If you are growing perennials in a region where the temperature drops below 0 degrees Fahrenheit, it's a good idea to take some special precautions to ensure that your plants survive the winter. After the ground has frozen, trim back any dead foliage left on the plants.
Rake up any leaves and other debris around the plants. Leaves tend to mat down over time and prevent air from circulating around the crown of the plant. This makes roots susceptible to damaging freezing and thawing cycles and prone to root rot.
The leaves that you remove from your flowerbeds make great material for composting.
The last step is mulching the perennials. Loosely pile evergreen bows on top of your plants. The bows will form and trap an insulating layer of air and they won't rot. Straw makes good mulch as well. Avoid using hay because it generally contains weed seeds. If you choose to use compost or bark mulch as insulation, mulch to a depth of 4-6" around the plants, but do not cover the crowns of the plants as this will foster root rot.