Franz Heymann wrote:

According to the RHS Encyclopedia it will take it down to -5 deg C.
I used to have a clump thriving outdoors for many years in the Woking
area.

Well, I'm on the outskirts of sunny Wakefield in West Yorkshire, and the two
small clumps that I have seem to have survived for several years, though
they aren't exactly thriving in that they are not spreading and they are not
flowering. Maybe I will try lifting them and storing them over the winter,
and replant them somewhere a bit sunnier next year.

Rick