I live in Western Maryland, (zone 6) and planted 3 windmills and needle palm in the ground this past summer. The coldest has been around 19 so far and they took it without protection (except for mulch) with no problems. The plants are probably between 3 and 5 yrs old. Being the first winter if it gets any colder i'll stick a garbage can over them. I get mine from Chilly palm..
Chilly Palm Tree Co. - Cold Hardy Palm Trees for the Carolinas AND ebay..but its best to get them from somewhere that tends to get a little cold.