I know; it is 25 degrees outside and winter is already here. What is the sense in mulching plants at this time of the year? Well folks, this is the best time to do it. The ground is frozen and… it is only 25 degrees. Most people think that the mulch is put on before the ground freezes. That is not optimal.
Once the ground is frozen, it is much more difficult for voles to dig under the mulch and down to your yummy roots. Furthermore, the mulch will help to insulate the frozen ground. Again, that may appear backwards, but that is what you want to do. You want to buffer warm and cold extremes at your root systems and keep the ground frozen around the plants. It is the freeze-thaw cycles that kills and tears roots. Therefore, with proper mulch at the right time, your ground is frozen at a reasonably good temperature and will remain frozen for some time throughout the winter.
The best mulch for this time of the year is straw. It can easily be purchased and it is not frozen solid. It can also be easily removed in the spring when the danger of frost is past. In the spring, be sure to compost it or even re-use it as regular mulch around garden plants that are not part of ornamental landscaping. Straw does not look as attractive as bark mulch for landscaping.
You do not need to winter mulch established trees and shrubs for the winter. The plants you will mulch will be perennial flowers, perennial herbs, trees and shrubs that have not grown for two years in your yard, and biennial vegetables that are needed to re-sprout in the spring so they will bloom and produce seeds. Examples of the latter are kale, cabbage, parsnips, carrots, and beets.
I do not let all of my biennials stay outside for the winter. Instead, some are put into the refrigerator in perforated Ziploc bags that contain a mix of damp peat, vermiculite, and perlite. I do not try to get seeds from cabbage and I leave the kale out in the garden to over-winter. If a catastrophic event were to suddenly take-out electricity, I would certainly change my method and have all of them over-winter in the ground with a good layer of mulch. However, I would also put hardware cloth around them that would extend into the ground to keep voles at bay. Or, if I had access to strong mouse poison, set poison bait stations out to eliminate the vole problem entirely. The latter is preferable if it is available.
Happy winter and the best of fortune with your plants!