Herbaceous peonies are a long lived perennial. Herbaceous means that the plant dies down to the ground in autumn. It is also interesting that herbaceous peonies form the buds for the next season underground. Buds are formed at the crown of the plant--this is why they are so hardy in northern climates.
Reasons to prune herbaceous peonies are to cut away any broken stems, remove spent blooms, remove diseased buds or stems, and prepare the plants for winter.
Check for broken stems. Prune the broken stem or stems and deposit the plant material in your compost bin.
Deadhead the spent blooms by cutting them off of the stems with your hand pruning shears. You can then place the spent blooms in your compost bin.
Prune away any infected/diseased spent blossoms, buds, leaves, and stems. Do not deposit the diseased plant material in your compost bin. Be sure to disinfect your hand pruning shears. To disinfect dip them in a 10 percent bleach solution or spray them with a 70 percent alcohol solution.
Cut the plant down to ground level after the first hard frost. You may then add/mix organic compost to the top 1 to 2 inches of soil around the plant. Do not put the compost directly on top of the plant. Peonies grow well in organically rich soil.
Cover the plant bed with mulch. Mulching materials include leaves, which are readily available in the fall, straw, hay, or evergreen boughs.