It sounds like you had some tissue damage due to the severe change in temperatures. Plants took the warmer weather as a sign to grow, but tender new cells died when the temperature plummeted. If you notice the older tissue of the plant is of a different consistency, it is tough, stringy and hard; it compensates for being frozen by wrinkling up a bit (leaves show some ribbing in the sub zero temps).
What to do? By scooping out the rotten parts you are doing the right thing. Maybe you aren't getting all the 'dead' tissue; maybe you have to not only scoop the rotten stuff, but cut away more of the tissue that looks OK but has been damaged and not yet rotted. Some people may say dusting with Comet or digging the rhizomes and bleaching them might help.
You could try that, but I think your problem is really tissue that looks good, is already damaged and has not yet rotted.
Laetitia

Try something to heal the tissue - powdered lime, sulphur or terraclor (if you can find it).

Walter Moores

Or something like Comet or Ajax with bleach.

Unfortunately, sometimes Ma Nature insists on helping us weed the weaklings out of our iris beds. She's done it to me several times in the past. For the first few years, I would try to battle rot and salvage infected rhizomes/clumps. After a while, though, I recognized the value in allowing the weaklings to die. There are, after all, nearly 60,000 named iris cultivars. If 600 die, you've still got more than 59,000 to try in order to find the ones best suited to your climate and growing conditions.

That said, you will find that a number of your irises will survive rot infections even if left completely untreated. The primary fans may die off, but a lot of those tiny increases will survive. The immature increases won't bloom this year, of course, but at least you won't lose the clumps completely.

Laurie

I appreciate the responses. Many of these are iris that I just added last year and those I can understand having more problems with, however many of them are ones that I have grown for several years and they have done good every other year so I don't think it has anything to do with them being weak iris. Unfortunately the weather here can be extremely unpredictable at times, and this basically created about a worse case scenario for me going from one extremity to another within a matter of a few hours. I'm wondering if in the future I may be better off putting down a straw mulch in early winter. I've always stayed away from this in the past because I've been worried about it causing rot all by itself, but perhaps its something worth trying out. I will have to try dusting the rhizomes with some of the things mentioned and see what works, I already can see that the vast majority of them won't bloom this year, I just would really like to be able to save the increases.

Schreiners and Cooleys bury their rhizomes in soil, and then remove it in early spring. This was written up in an AIS Bulletin several years ago.

At the Dallas Convention in 2000, pine straw as still on the clumps in April. I am assumed it was removed before the heat of summer.

I have experimented with pine straw here and have removed it just before bloom time. There has never been a problem.

Experiment and see what works for you.

Walter Moores

By "weak", I mean irises that can not survive the extremes of your own specific climatic and growing conditions. I, too, have had irises that grew well for me for several years under "ordinary" northern MN climatic circumstances, only to lose them under those horrible "worst case scenario" conditions that can occasionally occur.

I no longer mind when irises die in my garden because my goal is to find the irises that CAN and DO survive here under any and all climatic adversities - the ones that are truly northern MN hardy. I want to know that if the worst climatic conditions should occur, my iris beds won't be wiped out ... and I'm definitely getting there.

Mulch is a double-edged sword. A very airy mulch like evergreen boughs can help new transplants make it through their first winter, but if the mulch compacts around the rzs or isn't removed at the right time in the spring, it can, indeed, promote rot.

Laurie

I live in an area with alot of deciduous trees (esp oak and others too) I let all the falling leaves settle on the iris beds in the fall, do not clip old iris leaves, and only clean off in the spring (around now). Sometimes I even throw a few extra leaves on the beds to cover any bald spots. They do act as a natural buffer to severe temp changes. This year we had alot of winter rain which did cause some leaves to turn into a soggy matted mess, but I did not have any rot from moisture (strange??). You may get alot of little green weedy plants growing under the leaves, and slugs and other insects too, but they will not kill the iris. Another benefit: it 'hides' and protects iris shoots from deer and other browsing and stomping animals. I don't know if you have natural mulch like this, but your idea of mulching sounds like a good idea. You need to remove before the weather gets too warm because I am sure if it stays on in the warmer weather there may be rot.
Laetitia

I have been cleaning on my beds today and am amazed at how many of the mature rhyzomes (the ones that would have bloomed) are rotten. This is the worst I've seen yet. I'm just pulling off the fans and hoping for the best.... The offshoots seem OK as of today.
In the past, I'd wait until later in the spring when there wasn't such a threat of severe frosts and cold snaps, then I'd scoop out all of the rot and keep the portion of the rhyzome exposed to the sun and air. This seemed to heal it up and I wouldn't lose any more of it. I guess this year I'll wait and see what happens in another month.
Good luck with yours!
heidi

The odd thing is, I had maybe 30 varieties in 3 different beds situated around my yard, and I just finished cleaning out those beds yesterday. Not a single one in those beds show ANY signs of rot. Unfortunately I just moved almost all of my iris into one large bed last fall, and of course that's the one that for whatever reason everything rotted. Its on the highest area of my property so most likely the warmest, and since I know for a fact those in that bed had started to grow in mid February just before the cold spell hit I'm thinking that's why they got hardest hit. I have close to 600 iris in that area alone and from the looks of it I will be getting bloom out of less than 10%. I'll be ok with that as long as most of them have increases that make it.

