Fall Iris Care

jardinistaOctober 12, 2011

Hey all--I have a beautiful bed of Iris I planted about 4-5 years ago and really have done nothing to/for them. They bloom every year. This year they are looking a little crowded. Also,a friend of mine says she chops hers down to almost their "knuckles" every fall. Should I be doing this?? How do I know if they need to be thinned? and how do I do this? Thanks,Jardinista.

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The reason she does this is to reduce the leaf spot diseases that are common in Iris. If the leaves are healthy wait as long as possible to do this as they are still feeding the corms. There is an Iris forum which has more than I can tell you, and probably more than you will ever want to know about Iris.

    Bookmark   October 12, 2011 at 6:27PM
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Thanks,thought about specific Iris forum after I posted here,but wanted to hear what my local GWers do.

    Bookmark   October 13, 2011 at 5:38PM
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whitegarden(Z5 MA)

jardinista, what I have learned about iris over the years has been by making a couple of simple but critical (and yucky) errors.

First, if you haven't done anything to them in 4-5 years, chances are that they are a bit too deep in the soil now. Iris don't like wet feet. Somehow, every year, no matter how shallow I assure that they are in the spring, they are buried too deep by the fall. You need to make sure that the top of the rhizome is totally exposed or rot will set in. This happened to me the third year of my iris planting. Dozens lost and, believe me, the clean-up was gross and really smelly!

The next problem is iris borer. You will notice yellowy streaks on the foliage which is the immature borers that have tunneled their way through the leaves. You need to crush the larvae and trim down the foliage. Of course you will have lost a season of bloom by this time. I have found two preventative treatments. One is not particularly orgainic so now I am trying the second. The first is, believe it or not, a light sprinkling of Ajax powder (or any sink-scrubbing powder) over the rhizomes once in the spring and once in the fall right after you have cleared any leaves and organic matter away to expose the top of the rhizome. The other, organic solution is a spray with a solution of 1 part Murphy's Oil soap with 7 parts water. I started this this spring and don't know if it is as effective as the Ajax.

Dividing them is best done in the spring right after they bloom. Pull up the clump and separate the individual rhizomes from each other. Each should have 1 to 3 fans of foliage on it. Check each one for any soft spots and cut that area off with a knife if you find any. Dip them in a light solution of bleach and water and, if they are very wet, let them dry out for a couple of days. Clean the knife you are using in the bleach solution periodically as well.

Cut the fan down to about 6" with sharp clippers. Make a soil mound, Place the rhizome on top with the fan side high and the other side angled toward the soil. spread the roots over the mound and cover the roots and just the bottom of the rhizome with soil. You'll notice that there will not be a lot of soil holding the rhizome down. That is why you have cut the fan down to 6". Any taller and the fan will cause the rhizome to tip back and out of the soil. You want to plant in clumps of three in a triangle with the fan ends at the corners. When iris multiply they grow out from this end, so pointing them away from each other will help them from crowding each other prematurely.

Now water in lightly and either sprinkle or spray (see above). Finally, try to check them every day or so to make sure they haven't come "unplanted." They will take root pretty quickly and you will be all set.

Last step, give extra divisions to friend and/or bring to plant swaps. I have given literally hundreds of my iris "Immortality" to friends over the years.

    Bookmark   October 13, 2011 at 9:44PM
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Thanks for that great summary of iris care, WG.

I think Ajax - or maybe Bon Ami, which doesn't contain anything but an abrasive, is perfectly acceptable to organic gardeners. It presumably kills by cutting the soft-bodied stage of the borer. I used it (Bon Ami) when I had an annoying ant problem in a brick landing, at the base of outside steps.

    Bookmark   October 15, 2011 at 9:11AM
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Ditto everything white garden said.

I've usually found that 4th of July is about the right time to divide the rhizomes. My irises tend to bloom in late May and early June - so it is late June by the time all the blooms have died off and the stalks are starting to die off.

I've also found that I tend to divide the irises every 3 years.

