Winter mulch isn’t necessary for all garden plants, but it can mean survival for some less hardy ones. Winter mulch has a different purpose than summer mulch. The main benefits of winter cover are to protect against wide temperature fluctuations in the soil and to prevent extreme cold temperatures from harming plants.
The goal is to keep the plants dormant, rather than to keep them warm. If plants are kept too warm, they may break bud and the subsequent tender growth will be killed by winter conditions. On the other hand, winter mulch does provide some insulation against severe low temperatures.
It also protects the soil from wide temperature fluctuations, keeping plants from being damaged by heaving. Soil tends to heave when subjected to extreme temperature changes, pushing plant roots up out of the ground. Heaving is most harmful to shallow-rooted plants, such as strawberries, and newly planted specimens of any kind that have not yet had a chance to develop solid footing.
Timing is critical when applying winter mulch. Applying it too early can smother the plant and encourage disease development. Once the plants are completely dormant and temperatures are consistently below freezing, then the winter mulch can be applied. In most cases, 2 to 4 inches of organic material, such as straw, pine needles, hay or bark chips, will provide adequate protection.