Answer: Just prior to a hard freeze get yourself a wire cage and place it around the trunk of your banana, the higher the cage the more of the trunk you will be able to protect. Stuff the cage with DRY insulating material such as leaves, straw, etc.. Cut down the top of the banana so the remaining trunk is below the top of the wire cage by at least 6 inches then cover the top with more insulating material. Then you can either move the plant under a patio or deck covering or under an eave so rain does not reach it or get some heavy weight polyethylene and make a tent so water doesn't get the insulating material wet. Heap more insulating material on the outside around the base of the plant.
In the spring, if your winter was not too severe, the trunk should still be solid. If there is some rot at the top where it was orginally cut, cut the rotted portion off down to where you see solid tissue. Even if you lose the trunk, it will still resprout from the base and will likely grow even taller next year.
Hope this helps!
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