I use a grub killer with imidacloprid on my irises - which has been effective in killing the iris borer. 1 tsp per rhizome applied in the spring. Imidacloprid does not kill earthworms.

Irises do like well drained soil. I'm making a new bed by my pond for my irises - instead of just adding lots of compost to the bed, I'm also mixing a lot of sand into the bed - to assure good drainage.

    Bookmark   October 15, 2011 at 9:34AM
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whitegarden(Z5 MA)

Wow, diggin, that's good to know. I will stick with the powder then since it has been very effective in the past.

    Bookmark   October 15, 2011 at 8:46PM
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mad_gallica(zone 5 - eastern New York)

I've understood the reason to use cleanser on iris is that is often contains bleach, and bleach is a recognized way of dealing with the various rots the occur on iris that have been damaged. It's not to kill insects.

The alternative is to dig up the iris, then soak them in a bleach solution and replant.

    Bookmark   October 15, 2011 at 9:30PM
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hmmmm . . . I've never bleached my iris rhizomes.

    Bookmark   October 22, 2011 at 9:52PM
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molie(z6 CT)

Okay, this post I'll save and refer to next spring. Great information on care for my clumps of iris, that I've also been neglecting.


    Bookmark   October 23, 2011 at 11:59AM
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Hey whitegarden et al--thanks for the info. So it sounds like now,the fall, I don't do anything? Should I even cut the leaves down to 6 inches or shorter? Thanks

    Bookmark   October 28, 2011 at 1:09AM
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Steve Massachusetts Zone 5b


That was a great post on Iris care. I've saved it to my "clippings" file. I did some additional reading on Iris borer and found that certain types of beneficial nematodes will control Iris borer at 100%. Have you ever used those?

Here's a link (under Iris basics, borer control)


Here is a link that might be useful: Iris Societies

    Bookmark   October 28, 2011 at 9:25AM
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whitegarden(Z5 MA)

Thanks steve, I'll check that out.

The only thing I would do now is check your planting depth and check if any of the rhizomes are soft. Just push on them at the top near the fan with your finger. If they have soft spots, you shoud probably lift them up and cut out any of the rotted areas now and pop them back in the ground. I only cut back the foliage if I have lifted them, but it won't hurt anything to do it now. The foliage will die back over the winter and have to be cleaned away come spring anyway.

    Bookmark   October 28, 2011 at 9:00PM
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I'm afraid I missed all this spring and July activity. I had a beautiful iris garden in California, made up of mostly rhizomes I had transferred from my late mother's house to my new digs on a ranch in a dry canyon. The rancher transferred my iris to her garden at the big ranchhouse after I moved and promised to send me rhizomes. Well, she did -- I received a box with what looks like 30 or 40 of all sizes. I'm ecstatic to have my mother's flowers again. Trouble is, it's November. What is the best way for me to handle them?

Here is a link that might be useful: Canyon Communique

    Bookmark   November 12, 2011 at 10:09AM
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SilverBee - first of all, you are in Tennessee, so your timing may vary a bit from our timing here in New England. You can still follow the directions given by WhiteGarden, but just vary your timing depending on when your plants bloom.

As for the box of rhizomes you just received, I would just plant them in the ground now. They may not bloom next spring, but they will have next year to establish and you should get beautiful blooms the following year.

    Bookmark   November 12, 2011 at 11:04AM
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tree_oracle(z6b MA)

Some of the replies here are seriously overcomplicating the situation. Iris need to be thinned out when they have multiplied to the point that there is no empty space between the rhizomes. It's a simple as that. Just clip of the tops (some people use a lawn mower), dig them up, put about a third of them back with a few inches in between them and plant them so that the top half of the rhizome is above the soil. I've never used any sort of fungicide on them at all.

    Bookmark   November 12, 2011 at 1:23PM
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OK tree oracle--I'm with you. I must have planted them at the right depth to begin with since they bloom beautifully. I added a few new rhizomes that friends have given me to this bed and it just looks nice and tidy so far. Thanks,jardinista

    Bookmark   December 9, 2011 at 11:05PM
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