Journey into the winter underground

Unless my sewer line breaks or I’m planning my garden, I don’t tend to think about what is underground during the winter. The ‘underground’ feels totally inaccessible in the cold months: frozen solid and impenetrable, as indeed it is. I invite you, however, to journey with me briefly beneath the layers of snow and ice and take a look at what is happening, albeit slowly and quietly, right under our feet.

The first thing we will notice as we sink into the ground is that, yes, it is frozen. Any water that was in the soil when it froze has formed ice crystals of various shapes and sizes, from tiny grains to long needles of ice. This is, in fact, what makes frozen soil hard: perfectly dry sand or soil will get cold in the winter, but will not freeze hard. Frost heaving, caused by water in freezing soil, is when ice crystals become large enough to actually move the earth and anything on it. Frost heaves can push the ground up a foot or more, damaging foundations and roads, and shoving plants out of the ground.

In this frozen area of soil, we can’t see much that is alive, yet there is life. Grass roots and tulip bulbs exist here, waiting patiently for the spring thaw so they can grow again. Bacteria and other microscopic beings are there as well. Both bulbs and bacteria create extra sugars within themselves, which prevent their cells from freezing solid in the same way that road salt keeps our roads from freezing. This zone may seem dead, but it is teeming with life.

As we travel deeper, we come to a point where the soil is no longer frozen. In the southern part of Minnesota, this will (in an average year) be about three feet from the surface. This is the frost line, and is extremely important to wildlife and building contractors alike: below this line, hibernating animals can slumber without freezing into animal-shaped ice cubes, and builders can sink solid footings into ground that won’t heave. Earthworms have tunneled their way to this depth, and are curled up like fleshy knots in tiny burrows, waiting for the spring rains. We’ll also come across larger animals: chipmunks, moles and jumping mice which dig their own tunnels, and toads and snakes which will use any tunnel they can find.

Most of the animals in this frost-free zone appear asleep, but in fact their strategies for the winter vary widely. On one end we have hibernating groundhogs and jumping mice: these animals’ body temperatures have dropped to around 40 degrees, and they are unable to respond to any disturbance until the end of the winter. Chipmunks, too, have cooled off and are sleeping, but only for short periods at a time. Every few days they rouse themselves, bring their body temperatures back up, eat a snack, and then go back to sleep. Snakes and toads, on the other hand, are not asleep:  their bodies are cold, but they are alert and able to move around (however sluggishly) and respond to what is around them.  Finally, if we are lucky, we might see a very much awake and active mole, a tiny predator that can dig a 100-foot tunnel in one day. These toilet-paper-tube-sized animals dig throughout the winter, searching for worms, beetle larvae and other mole delicacies.

Depending on where we are in Minnesota, this might be the end of our journey—as we go deeper into the unfrozen soil, we see fewer and fewer animals, until we hit bedrock (around here, that’s perhaps 100-200 feet deep). But even with this short imaginary exercise, we are better able to understand a few of the mysteries the winter underground may hold.

Join us at Kroening Interpretive Center in North Mississippi Regional Park for the following nature programs. Reservations are required. Call 763-559-6700.

Snowshoe Date Day, Saturday, February 13, 2-4 p.m., ages 21+, $25/couple. A Day in Frozen Wonderland, Monday, February 15, 10 a.m.-noon., all ages, $5. For young children and their adults:

Groundhog Day Puppet Show, Tuesday, February 2, 10-11 a.m., ages 2-6, $4 per person; and Making Tracks, Thursday, February 11, 9 a.m.-noon, ages birth-6, $5 per person.

 Free Family Fundays: Come by on Sunday afternoons anytime between 1-3 p.m. for a free family program, all ages welcome:  Snow Forts on February 7; Backyard Birds on February 14; Knots on February 21; and Tracks, Trails and Tales on February 28. In addition, there will be a free river walk on Saturday, February 6 from 9-10 a.m. Reservations are not required for the above free programs. Call us at 763-694-7693 for info or visit