One of the final tasks of the gardening season involves mulching perennial plants. While snow is likely the best form of winter insulation, we cannot always rely on consistent and adequate snow cover to protect our plants. Therefore, straw, leaves, or other organic material is often applied to perennials to help protect from extreme cold and to shield the plants from fluctuating temperatures and breaking dormancy too soon in the spring.
The best time to apply protective mulch is after the ground has frozen. If unsure of the proper timing, applying mulch later than ideal is better than spreading it too early. In fact, mulch applied on top of existing snow cover is still worthwhile since the snow adds to the insulating properties of the mulch. The benefits of the ground cover also continue into the late winter and early spring, so it is still worthwhile even if the mulch cannot be applied until much later in the year.
On the other hand, applying mulch to plants too early- before the soil freezes- can trap warmth and encourage plant growth beneath the mulch which will ultimately weaken the plant. In addition, trapping heat below the leaves or straw creates an appealing environment for rodents, which may seek warmth under the mulch and in turn begin feeding on the plant material.
Before applying mulch, trim back the dead plant material. Then, add approximately six inches of leaves or straw to provide proper insulation for most plants. Mulch should be removed in early spring to prevent fungal diseases, but may need to be temporarily reapplied to protect from a late spring freeze.