Answer: Musa basjoo is one of the hardiest bananas, but in your cold climate it may be difficult to insulate the trunk well enough to withstand the cold winters your area experiences. The higher you go the less residual heat from the ground so the more exposed tip is going to be colder. It is also cumbersome to wrap a tall tree. That is why most gardeners will trim it off before trying to insulate it. And, most important of course is that the roots be well insulated so they can survive to grow again the following year.
The first step for wrapping/insulating the trunk is trimming off the leaves (trim them off once a frost has killed them) so you have just the bare trunk to wrap. Next, whether you cut it off shorter or leave it tall, the general idea is to surround it with a generous layer of dry insulating material such as straw or oak leaves, hold the insulation in place and keep the whole thing dry until spring. Some gardeners will also apply a fugicide to the trunk prior to covering it up.
There are many ways to do this. One is to build a surrounding cage several feet in diameter out of wire mesh or wood lattice or whatever you have on hand, stuff that with the straw/leaves, then top with a plastic tarp. Or you can make a sleeve of fabric such as burlap or row cover and stuff that with the straw. Next, the plastic layer such as a tarp is used to cover the top to prevent rain and snow from wetting the insulating material. It should have gaps on the sides to prevent any condensation from building up inside and also to prevent any excess heat buildup during mild spells.
It is important to wait and do the wrapping after frosts to allow time for rodents to find themselves other homes, and so the plant will have naturally slowed its growth with the season. Do not let it be exposed to temperatures below 20 degrees though, or there will be freeze damage.
I should mention that this plant regrows its height quickly if generously fed and watered. It will probably look very unpromising by spring, but hopefully the roots will be alive and send up new growth. I hope this helps, good luck with your banana trees!
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