I have been requested to re-post this topic. I previously had it posted in response to another enquirey.
I am not an expert (yet), but I live in southern Saskatchewan (Zone 3a) and, as such, have quite a bit of experience with winterizing tender roses. I have about 250 roses, of which at least 50% can be considered as tender and about 20% very tender roses. I also believe that my winterizing method helps significantly reduce blackspot and other fungal diseases in my rose garden from re-occuring the following year.
The commonly understood criteria for protecting roses in our climate is to prevent freeze-thaw damages in the canes during the spring time when night time temperatures drop to below freezing and when daytime temperatures are high enough to kick start the plant. It seems that the extreme cold is not overly harmful, except for the most tender of roses and when temperatures are below -40 to -50 (brrrrrr). I have been almost 100% successful when our winter temperatures have not been lower than -30 and at least 85% successful if they happen to drop below -40.
Here is what I do, which is a bit different than what you might read in other publications:
1. The first thing that is most important is when you first plant the rose. I dig my hole, as per usual practices, but the depth is such that the bud union (swelled area where the graft meets the root stock) will be positioned at least 4 inches below the adjacent ground surface (i.e. the area well beyond the rose). When you are finished planting the rose (including backfilling), there should be a depression remaining around the rose about 4 inches deep and the bottom of the bud union should just be into the ground surface at the base of the plant, but do not cover the bud union with soil. As will be seen below, during the winterizing process, the bud union should be surrounded by dry peat moss which is an excellent insulator, better than moist soil.
2. I then use a 4 or 5 gallon plastic pot (the kind used to sell large plants at nurseries) with the bottom cut off and placed around the rose, stuck slightly into the ground at the base of the plant and sticking about 1 or 2 inches above the adjacent ground surface. If your rose has large branches sticking out, you may have to prune a bit and/or bend the branches so that the pot can be inserted over the rose once it is planted. This ring will be about 8 to 12 inches high (depending on pot size) and serves as an excellent water reservoir when you water the rose during the summer. It also serves as a backstop if you use bark mulch, stone, etc as a landscape material in your rose garden, preventing this stuff from falling into the hole around your rose.
3. Let the rose harden off as much as possible prior to sustained cold periods. In my area, this means leaving them harden off without protection until at least the third week in October and it also means not deadheading the buds (the presence of rose buds on the plant tells it that winter is approaching). The hardening off period could extend later in your area. I also generally hold off on the watering a bit during this period. Some notable rose growers in my area apply a fertlizer known as super-phosphate which is supposed to help expedite the hardening off process by preparing the canes for winter.
4. When the hardening off is completed, I (in sequential order): (a) thoroughly water the roses down; (b) remove all debris and leaves (for burning); (c) prune back the canes to between 12 and 18 inches (this may not be necessary in your area); (d) spray the canes thoroughly with lime-sulphur (helps eliminate over-wintering insects and disease) and let this dry thoroughly; (e) spray the canes down thoroughly with an anti-desiccant (to prevent dehydration of the canes - I use hot pepper wax for this purpose); (f) place dry peat moss around the rose to at least 12 or 15 inches above the bud union; and (g) place a styrofoam encasement above the peat moss with a weight on top such as a brick or stone to keep the hut from blowing away (Note: The styrofoam cover must have holes in the side to allow moisture to escape). The styrofoam encasement sold in stores is called a "Rose Hut", but may not be available in your area. It is important that you use dry peat moss because it has superior insulation properties relative to moist peat moss and/or soil. If you don't have a "Rose Hut", maybe try using something else such as a large pail or garbage can. Whatever works, but make sure you have ventilation.
When spring time arrives, do not be too quick to remove the protection. That was my first mistake when I first started this system. Wait at least for the native tree buds to begin popping out. That generally is the signal that you can start removing the cover, including the peat moss (carefully so that you don't break off any sprouts). For the large number of roses that I grow and in recognition of the large volume of peat moss that I use, I have discovered that a large 6 to 7 Hp shop vac works well for removing the peat moss - extremely fast and no damages to the sprouts.
The above may be too extreme for your area and some steps may not be necessary. Admittedly, it can be quite time consuming, but it is one of the best and, if you have that special/extremely tender rose, you might want to try it. I guarantee you that, if you use all of these steps, you will have protected your rose against the harshest winter and spring freeze-thaw conditions, to the extent that is practicable.
If anyone wants some photos showing this method, let me know.