Tree peonies don't need to be pruned/trimmed. You can deadhead the blooms as they finish, but only cut just below the bloom, removing excess stem can cause less productive growth the next year. Removing the spent blooms will allow for more root growth instead of putting the plants energy into seed production.
The only time you would prune is to shape a plant, but that will remove bloom potential for the next year, as they set bloom for the following year, right after blooming.
