have no hesitation ... just do it ...

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its called a fairy ring ... and it happens with lots of plants ...

you have huge clumps .. just take a shovel full off the back side.. bare root it ... and it will explain it itself ...

have no hesitation ... jsut do it ...

in every post.. tell us where you are ... big city name ....

timing might matter.. depending on how far you divide ... in my ground freeze MI ... a plant must either be heavy enough.. or able to grow fall roots.. to hold themselves in the ground ... before the soil gets too cold.. or else they might all heave out of the soil during winter.. and then freeze dry dead ...

you are well aware of how aggressive they can be.. these are not foo foo plants.. so just go dig them up when the time is right .. and dont worry about it ...

the usual default for timing is just after flowering ... which is usually root growing time to store energy for next spring ...

you can probably search either plant.. and flip to videos.. and find tutorials to guide you ... after watching a couple of those .. bring us back whatever questions are left ...

ken

oh.. as to the fairy ring... sharpen a shovel.. and just dig out the dead center... fill it with fresh soil from elsewhere.. and by next summer.. the plant will grow back into the center ...

on examining what was dug out... you will find old rotted crowns.. and probably the potting media that wore out ... making it water repellent on some level.. and thats why the center died .......

usually its just about the size of the pot of plant you originally planted ... further proof that potting media wears out.. and should not be used in the garden .. in planting holes ...

ken

The dead or empty center of an established clump of Siberian iris has nothing to do with any previous potting media! You will get exactly the same condition eventually from planting bare root starts. It is just the way Sibes grow....as is the case with many ornamental grasses and a few other clump forming perennials. Healthy vigorous growth expands from the perimeter of the plant, while the old center becomes congested and loses vigor, eventually dying out.

The time to divide irises is not immediately after flowering but in late summer or early fall, as that is when the plants are no longer in active growth. You could also wait until early spring as soon as the ground is workable but risk losing that season's flowers.

There is a pretty reliable rule of thumb for when to divide and/or transplant perennials. For those that bloom before the summer solstice (June 21), dividing or transplanting is best done in fall. For those that bloom after the summer solstice, divide or transplant in spring.

The dead or empty center of an established clump of Siberian iris has nothing to do with any previous potting media!

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you accuse me of making absolute statements ... and then you trip right into the same .. karma... lol ..

when you put a pot of media in sand.. with a hosta in it... it will be dead in a few years unless it can grow out the sides into the sand..

the sand literally wicks all moisture out of the peat .. .and once dry.. is near impossible to rewet in the ground ...

ibso presto.. a few years later.. presuming the host lives... you have a fairy circle ...

when i moved and planted 1650 pots of hosta... i would say it happened a with a couple hundred of them ... go figure ... and i discovered the issue.. when i went to dig a hole to plant something else there.. and lo and behold.. it was a pot sized shape of media.. with dead hosta in it ...

i would presume .. since i have nothing but sand that it wouldnt happen as often in a good friable soil ...

ken

Just updated my profile to show I am in Zone 7, southern NJ so fall is long and mild. So I can just dig up the dead center of my Siberian and refill with fresh soil to let the ring to grow inward.

Strangely, my 15+ year old Morning light, Zebra grass, and Tall bearded Iris clumps are huge, but show no bare center. They are in sunny location outside my irrigation zone, and I never fertilze nor divide them once. I have clay soil with poor drainage, decent natural rainfall and no drought for at least a decade. I hope I never need to divide my ornamental grss as it needs the help of a bulldozer. I may forgo dividing my tall bearded iris as it has no bare center, just less flowering. The foliage is nice and tight that leaves no room for weed.

Bearded iris are completely different creatures. They get divided in July/August - when it is broiling hot, and most plants have no desire to be disturbed. IME, if you divide them in fall, they don't have time to grow the roots they need to manage the winter rainfall, and will rot.

Bearded iris never develop that dead, 'empty' center so it is harder to know when to divide. Generally, when the planting bed is congested with the rhizomes and flowering drops off, it's time :-) Lots of the rhizomes will be dry or dead or producing minimal new growth and those should be cut off and discarded. This is a perfectly acceptable time to do so now in my climate but earlier may be advised for colder winter areas.

Ken, that dead center will develop with well established Sibes regardless of what they came potted in and what kind of soil they are currently being grown in. It is just the way these plants - and many ornamental grasses as well - grow. And yes, I can safely make the statement that it has nothing to do with any previous potting media as it will happen exactly the same way with bare root starts (no potting media involved).

There is a big difference in making absolute statements (all conifers are trees, etc.) and those that accurately reflect the conditions involved.

You know just enough to be dangerous and often misleading in your pronouncements. And I say that with almost 50 years of gardening personally and professionally and a degree in horticulture behind me.