While much of the nation sits in a deep freeze, here I am, in my nice warm family room, looking out at a sunny blue sky day. The ground is covered with snow (four inches of it), and the thermometer reads 9°. With temperatures like this, I'm glad we've got the snow cover, because it helps insulate the plants from the cold.
Mulching helps keep roots warm, and I'm glad I added more to the base of some of the more tender plants this fall. But it's still pretty darn cold under that mulch and snow, and the ground is frozen solid. How do the perennials live to see another spring?
Of course, some survive better than others. That's what the USDA hardiness zone ratings are for; a plant that's only hardy to my zone 5 won't make it in Alaska's zone 2. But the mechanism that allows a plant to return in the spring after a winter of below freezing temperatures is the same.
Many perennials lose the parts of their plants that are exposed as the temperatures drop. The roots in the ground move as much of the the water in their tissue's cells into the surrounding ground as possible. With less moisture in the plant's cells, a certain amount of freezing and thawing can occur without rupturing those cells.
Also, sugars and salts in those roots act as a natural antifreeze. By lowering the freezing point of the water in the plant's cells, it allows them to survive lower temperatures. Think about what happens to ice and snow when you salt the sidewalks. It works much the same way in plants.
I'm not a botanist, but that's basically the gist of it. Pretty cool, huh?