Does anybody know the true in the ground cold hardness of a Nephrolepis exaltata? Specificaly Nephrolepis exaltata `Bostoniensis' if that matters.

I know Nephrolepis exaltata fronds are killed to the ground by frost during the winter in their northern Florida native range and then come back in the spring, but what is the coldest they can take when in the ground and still sprout back in the spring every year like so many native ferns do here?

I know for a fact they can grow in the ground in USDA 8a zone or down to around 13°F (-11.5°C) and still come back each year in the spring. But can they survive in USDA zone 7a/7b outside in the ground? Here in AL we get an average low each winter around 5 to 10°F (-15 to -12°C) but some years it can get down to around 0F (-18C).

I just thought it was strange that all the sources I've looked at show it hardy just to USDA zone 8b or 9 but it seems like they might could grow outside in the ground in colder zones such as 6 or 7 at least and come back from the ground each year.

Also does anybody know a online source to buy the exact original classic cultivar: Nephrolepis exaltata `Bostoniensis'? It seems like every single nursery and store sales all the cultivars of Nephrolepis exaltata, except `Bostoniensis'.

Any help would be very appreciated.