My basjoo overwintered in-ground for the first time this past year. It was an extremely mild winter here in terms of lows.

I know one thing that seems to really help is if you can plant it right against a sunny foundation wall of your home, as the extra heat leaking out of the foundation during the winter moderates the soil temps during the cold.

Otherwise, really DEEP and DRY Mulch -- they die from the moisture promoting rot as much as from the cold.

Now, as far as palms, two things -- stick with Trachycarpus for the most part -- they are cool season palms and do far better in our climate than most of the others, that need a lot of summer heat to thrive. Same rules for overwintering, lots of shelter but the top must be kept dry.

I overwintered mine in an unheated greenhouse last winter by enclosing them in foamboard boxes and running 3 13 watt CFC bulbs for a box roughly 4 feet x 2 feet x 4 feet from early Dec to late March. They came through in pristine condition and are growing a lot now.

Check out the posts on the banana and palms/cycads forums.

In particular, check out the posts by Artictropical (Kevin) from Zone 4 N. Utah on how he overwinters massive palms in his climate, that can get down to like -45 F air temps. It's inspirational.

I live in Chicago and I have overwintered musa basjoo for the past 5 years. Last year I didn't put anything on top, no mulch, nothing and it came back fine. I am zone 5, however, last year was a warm winter. It survived below -20F temps with 6 inch mulch, nothing else.