I would wait until fall to separate. You can let the rootstock bloom, normally herbaceous of some sort but tree peony seedlings may be used. Some rootstocks are more atractive than the tp. If you like flower. Cut the leaves off the stems and then this fall when you dig follow the stems to where they emerged from the root. Hopefully you will have roots from above the graft. You may want to add a mound of soil around the tp to see if it will develop extra roots.

maifleur, thanks for reply.
Will do as you say.
One more question, though.
Normaly I would of transplant TP with rootball intact. In this case, when I need to separate HP from a TP should I wash out all the soil to get to the HP roots or it could be done in a some other way without much disturbance to the TP

Wash with a soft spray of water to remove the dirt. Do the separation then allow cuts to dry before replanting. A couple of hours should work. If you replant while cut is moist you may develop fungus or rot.

If you are lucky the dirt will fall away from the roots when you dig and no washing will be necessary.

Got it.
Thanks.