Why Bulbs Don't Freeze in Winter
While winter soil may actually freeze to depths beyond which the bulbs are planted, soil temperature will rarely fall below 29° F or 30° F (-1°C). At these just-below-freezing temperatures, water in the cells of the bulb may freeze but the cells will not be harmed. Also, as is true for many hardy plants, cold temperatures trigger starches in bulbs to break down into glucose and other small molecules. This simple sugar or glucose, interacting with other small molecules, acts in much the same way as salt on a winter sidewalk. The sugar in the bulb, like the salt on the sidewalk, lowers the temperature at which water freezes.
This fortunate chemistry helps to keep bulbs safe and snug in their winter beds. Other factors that help keep soil temperatures within tolerable limits include an insulating snow cover and, in colder areas, a nice layer of mulch over the bulb bed once the ground temperatures have dropped.