I wouldnt rely on a website for "the truth"
"said that the plant could take temperatures only as low as 10° F."
so that means at 9° F the plant will die??
I think not!
a much more reliable indicator than cobraplant.com is nature itself!
think about where S. Purpurea lives in the wild, we have them growing here in western NY, and they grow into Canada!
I know for a fact it has fallen to zero, and below zero, many times in the last few million years that the plants have been growing around here..it falls to zero or lower every winter!
the plants always survive..
yes its not GREAT when temps get that low..and its risky, and your plants *might* die..
but, they CAN take it..
I would leave it outside under heavy mulch and a thick layer of snow..
if the air temps is zero, but the plants are buried deep under snow, it wont be that cold where the plants are! snow is a great insulator..
they should be fine..
Scot