Should irises be mulched?

bren5kidsMay 10, 2012

An employee mulched our flower beds with some lovely compost, but he also completely covered-up the iris roots. Is this a bad thing? The irises are blooming well now, but they already had their blooms set when he mulched. I have heard that iris roots need to be exposed to sun to bloom well, so I'm worried that my reblooming irises won't put out a second batch of blooms if I don't go out there and (laboriously, at 6 months pregnant :-) uncover all the roots. He must've thrown shovel-fulls of compost over the irises because the are well covered, even in the middle of the patches.

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newyorkrita(z6b/7a LI NY)

Not only will they not rebloom all covered up like that but I would expect they might just die on you. Yes, those rhizomes need to be uncovered.

    Bookmark   May 10, 2012 at 6:54PM
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I suppose you should worry some, but not about the re-bloom this summer. I think the rhizomes will be fine as long as the soil drains decently - I remember it being quite dry in CO - so I don't think rot in summer is going to be a problem. Next Spring may be another issue as the rhizomes don't want to come out of dormancy covered in a potentially rot friendly environment.

The idea that the rhizomes need to sun to "ripen" is a myth repeated in many Iris books, it may have started in the UK where hot sunny days are rare. They just require good drainage. Too much nitrogen, water and a thick covering are the recipe for rot in Bearded Iris.

    Bookmark   May 10, 2012 at 8:14PM
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hosenemesis(SoCal Sunset 19 USDA 8b)

I would fear that the compost would rot them, since it is so rich in nitrogen. Iris rot bacteria seems to correlate with high nitrogen levels. You may want to wait until after the bloom to get in there and brush some away. Do you have an air compressor and a pair of goggles? That would work and you wouldn't risk accidentally breaking off stalks.


    Bookmark   May 10, 2012 at 11:10PM
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Thanks so much for your input and advice. It is quite dry here in our neck of the woods, but these iris do get hit by the lawn sprinklers. I've pulled mulch away from the edges but was afraid of breaking stalks if Itried to get into the middle. I like your idea, hosenemesis ;-) We do have an air compressor and I think that might be a good job for my 14 yr old son!

    Bookmark   May 10, 2012 at 11:59PM
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ken_adrian Adrian MI cold Z5

so none of you would question what the compost actually is.. nor how deep it is ...

on my mineral sand.. which is extremely hot and dry come summer.. a 2 inch coating of a fine compost would disappear ... before august was gone ... just poof.. gone ...

and i would suggest that it would do less harm.. than arming a 14 year old rocket scientist.. with a compressor capable of 150 pounds of aerosolized terror .. [though my 14 year old is a brilliant student .. an air hose would be a nightmare of fun and joy .. so forgive me for potentially insulting your son.. lol ] .. most likely leading to paint loss on the house 150 feet from the iris bed.. and considering i have siding.. it would be a mystery as to how she accomplished such ....

based on your words alone ... i would probably clean out the centers ... maybe with just a little whisk broom ... i would not be anal retentive about it... and i would find a new gardener ...

the compressor.. sounds a bit overboard ....


what about a shop vac.. with the hoses reversed..

    Bookmark   May 11, 2012 at 8:00AM
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Ken adrian, thanks! You made me smile! The compost is no more than 2 inches deep at most places, and I am planning to dig , divide and replant this summer/fall. So it sounds like I maybe don't have to worry about it. As to my 14 yr old son with an air hose......we live in the country with 14 acres irrigated with wheel line that need to be moved every day, have a big shop with all kinds of tools that he regularly tinkers around in, two horses that he saddles up and rides,and he also helps his uncle with harvesting in Oregon during the summer, so an air compressor is the least of my worries :-D
And the "gardener" ? One of my husbands construction employees that needed a little extra work so we thought we would put him to work in the yard :-D

    Bookmark   May 11, 2012 at 1:30PM
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ken_adrian Adrian MI cold Z5

i hope you noticed that i said .. ON MY SAND ...


my 'idea' might not be the best idea on clay.. or a soil that holds a lot of water..

because.. and correct me if i am wrong.. the whole point of the rhizome being on the surface.. is that it basically the rhizome wants to be bone dry ... with its roots in a good soil with some dampness ...

i think a lot of you would pass out.. if you saw how mine are nearly buried in COARSE wood chips ... but w/o .. the sand is so barrenly dry and hot in july and august.. that the rhiz's would shrivel and die ...

its all about your soil .. more than the compost .. IMHO ...


    Bookmark   May 11, 2012 at 5:04PM
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hosenemesis(SoCal Sunset 19 USDA 8b)

Yes- Ken, my dad's soil is all sand and gravel with large boulders, and his irises NEVER rot. He has to water every day, too. It sure is a joy to divide them in that kind of soil.

I have rich soil and I overhead water, so two inches of compost would mean trouble.

