them work in Toronto or here. [Coincidently, there is a company which raises these nematodes for both wholesale and retail sale within a few kilometres of where I live and I have toured their facility!]

There is a good reason why they don't work. The nematodes have to get down into the soil where the grubs are in order to work. They seem to have great difficulty getting through the thatch produced by our predominant bluegrass lawns. (That's the same problem the chemicals such as Diazinon and Chlorpyrifos used to have and GrubOut [Sevin] has had more recently, and why the instructions were changed for them--you now must water the lawn for at least two hours before application, and another hour after application if the chemical is to get down to where the grubs are.) However, in California, where the nematode idea originated and where they actually work, they do not have rhizomatous grasses, they have clumping grasses. It's much easier for the nematodes to get down into clumping grasses. Generally here, the nematode solution is ONLY valid for use against such insects as Black vine root weevil and Iris borer, where they can be applied to soil vs. turf.

Over the past two decades at least, each time I speak at an exposition, such as Canada Blooms, inevitably there will be a question about nematodes, and I respond with much the same response each time. This has happened to me from Ottawa, throughout Ontario, and all the way through Saskatchewan to British Columbia. Often that response will lead to an exhibitor at the same show coming to me after the talk, often with a container of his nematode product, to tell me literally that I don’t know what I’m talking about. I then have to go into a long explanation of why the nematodes do not work, and why I do not recommend them. Often, of course, they go off in a huff not really wanting to hear what I have said to them.

Currently, Nematodes have not been widely–adopted for use by the turf industry because of their high cost and because the environmental conditions necessary to guarantee the success of the Nematode treatments rarely exist.

But, numerous Iris growers report Nematodes do work on Iris borers, however you have to be certain to get them watered into the soil immediately after application.

My second complaint about Nematodes is that they are expensive. For Iris borers, I think it is far better to take the advice of Iris grower/hybridizer Chuck Chapman for the control of this pest. Actually he has two methods he suggests and knows they work well; one is simply burn off the foliage of all Iris plants as early in the spring as possible no later than April. This destroys most of the invasive pests. The second method is what he calls hunt and squish.

Let me deal with the burning first. According to Chuck (see ), for Siberian and Spuria Iris, it is quite easy to burn off the foliage early in the spring before any of the new foliage appears. One way of doing this is to use a propane-fired torch such as is used for burning off the tops of weeds in driveways and along curbs [by the way, I definitely do not recommend using those torches for that; I think it is far more dangerous than using a chemical such as Roundup which you can still obtain in the U.S.A.].

For the more common tall bearded and dwarf bearded Iris, Chuck says that most gardeners might well have to cover the plants with old dry (tree) leaves and set fire to those in order to achieve results. Again, use of a propane-fired torch would work well, and he has never found any damage if the burning is done early enough in the season. However, it would likely be a good idea to have a garden hose at the stand-by!

Now, let’s get to the hunt and squish method. Since Steve Paul specifically asked about something that can be done right now, this is his answer. One needs to look closely at all the growths on all Iris plants—you are looking for a tell-tale jagged marking on the edges of the newly ‘unfolding’ foliage, and also perhaps some moisture and browning on new foliage. The insects will soon be going down the stems and settling in the rhizomes but as of now, according to Chuck, they are still in the new stems and foliage. When you spot the signs of jagged edges etc. simply squeeze! Chuck says it gives a huge bit of satisfaction when you do that and feel a slight pop-ping and you know you’ve got the critter!

Good Luck Steve, and thanks Chuck.     

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