All of my bulbs go through the winter with a layer of mulch, leaves, pine needles mostly, and it doesn't prevent them from blooming. As long as the soil temperature remains below 40°F for several weeks, and it usually does around here. Mulch helps keep the temperature of the soil fairly constant, once it becomes cold. Ideal way would be to wait until the soil temperature becomes cold and then add the mulch, but who wants to be out there working in the garden on cold days? :-(