I wonder what your daughter could do with a propane weed torch...

    Bookmark   May 12, 2012 at 1:01AM
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I laughed and laughed at ken's answer. But it is important to consider the conditions when folks have questions, of the soil, and of the kids. I had a son who really liked hitting plants with sticks to see them kind of explode. He was starting with the lilacs but I did aim him at the prickly lettuces by the garage, and it turned out okay.

When folks give us recommendations, we have to do some considering about whether it would work for us, and not just blindly act on things. I just moved from a place with heavy clay soil to one with silty loam. Whenever you dig up a plant here, it just bare-roots itself, the dirt falls away. So I have to rethink a lot of things I did in the past. I may start a little mulching in the winter, but I am not sure. Since it is silt and not sand, the water can still pool up when things thaw. Lots of frost heave here.

    Bookmark   May 12, 2012 at 9:05AM
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berrytea4me(Z5 CO)

In our climate up to an inch of mulch will not harm your irises or prevent bloom. In fact it will help retain water. When I'm planting rhizomes late in the season I usually plant them deeper on purpose to prevent heave during the winter freeze/thaw cycles. I remove the excess so it is no more than 1' deep in early spring as soon as new growth starts (Feb/Mar).

Also, our soils tend to be nitrogen deficient so it will benefit your iris to add any slow release nitrogen whether in the form of well rotted compost or something like Osmocote time released fertilizers. I often ammend with composted manure (a no-no in other climates) when dividing & replanting. That said we usually add fertilizers to the outside of the clump where feeder roots are.

What causes rots here is uneven watering, like when we get thunder storms w/several inches of rain between long dry spells.

Irises do absolutely fabulous here in CO. Hope you enjoy your blooms and that you'll post some photos.

Here is a link that might be useful: my website

    Bookmark   May 12, 2012 at 2:42PM
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I know I'm late to this question, but I need some input. When I mulched irises a few years ago with pine straw, I ended up with hoards of aphids on the plants. That was back when I lived in a house that had GOOD garden soil, and I mulched my gorgeous beds to help keep the weeds down. Now my entire yard is basically red clay (during the late 1800s and early 1900s, there was a brick factory a few mile from me), and when I moved here, I put the few irises I could move with me in the tiny bit of sandy in and around some rip rap. They are doing fine, but I want to move them to my new little garden haven that is sheet mulched and closer to the house. I'd appreciate any thoughts regarding irises in sheet mulch.

    Bookmark   June 27, 2013 at 8:44PM
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hosenemesis(SoCal Sunset 19 USDA 8b)

Irises are prone to bacterial and fungal rot, and sheet mulching relies upon bacteria and fungus to work. So I think it would be a bad combination. The secret to growing bearded irises is great drainage and nothing to hold moisture over the top of the rhizome.

But since irises are cheap, why not try it and see what happens? Please report back if you do, and good luck.

    Bookmark   June 29, 2013 at 2:14AM
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Way back when I moved here 46 years ago, i saw a man who lived down the street from me litteraly mulching his iris bed with manure. I nearly had a stroke thinking I knew everything there was to know about growing iris. Thankfully I kept my big mouth shut and waited to see what would happen in the spring. He had and did have for many years, the most gorgeous iris I ever saw. Through the years he shared a lot of rhizomes with me and we became very good friends. One day I told him how horrified I was to see him shoveling manure on his iris. He said it was a good thing he didn't know the rules. He is gone now and I miss all the good gardening advise he gave me. Here in southwest Ks we cover the rhizomes with a little soil so they don't bake in our hot summer weather. Not every one gardens the same.

    Bookmark   June 29, 2013 at 10:41AM
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Thanks for the input, folks. I too would be horrified by the manure mulch idea, but bearded iris do tend to be fairly carefree as long as their feet aren't wet all the time. A friend once told me she'd discovered the secret to growing irises, just toss them onto the dirt, period. Because of the hot, humid and drough-prone area where I live, it's not quite that simple for me. I'll stick a few in the sheet mulch bed and let y'all know (in about a year, I suppose) how they do. The rest will just have to stay in the rip rap for a while I guess. :(

    Bookmark   June 29, 2013 at 2:45PM
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hosenemesis(SoCal Sunset 19 USDA 8b)

Flowergirl, thanks for pointing this out. Iris growers in California report the same success with planting the rhizomes beneath the dirt a bit to shade them. Some of the biggest growers do this, so having the rhizome exposed, as you note, is no longer the end of the story.

I only have trouble when baby tears or other moisture-laden plants grow over the rhizomes, or moisture retaining compost traps the wet on the top of them. In this heat, it is perfect growing conditions for bacterial rot. I wonder if cardboard wouldn't do that in the same fashion.

    Bookmark   June 29, 2013 at 5:33PM
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