We live in Minnesota, where it gets quite cold in the winter and the ground freezes well below the depth of the roses.
We have a row of rose bushes which we inherited from the previous owners of our home. For the first two winters we have followed the instructions given us by the previous owner: prune back to about 18", mound up dirt on each plant, encircle with chicken wire, fill chicken wire with straw (or with leaves and top with straw). We have not lost a bush yet.
My questions are: Why does this work, and what parts are necessary?
It's not preventing them from freezing, since the ground freezes down pretty deep. And it's not protecting them from wind chill, since they are roses and not warm-blooded animals who would perceive wind chill. (If the air temperature is -5, and the wind chill is -20, anything exposed to the air will tend toward -5 and not get any colder.)
The portion of the plant that is beneath the ground or the mounded up dirt will certainly be below freezing, and well below freezing as the winter goes on. It will, however, be marginally warmer than the air temperature. If the ground is frozen down to about 3 ft, and the average air temperature is, say, 15 degrees, then the ground between the surface (15 degrees) and 36" down (32 degrees) will be on a continuum between those two temperatures.
So does mounding up the dirt help to keep the plants slightly warmer than they would be without it (e.g. 20 degrees instead of 15)? If that is the case, I don't think the straw on top would contribute anything to this, or at least would be no better than snow piled on top.
It is to reduce the temperature swings? If so, are any temperature swings hard on the plants or only crossing the freeze/thaw line?
Why does covering the roses work, and what parts are necessary?