|when is the best time to cut back|
|Peonies are not usually cut back until after the first frost of the season kills the foliage. So, if you mean you want to cut off the spent flowers, go ahead and cut the old flowering stems down below foliage level. Leave the foliage on the plant until it turns black (from the first frost) and then you can safely cut the foliage down to ground level.
Best wishes with your garden!