Rpr ...
I have to say it ... sorry ... there are far to many variables that come into play to make that kind of generalization. Also, it depends on the rose itself.
If a rose goes into winter stressed, it will be more vulnerable to a cold snap. If it goes into winter healthy and is not a "tender" rose, it can handle a cold snap of of 20s, low teens and even single digits. Roses are tougher than most people think, but the gardener has to select the right roses for the conditions for his or her garden.
If you chose a rose with a lot of tea rose in it's lineage, it will be a "tender" rose in that the tea roses brought the repeat blooming genes into the rose pool. Along with that wonderful characteristic, they brought the inability to go fully dormant. What that means is that the canes hold too much moisture within and between the cells in the wood and those water molecules can freeze and expand. When it freezes, the cells burst causing die back, which impairs the health of the plant. That's one of the criterial factors the gardener needs to keep in mind while selecting roses for his or her garden. It was more of a factor when you are looking at the early hybrid teas, grandifloras, and polananthas, but as time when on and these roses where continuously crossed with more hardy roses, the genes became more homogenized and hardier plants were developed.
However, the rose industry continued to bud these plants to Dr. Huey, which is cold hardy only to zone 6, so often the graft would fail. The solution was often to bury the graft or to bury the whole rose for the winter. Uncovering the rose too soon left the roses vulnerable simply because they did not have the tools to survive the colder temperatures.
Other variables included siting of plants. The age of the roses ... meaning how well they were establish going into winter. Juvenile plants without established root systems are always more vulnerable to all of the stressors mother nature can throw at them. Also, if the are sited in a part of the garden in a micro climate where cold air pooled, the zone information was distorted by nature. Drying winds also have an impact.
Mulching with the wrong material can make a difference.
The list of things that can impact the process goes on and on. Not just for roses but for all plants.
When it comes to roses, there is always more than one variable that can make a difference as to why a rose fails. It's not just about cold tolerance. Often people think its about cold tolerance when the problem that caused the rose to fail was something completely different.
Roses are actually highly adaptable, but if you push the zones by selecting the wrong rose for your garden, you are setting yourself up for a casualty.
I think it's important to qualify statements like "Roses can go through hell in late fall with little harm but uncover them and get a cold snap into the low twenties or teens and some times that is certain death, even though they die slowly.
They kind of just fade away."
In my rose life, roses continue to surprise me by not following the rules ...
I am not a professional, nor would I ever claim to be an expert. No one knows it all. Even the most expert rosarian has never grown every rose in every kind of condition in every type of soil.
I always fall back of Ralph Moore's statement, "As soon as you think you know everything there is to know about roses, along comes a rose to prove you wrong."
Edited for another of my famous typos
