Here, one would pull out and check the roots because the gophers eat them. Cold should not do that in z8.

I am highly aware of gophers and this was not they. I trap them and plant in cages.

Do you get a lot of wind and exposure where you are that might be creating a colder than normal microclimate? It does seem unusual to have that many roses die even with our colder than normal temperatures last winter.

Since you mention gopher cages I would still check the roots if you havent already, my dad has to use cages as well for ground squirrels and still they sometimes get through.

I agree I would be surprised if it was the temperature. I know that at the very least, Sally Holmes and MIP are both hardy here (zone 5 Colorado).

Yes. ALOT of wind exposure

Sally Holmes is my test rose - if she won't grow in a particular location, no rose will. The only thing I've found so far that she's sensitive to is salt. I would get a soil test to see if there are any clues there.