After temperatures hit freezing and the plants die back, cut the stems on most perennials to within an inch or two of the ground. Dispose of the cuttings; they can harbor diseases that could survive the winter and return to the plants in the spring. Some plants, such as Oriental poppies and iris, produce a cluster of green leaves in the fall. Leave these intact. Remove only the older, brown stems that remain form the spent flowers.
As the season progresses and the weather becomes colder, mulch the soil around the plants. This is generally done in mid-to late November. Mulch keeps roots cold. It doesn't protect them from the cold. A plant can be hardy in more northerly latitudes where winter temperatures are severe but can be injured here, where winter temperatures fluctuate considerably. The alternate freezing and thawing of exposed soil can damage roots and even heave them out of the ground.
Recommended mulching materials for perennials include hay or straw, evergreen boughs, pine needles, peat moss and cornstalks. These mulches are light and won't pack or suffocate roots. Apply to a depth of 4 to 6 inches. A few plants, however, such as peonies and bearded iris, don't require winter mulching and , in fact, do better without it. Mulching can cause their thick, fleshy roots to rot. As with other perennials, though, they require watering during dry winter conditions.