Here in Minnesota one of the things that we need to worry about is the cold. Over the winter we can see temperatures down into the -30s (even the -40s in the Northern part of the state) and it can damage many of the plants that we grow. The tops of the trees are usually able to handle these types of temperatures — though a good heavy snowfall can cause a limb to collapse now and again.
The bigger problem is with roots which aren’t able to handle the cold like the top of a plant can. Once you get 10 degrees or so below freezing you’ve killed the roots of most plants. Fortunately the ground is a great insulator and doesn’t get nearly as cold as you’d think. Once you get two or three inches under the surface of the soil temperatures will hover right around freezing for most of the winter. The plants that are most susceptible to cold are those that are in containers because their roots are above the soil’s surface. Nursery growers usually protect containers from the cold by consolidating them (pushing them together as in the picture below).
These containers are then covered with a layer of polyethylene fabric, about 6 inches of straw, and then another layer of polyethylene fabric. Temperatures under the fabric rarely go below 26 degrees or so — even when outside temperatures stay around -20. Most plants come out fine — our biggest problem is that sometimes voles and mice will take up residence under the tarp and eat the plants — which isn’t a big deal unless its a research project — in those cases we will often use poisons or, more frequently, repellents.
The only thing better than this method is snow. If we could count on snow every year we wouldn’t bother covering the plants at all. Snow is the best insulator that we have. Under snow temperatures rarely go below 29 degrees or so. So, despite the traffic problems that snow causes, nurserymen and landscapers are always happy to see snow on the ground before the really low temperatures hit.