I'm a little worried about the other plants, mostly daylilies I have in that area, but they're still waking up and its too early to see what damage was done.

Really makes me want to look at warmer areas. I've lived in Montana all my life, but this just really is disappointing when something like this happens. I guess it can happen anywhere but Montana is very good at extreme temperature fluctuations and they happen pretty frequently. Not exactly prime climate here for plants.

I'm new to iris gardening, started a couple years ago, decided they were my favorite flower and I was doing well with a couple hundred out in the yard. So going through greenery withdrawl- I bought some rhizomes from Ebay in Dec. and had a problem almost identical to cls009's. I planted them in Miracle grow soil and they were doing great in the basement, until strange little sucking bugs developed on them (maybe aphids- not sure). I thought since the other iris plants were okay in the yard over the winter, if I put mine out on the porch, the cold would kill the bugs and not the plants. (Made sense at the time, but now I see the huge error.) Mine must have tissue damage as well- had them growing and they froze. I snipped off the dead leaves, and they aren't growing any new leaves yet. The rhizomes also feel semi-soft, but they don't smell - yet. I saw the recommendation for the materials to heal the tissues, but is there any hope for these to recover? Is there a way to tell if they will heal?

For some reason, if you plant iris in the garage or basement or house in the winter and they start to grow, the growth is very tender. You then cannot put them outside until all danger of frost is gone. Unlike the tough growth you see on the iris in your yard at this time of year, the tender house-growth cannot survive freezes.
Been there done that!
Will it grow back? Perhaps, you can just be patient and keep an eye on them, but I have not had much luck getting such an iris to recover. In any case, you don't have anything to lose by trying to take care of them and see what happens.
Laetitia

I was wondering, cls009, how your iris are doing and how many you ended up losing. I ended up losing entire, established (2-3 year old) clumps, plus some that were just planted last year. There were some favorites that I lost (which were marked) but I'm hoping I planted an extra rhyzome here or there from them (unmarked). I probably won't know until next year if I did or if they were sold/traded/given away. Most everyone around here lost some iris to this winter/spring. It IS heartwrenching, especially when they're favorites.
Anyway, I pretty much know which ones I've totally lost and which ones I atleast have an increase or two that made it. I do have about 60% that look great so I'll atleast have some beautiful blooms to enjoy this year. Out of about 100 seedlings (ranging from 4 years old to 1 year old) all but 4 made it fine. I was pretty excited about that.
Oh, and 90% of my medians made the winter great. I'm thinking I'll buy a few more of them this year. :o]
heidi

I been there too. I had iris many since 1969 that survived until the winter of nineteen below zero and no snow cover. Then it warmed up and rained. Some I do not expect to find again like Dawn Crest. That was in 2003 and I was discouraged and started buying daylilies to fill in the blank spots.

I joined Garden Web last year during a very rainy and cold spring and was encouraged to stick with the irises even though many were rotting and few blooming no sun that spring then it turned ninety degress and summer, Maine is very unpredictable too.

I decided in June to try new ones, id the noids that survived that winter that have been with the farm since the 1800's. I have bought many last year and am having the warmest spring, very sunny weather and might see many first years bloom.

Winter was strange though. Normally they do not start growing in January. Then we had cold spell but not below zero. Damaged some with rot but only the rebloomers as usual. I think it will be a reasonable iris year finally.

Zintal

I have had a problem with rot too. I am blaming the location rather than the weather.

I put them as a boarder plant along the side walk where I though the drainage would be ideal.

Many of those have rotted however.

While some I have in a raised bed are doing unexpectedly well. In my raised beds I grow brown eyes susans and other lush things like cosmos. I hevealy compost in there -- kitchen non-meat scraps. I put the iris in the back last spring just to try.

To tell you the truth I expected never to see them again. I expected to rot but they are doing superbly -- at least so far. Their fans are the biggest. So I am sticking lost more in there this year.

I guess the great drainage makes up for all the composting material that would ordinarily make them prone to rot.

Stupid late freeze. I have rotting problems this year with old iris I have neglected for years with no problems.