Interesting that you say this about it being cold hardy.
I had 3 or 4 varieties last year (and I did not bring them into a garage or cover them in any way while in pots). I feel bad about this, but just did not have time.
I figured it was too late, but I brought them in sometime in late January (when we already had a pretty harsh winter). They all died except..
The EL Celeste, (I just noticed today, May 10)... is leafing out from the bottom.