It is not the prolonged below 40 F, it is just the fact that it gets that low. We typically don't stay below 40 F either, but we have lots of AM's in the 20's and 30's and occasionally we get into the teens. I worry about my garlic when it is in the teens though. I prefer our lows never get below the mid-20's for best growing.
Certainly both the Gulf down here and the Atlantic up there moderates the weather.
