Perennials can suffer when temperatures fluctuate in winter. The soil freezes when temperatures dip low, and it contracts as it freezes. If a warm spell occurs, the soil thaws, and as it thaws it expands. This disruption often dislodges plants, forcing their roots and crowns upward. Subsequent cold temps and the exposure to the elements can damage these exposed parts.
Prevent this damage by applying a mulch to the soil around perennials, particularly those that are marginally hardy in your area or that have shallow root systems. Don't do this task too early—wait until after the soil has been subjected to a few hard freezes. Then lay two to three inches of mulch over the soil. The purpose of this mulch is not to keep the soil warm, as you may assume. Instead it serves to keep the soil frozen and in place until spring's true thaw.