Like Bigdog mentioned, they seem to be very tender, at least the foliage and stems. We have them here at Leu Gardens in Orlando and the dieback/go dormant almost every year once it gets below 40F. They are under shade canopy and in a very protected location so they never get frost. Maybe they are root hardy but they are also the last banana to really get growing in spring even though it starts getting hot here in late March/early April. They also flower late in the year, late Oct. and Nov. But the fruit never ripens (for seeds) as it cools down mid to late Dec.
Here are a couple of photos from last year, they have yet to flower this year;
http://pg.photos.yahoo.com/ph/leu451...49.jpg&.src=ph
http://pg.photos.yahoo.com/ph/leu451...e2.jpg&.src=ph