Iris Borer (growing, fertilizing, insect, spring)

[BuffaloLight]

I am managing a community garden and last year we had a guy donate a lot of siberian iris. He told me they had stop blooming some years ago and were just taking up space. Me not really knowing much about iris took them and planted them assuming they just needed dividing to bloom. They looked beat up and half dead and I figured it was from transplanting in mid summer.

This year, they bloomed but not as much as I felt they should have. The leaves looked big and healthy until about 3 or 4 weeks ago and things started to go south fast. I took a pair of clippers just to take all the dead leaves out and notice a white grub with a red head that is not even a half an inch. After finding out if was an iris borer on the internet I went back to take a closer look and a lot of them were infected.

At this point it's super easy to tell which are infected and which aren't. The leaves are beat up and if you peak in the fans you can seem them in there. If the grubs haven't made it into the rhizomes can I just hack the leaves off at the base? Or would this do more damage to the plant? The rhizomes where the grubs have made it down into the rhizomes looks like they've completely killed it but I have to dig them out and do the entire soaking thing. Is mid July a good time to do this or should I just wait? Anyone have any experience?

[J&Em]

I haven't had the problem with my irises, yet, being just on the edge of their range, but know a few iris lovers who have had to deal with it where I use to live.

You could just pitch the whole lot to avoid the borers spreading to any other iris you may have. If that isn't a problem or you really want to save a few then get rid of the obviously diseased ones, once heavily infected they tend to be hard to get healthy. Check rhizomes of what is left for tunnels and obvious borers and get rid of anything that is hollow or shows signs of multiple borers. Do not compost the stuff you get rid of, put it in the trash. Cleanup every last piece of leaf and dried up materials from last year the same way to avoid any new infections. For those that appear alright you will need to treat them now.

A lot of the older iris experts used to recommend a 10% bleach soak of those rhizomes you think are clean of borers. Ten minutes should be good. One older gardener told me she found it was enough to "drown" them in a water bath for 10-15 minutes! They used to recommend following up the cleanup with using insecticides to treat the rhizome but in the last few years biological controls in the form two beneficial nematodes have been found to have as much or better effect.

If you feel like learning a bit about them the following is an excellent, if long article. Near the bottom is a list of places that you can buy the nematodes. Gardens Alive is probably the best known source for home gardeners.

Nematodes


A nice short article on iris growing and prevention of disease is here:Gardening with Bearded Iris: Spring Cleanup, Fertilizing Irises, and Iris Borer Control

[BuffaloLight]

That was very informative. I have been using straw to treat all my beds, lots of clay. I do not want to spend money and do not want it to be time consuming because this a community garden. I still am not sure what to do now, in mid July, where a good many of the iris borers haven't made it down to the rhizomes. Logic tells that more harm will come to the plant if the borers get to the rhizomes but on the other hand will the iris survive with out any leaves in July? Or do I just dig them all out and drown them and to hell with cutting the back at all? I am hoping these things will look half way decent because Garden Walk Buffalo is at the end of the month.