The Green Fuse?

 I await an explanation for the hardiness of my sedum and parsley (previous post) from some all-knowing botanist who---mirable dictu!---descends on my blog.  Meanwhile, as I rake in the garden, I'm thinking of a line from Dylan Thomas--"The force that through the green fuse drives the flower" (from the eponymous poem).  Maybe there's something particular about the living force of them that protects the sedum and parsley from freezing? 

This train of thought doesn't seem promising. I wish it were, because, wouldn't that be wonderful?