Average lowest ever used to be around 25*, occasionally it has gotten to 23. Lately it hardly ever gets to that. I know those have been through that or very close to it while I've been here for the last 20 years. They are on the North side of an 8-story office building. I would think that you could give them some winter cover when it gets that cold for a few years and then they should be able to take it with some damage but not death. There are numerous Dracaena marginatas around that get nipped back every few years, but they come back too.
