Answer: You can prune off spent flowers during the summer months. Wait until frost kills the tops and then cut them off at ground level. When they experience a frost it signals the roots of the peonies to go into dormancy. If you simply prune everything down before the first frost, the roots will use up energy trying to replace the lost stems and leaves, which will then be killed by frost. So, just wait until the tops are blackened by a frost and you can safely cut them off.
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