I don't think they'd survive long term if your temperature drops below 20 degrees. Here in zone 9, Phoenix dactylifera is damaged on the rare occasion that the temperature drops below 25. They will recover as soon as the temperature warms back up, but they're also not having to deal with freezes year after year. Some years are almost frost-free or completely frost-free so they have a very long, warm growing season.

Planted outside, it may survive for a bit and then have a long, slow decline.

I grow a number of different types of date palms here in Florida. About as good as you'll do on the east side of the Rockies is growing them in a strong zone 8b. There are some areas that plant them where record cold will eventually take them out (coastal GA, SC, OBX NC). They are slightly more hardy in arid and semi-arid places, like the west coast and southern desert west.

My medjools have seen 24F in Jan. 2010 with no damage or negative effects. That being said, probably another 5 degrees lower and the story changes dramatically. I have 7 Cretan Date Palms that are supposed to be a little more hardy. I've heard the reports of this palm being a zone 7b palm, but my belief is that this is largely being pushed by palm sellers. This is their first year in the ground, so no experience to speak from on my end.

Even if you had a plant that tended to sucker profusely, it would be a stretch at best for even one well protected offshoot to survive in zone 7 for any length of time. There are claims of date palms surviving high single digits in places like Las Vegas, Nevada or St. George, Utah. How reliable those claims are, I have no idea.

In TN, you're looking at having to build some kind of shelter around it in the winter if you keep it outside. If you have any friends living in a solid 8b climate or better... it would make an interesting Christmas gift.

If you're really into the idea of growing palms outdoors in TN, you should check in to getting some sabal minors ('McCurtain', or southern Arkansas ecotypes), or needle palms. They are both good understory palms that can grow under live oaks and other large trees that provide overhead canopy (good freeze protection).

Some of the dates apparently recovered from around 0F in areas around El Paso TX. (and in the 60s, possibly even lower temps in southern NM) They are hardier in desert climates with the types of freezes they have those climates: radiative. Also, the next day or so after that 2011 freeze it was up in the mid 60s and sunny and dry again with blazing desert sun. (I know, I was there) That is why their northern limit on the east coast is around Charleston and not, say, Virginia Beach.

davidrt28,
Interesting to actually hear from someone on the forums about date palm hardiness in desert climates. I usually just read the stories on the net. 0F... wow... surprising! All I can say is that I hope I never have to find out if mine will survive that here in central Florida (not likely).

Just so it's clear I was only visiting El Paso around the time of the freeze. I've read the reports of the recovery on other forums, like this one: http://www.palmtalk.org/forum/index.php?/topic/22599-phoenix-canariensis/ As you can see they came back from -1F.

Apparently though, a lot of the injured dates there got removed/destroyed because people thought they were dead. When I drove through the SW in 2011, I saw injured palms from high elevations outside Phoenix, Arizona all the way over to roughly 1/2 way between Houston and Austin...a huge swath of almost 1000 miles. The Washingtonias and Phoenix in El Paso looked especially terrible, and uninformed gardeners might have thought they could not recover. Some upscale neighborhoods of El Paso had, apparently, had 'skyduster' Washingtonias imported from Southern California or Arizona. They were big brown dead looking blobs in the sky so I bet a lot of them were cut down before having a chance to regenerate. Big phoenix were rarer but I saw a couple with say 8' trunks, all the foliage completely brown. Other landscape plants showed damage too...huge oleander bushes with severe foliar burn, etc.

build a green house

they can come back becasue the growing spear doesnt have ice in the desert bc the humidity is low