Are house leeks supposed to be hardy?

Some people grow them on roofs so I'd expect them to be hard as old
boots? I've got some in pots with fierce drainage (more gravel and sand
than anything else) but the Winter frosts seem to have killed them off.
They are all completely black, limp and shrivelled and look like they
have departed the Earth, shuffled off the mortal coil and gone to live
on a roof in heaven.

Should I have brought them indoors for Winter?

--
David in Normandy.
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