Hardy tree peonies

Coldhardy tree peonies.. Is tree peonies that can survive temperatures down to minus 20 degree celcius – some of the types called Paeonia delavayii lutea and Paeonia delavayii delavayii..Is this hardy Also the most of the type Paeonia Rockii is hardy too.. I have grown theese tree peonies for over 20 years now