.... 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.
