Cynthia, I don't know. They are supposed to be hardy to 20 deg., but I lost my purple ones at 28 deg. last winter. The white ones came back with no problem. Since you have a greenhouse, I'd take some cuttings now, and root them to keep in the greenhouse just in case. They root very easily and very quickly.
Jim

Thanks Jim
I didn't think about rooting some cuttings....DUH!

Mine are the white ones, and I would like to have more next fall. They are like little sparklers

thanks again
c

mine came back with no problem.

Don't forget to harvest seeds!
None of mine came up this year because I mulched and then didn't plant any of the seeds I harvested last year.