Depends how much time below 20° you have. That seems to me to be about the breaking point with covered stems. In 2003 I bottomed out at 9° and lost at least 4' on each stem. You might try adding christmas lights to provide some additional warmth. I think its going to be very tough to winter over stems that far north, but it won't hurt anything to try.

Island breeze, a simple method some membrs of this forum use is the leaf bag method.They fill plastic garbage bags with leaves and place 4 bags around the pseudostem and then place one more bag in top.You can maybe tie the bags togeather with some twine to keep them in place. Certainly not attractive, but agreat insulation method, cheap and fast.

My experience is gained from protecting figs, not bananas....but I have found that plastic seems to encourage rot and mold formation (deadly).

I'd surround with paper lawn bags, not plastic, were it me...but again I have no direct exp with nana's (yet) only figs.

something to think about

~Chills

Islandbreeze, Last year I overwintered a banana (musa basjoo) and three palms and two elephant ears. I piled leaves around them, then wrapped the leaves with burlap. The banana and two of the palms survived last years -20F winter. I did lose about half of the pseudo stem on the banana. This year I'll do basically the same with more plants, but will put plastic over the top of the leaves to protect them from too much wetness. If you do use plastic, be sure it doesn't touch the plant because the cold will transmit to the plant though the plastic. The palm that I lost was because of dampness, and rot.

I'm technically in zone 5, but because I'm so close to Lake Michigan, I think I'm closer to a zone 6.

Better plastic burlap outside the leaves for protect the stem?