Yan, I would definitely mulch your plants with some leaves and/or evergreen branches (the object is to prevent repeated freezing/thawing during the winter), and even if the tops of your plants do die back they should re-grow in the spring; you probably won't get bulbs as large as you would have otherwise, but they should still do okay. Have you had unusually mild weather? You could also consider covering your plants with a "low tunnel" by covering hoops of PVC pipe with clear plastic; I planted onions that way last fall and the tops stayed green all winter and began growing again very early in the spring -- and garlic seems to be much tougher than onions!
