I don't grow Crown of Thorns, but my plumeria has already been subjected to 48F on a couple of occasions. They are as healthy as can be. My plumeria will begin to go inside when the temps get down to the mid or even lower 40's. They are going to go completely dormant regardless, so what's the big deal if they get some cool temperatures? I just think the coolness better prepares them for dormancy.
