I'm not sure what -9C translates to on the Farenheit scale, but I am also in
zone 8 (PNW) and we had a few lows this winter into the lower teens,
approximately 15-20 degrees below freezing. Rosemary should be perfectly
hardy in our areas even under those occasional unseasonal circumstances so
I'd guess that perhaps drainage is not as sharp as it could be. More often
than not in our mild climates, it is not the cold that kills plants but
winter wet rotting the root system.

Good thought - it might not be the best drainage.

If it survives where you are (Seattle?) it will here (a warm part near
Vancouver) too.

I cut it back hard and we shall see if there is any life left in the poor
thing. Lucky the other one in a spot under a tree is growing all over
everything else or I'd be facing lamb without Rosemary (or with store bought,
which just doesn't seem the same).