In Wisconsin, roses must be protected against not only low winter temperatures but also fluctuating temperatures and winter winds. Different types of insulators such as soil, cones, and mulch can provide the necessary protection.
To protect roses using soil, mound soil 8-10 inches high around the base of the canes after the first hard frost while you can still work the soil. First, either shorten the canes or tie them together to prevent them from whipping in the wind and loosening the soil around the base of the bush. Bring soil from another part of the garden for mounding because you may injure roots if you remove soil from around the rose plant or bed.
After the ground has frozen, usually around Thanksgiving, pile hay, straw, pine needles or leaves over the mounded canes and cover with more soil to hold the material in place. These materials help to keep the soil temperature constant. It is important to wait until the ground has frozen so mice don’t make their winter home inside the mound and snack on the canes while they’re there. For additional protection, place twiggy branches or evergreen boughs from your Christmas tree over the top of the mounds. The branches will help accumulate snow around the bushes which helps insulate the roots while still allowing air to circulate.
You can also make or buy your own rose protectors or cylinders. Cylinders 12 inches in diameter, styrofoam or biodegradable cones or similar coverings will work. These items also should not be used until after the ground has frozen.
To use an open top cylinder, tie the canes together, set the cylinder over the plant, and fill it with leaves, straw or pine needles. Cut the canes even with the top of the cylinder and cover it with heavy plastic to keep the insulation material dry. Anchor the plastic to keep it from blowing away.
To use a cone, mound the soil as above, prune the bush to fit inside the cone and place the cone over the top of the bush. Anchor the cone in place by placing a rock on top or soil around the base. Poke a few holes hear the top of the cone to prevent moisture buildup and mold.
In spring, remove protective materials as soon as danger of hard frost (several degrees below freezing) has passed, but before new growth appears. If new growth has already started, the plant may need protection in case of frost, so replace the protective covering if frost is predicted.
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