Providing good mulch the year around is very important. It helps the soil retain moisture, keeps the winter weeds out, and in winter helps prevent deep freezing of the soil and plant dehydration. Many materials are suitable. These include: pine needles, ground bark, or pine, hardwood, oak, or other leaves (preferably partially rotted), and living ground covers such as myrtle (vinca minor). Cover to a depth of two to four inches.