The time is right for overwintering your flower and vegetable garden. This mulching process will keep ground temperatures constant through the winter, so your perennial flowers and herbs do not experience root damage due to repeated freezing and thawing of the garden soil.
Although the materials used for overwintering are insulators, their purpose is not to keep the ground warm. On the contrary, the layer of mulch is intended to keep the ground frozen until spring. When frozen ground thaws and refreezes, it can buckle and push roots out of the soil. Exposed roots can suffer injury from direct exposure to the elements. Perennial plants have long roots, developed over years of growth, so this damage can be very serious.
Wait until plants have gone dormant and the ground is frozen before applying mulch. You may be familiar with the look and feel of frozen soil, but a probe thermometer is always a safe way to check. Straw or evergreen boughs are ideal materials for overwintering because they are thick enough to provide insulation, yet flexible enough to bend if covered in snow without compacting and damaging plants underneath. Lay straw or evergreens in a 4-inch layer over the garden bed.
One thing to watch for when applying overwinter mulch to your garden is the sudden presence of rodents looking for a warm place to spend the winter. Waiting until later in winter to apply mulch is one way to deter their sudden appearance. By the time temperatures are regularly below freezing, mice and other critters will have already found a warm location to wait out the cold.