Paul,I think you can probably grow those outside in your zone for just about the whole year. I am in 5a and have taken my in 2 or 3 times overnight when it went into teens. If you keep the container from freezing solid for prolonged periods of time it will be fine. Even a few times freezing will not kill it,won't even drop leaves probably. I bet in your greenhouse it will live without heat 365 days a year.
I had a few plants of different species freeze solid for 6-8 hours without dying. Even one that I think might be more cold tolerant than C.flexuosum. This isn't something I have ever had interest in but having a few accidents lately has opened my eyes.