The general advise is when you're dividing rhizomatous irises (sibirica, ensata, but especially germanica/bearded types) to throw away the old rhizome because it wouldn't bloom anymore.

Has anyone actually ever tested this?

I can understand that new sections have a supressive action on the old rhizome (comparable to apical dominance along a stem, but this is just a guess...) so that would explain why the old rhizome won't produce leaves and blooms. But once they're separated, what holds the old part from growing and blooming again like before?

new increases might start growing off the old rhizomes.

.... as the rhizome matures, especially after it blooms -- a thick layer of cork cells develops on the outside of the rhizome sealing it off from the root system attached to it, as it re-directs energy to the new, developing rhizomes on each side of the apical fan/bloomstalk. This process is likely hormonal driven as the plant goes through its phases of growth/maturation. Also changes in the tissue composition of the mother rhizome occur, as all the sugars, carbohydrates and auxins (growth hormones) become just starch and woody cellulose.
This process usually happens after the bloom period, however, sometimes there are lateral and intercalary groups of meristematic tissue (groups of live plant tissue at different points) on the mother rhizome that remain active despite the cork sealing process and these can give rise to new offshoots on the mother rhizomes.
These new off shoots get their sustenance from the residual stored starches and sugars in the mother rhizome until they are large enough to develop their own root systems. I've tried planting the mother rhizomes after i've dug and separated the clump, especially on varieties that were hard to come by in commerce or that i valued. My results were very mixed, - some responded by initiating new offsets, others merely withered away, so the above observations may not hold true to all TB iris varieties. Treating the mother rhizome with a liquid rooting auxin (Root Stimulator) seem to help in some instances, but not all.
I've not tried to propagate mother rhizomes on beardless classes, so my observations are for bearded only.

Timothy ~ Thank you for sharing your experience, as I have tried and failed at that. However, I did not go to the trouble of using rooting hormone...I figured either it will work, or not. I have to admit, it was a "mother" that someone sent along with several irises.

for that detailed information Timothy

Tim. Very interesting.

Thanks, Tim. That was fun to read through

For bearded irises..... My understanding, and I am by no means very knowledgeable about such things, is that a rhizome only has only so many little buds, or places on the rhizome where a new plant/fan will grow.

If you have noticed a potato has what we call 'eyes' where a new potato plant may sprout. Once all of these areas have exhausted themselves the potato cannot make another plant. A rhizome works the same way. Once all of the buds have produced a new fan the rhizome will not produce any more. I hope this is not confusing. I don't know the technical terms for the buds, or places on the rhizome where a new plant/fan will sprout from.

grannysgarden said:For bearded irises..... My understanding, and I am by no means very knowledgeable about such things, is that a rhizome only has only so many little buds, or places on the rhizome where a new plant/fan will grow.

If you have noticed a potato has what we call 'eyes' where a new potato plant may sprout. Once all of these areas have exhausted themselves the potato cannot make another plant. A rhizome works the same way. Once all of the buds have produced a new fan the rhizome will not produce any more. I hope this is not confusing. I don't know the technical terms for the buds, or places on the rhizome where a new plant/fan will sprout from.

I don't think that is a correct explanation because plants retain meristematic cells all around their tissues which can divide and differentiate according to needs; and I don't think they run out of meristematic cells either.

Interesting Lee-Roy. Then some of my bloom outs may start some regrowth.

The potato is a tuber -- the iris (bearded and most beardless) is a rhizome. The two plant structures have some similarities, but quite a few differences in composition and structure, so the two cannot be compared.

grannysgarden said:Interesting Lee-Roy. Then some of my bloom outs may start some regrowth.

that is true, if the mother rhizome has active sets of live plant tissue in addition to the increases on either side of the bloom stalk. I have always allowed a bloom out to stay in place for another year, at least. About 60-70% of the time, i'll see new set of shoot(s) many time those resurgent new increases will come at the "toe" of the old rhizome, and some towards the middle of the old rhizome.
Additional evidence that i've seen, for persistent live groups of plant tissue on the old rhizome, is from my battle with wintertime botrytis -- (which i've won, for the record). The malady will completely destroy the above ground rhizome but new increases will show up the follow spring from locations nowhere near the usual spots near last years bloom stalk. Digging down a bit i discovered new live shoots from the middle to even the bottom side of the old rhizome. I also found new growth from new tiny round marble like rhizomes that i didn't even know were developing.

One thing i didn't mention, from my initial observation post in this thread, is the varieties that were rapid increasers tended to have more active site along each side of the rhizome for shoots to appear. Cultivars who had a slower rate of increase ive had more of a failure rate trying to get new shoots to appear. So trying to re-cultivate old mother rhizomes for increase is hit or miss depending on the variety.

great information to keep in mind, timothy. i often have bloom out, but haven't been lucky. my rhizomes usually dry up. but it maybe the fault of my aftercare? i may have to water them more, maybe as much as a newly planted rhizome? i will continue, to experiment, and take notes.

TBManOR said:The potato is a tuber -- the iris (bearded and most beardless) is a rhizome. The two plant structures have some similarities, but quite a few differences in composition and structure, so the two cannot be compared.

Yes, I would definitely agree, a potato is not an iris. I was referring to the 'eyes' on a potato being similar to the 'eye buds' on a rhizome. GuiseppeJ wrote on the thread "Bloom out survey and problem solving" post #1926652 on March 12, 2019 the following:

This is a partial quote......
Bloom out is when every fan blooms and that is it. There is no increase since every increase has bloomed. It is also referred to as a bald rhizome. The Mother (the original rhizome blooms) and there is no increase. Digging the rhizome will show no sign of an increase. Nothing you can do will cause it to produce a growth since there is no bud eye. Certain varieties and breeding lines are prone to produce "bald" children.

From my experience I leave, what I refer to as bloom outs, in the ground for at least another year. When no new growth begins in the spring of the following year it has always been futile for me to cause new growth on that rhizome. Again, I am no expert and learn something new all the time therefore I yield to your knowledge and experience.

I have much respect for Joe Ghio's pronouncement on the matter, but i honestly can say, that the above quote from him has not been my observation, 100% of the time. I've had 50 years to experiment with it, and I my own experience has taught me otherwise. I've consistently had some success rate in holding over old rhizomes on certain varieties. The key is that ONLY certain varieties can develop new shoot from old rhizomes, which i think i made clear from my above post.
I also grow hundreds and hundreds of cultivars like he does, but in a climate radically different from that of San Jose Ca. The climate factor may have something to do with it, im not sure. I do apologize if i made my own experience/experimentation sound authoritative -- i'll make sure to add the appropriate caveats to my technical responses in the future.

How much does the freeze, hard freeze, thaw cycle of Minn. affect Iris.

My old ones, decades old keep on going but new ones , the rare few best , very best, three or four years and then they go away.
I may try planting next to the foundation which is warm enough that even with sub-zero snow often melts away in winter.

You might find that mulching them over winter helps in your zone, but take the mulch away in the spring.

My Indian Chief and Unknown Yellow keep on going
They come up all over the place from rhizomes I have tossed
On this subject my Frank Adams has a bloom stalk on every fan. No fan in sight with out a bloom stalk. Will I loose it? I assume I need to cut the bloom stalk and associated leaves right down to the ground. There will be nothing left!