No idea. I think they're hardier than they're given credit for. I had a huge one in Switzerland that used to survive well through most winters when it would hit -10 and below from time to time. It finally succumbed two winters ago when we had a really cold spell that lasted about three weeks - I think it didn't get above -5 for the entire time.
I've got quite a few mature ones in the garden here - trained into rectangular pillars - and I do nothing to them except cropping them back every so often (and using them for cooking).
Mind you, most times when a plant dies I have no idea. Try another one and if that doesn't work, maybe try it somewhere else (2 feet apart is quite close given how big they get once mature).
H