Gardening experts split over the proper time to cut back perennials. The traditional view is to do so in the fall and there are benefits to that course of action. If the plant is suffering from disease then an autumn trim followed by a long, dormant winter can help the plant come back healthy next year. On the flip side, leaving perennials untrimmed gives birds access to a meal in winter according to Master Gardener Carrie Sherrill.
Sherrill says that she goes about half and half each year as far as the number of perennials she trims back in the fall. She says she will always keep the occasional stalk to ensure what she calls "winter interest." For gardeners, just having something poking out above the snow is better than nothing at all. If plants are not cut back in autumn, they can be pruned back through late April when new growth is just beginning to sprout.