As far as I know there are no descendants from the original four Itoh peonies and even from other intersectional hybrids registered in APS although I have not checked all 141 cultivars. I read that some people got seeds from intersectional hybrids but I doubt they were registered. I know two "Itoh" cultivars originated as mutations from a parent "Itoh" peony (White Emperor from Yellow Emperor and Love Affair from Prairie Sunshine), probably all the rest are results of crossings between a herbaceous and a tree peony.
