What is the cold hardness of a Nephrolepis exaltata planted in the ground? I know the fronds are killed to the ground by frost during the winter in their northern native range, but what is the coldest they can take when in the ground and still sprout back in the spring?

I know for a fact they can grow in the ground in USDA 8a or down to about 13F and still come back each year in the spring. But can they survive in USDA zone 7a with an average low each year around 10F but some years it can get down near 0F.

Any help would be very appreciated.