Answer: Most gardeners will cut back perennials in the fall, but not until frost has killed them back and the foliage is dead. When you do so, leave in place any basal foliage that has remained green.
Some gardeners will leave some of the faded flowers/stems intact until spring to provide winter interest and allow birds to savor the seeds -- plants favored for this treatment include sedums, purple cone flowers, black eyed Susans and ornamental grasses. These would then be cut back in the spring.
It is partly a matter of personal preference, partly a tradition based on the idea that gardeners have more time to clean up the garden in the fall than in the spring. However, it is always a good idea to remove any diseased or infested foliage in the fall to try to limit its reappearance the following year.
Enjoy your perennials!
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