If left in place and not divided most intersectionals will start going downhill after about 5 years. Most will dig up at this point and divide. I personally do not know of anyone that has left in place longer than 10 years and that person complained to me that they felt lucky if they had one bloom.
I am waiting to see what happens as the plants stay in place. Herbaceous peonies will hollow out their roots in that length of time. No scientific evidence to back it up but I believe that the regular peonies when left in place for years utilize the fungi in the decayed material as food or they would decline as most plants will when left in one place for years.
With the necessity to refresh the potting medium treating as pot plants seem to be the best usage for the intersectionals. As with many potted plants the roots should be trimmed and best portions kept at repotting.
