Answer: Musa basjoo, or the Japanese fiber banana, can be reliably grown in temperate gardens, having survived temperatures of 0F without protection and -20F with protection. Coming cultivation in the west from Ryuku islands, off the south Japan coast, a mature Musa basjoo will grow 18-20 feet tall with leaves on the order of 2 feet wide and 6 feet long.
Protecting your banana plant can be as laborious as you want. Several methods have been successful. The easiest is to allow the first hard freeze to knock the plant back, then cut the trunk to the ground and mulch over. The mulch should be 6-10 inches deep and can be leaves, bark, straw, or anything else that will provide insulation. The draw back to this method is the size of the plant, and it most likely will not flower. The most extreme method is to cut off the leaves after the first frost and build a cage around the banana clump, preferably a little taller than the plant. Fill the cage with insulating material and wrap with plastic. Such a procedure is especially good the first winter. The cage technique is not advisable in areas where you rarely get below 20F and most of your winter is mild. We have lost more bananas due to fungus caused by being wrapped up than we have ever lost to cold. The middle-of-the-road method (the one we use) is to do nothing until it is going to be below 20F for a period of time. We then wrap with household insulation and plastic until the cold spell is over. Then we pull the wrapping, preventing heating and cooling inside the tent. We don't even cut the damaged leaves off until spring. The method you choose depends on your expected low temperatures and the age of the plant. A new banana plant should always be protected the first year.
In very cold areas, here are the steps to ensure successful overwintering:
Remove the leaves to the top of the trunk, don't shorten the trunk is possible
Spray the trunk with a copper based fungicide and if possible algaecide
Wrap the trunks with burlap
Spray the burlap with fungicide
Try to have your cage much larger than just to the edge of the trunks
Use dry insulating material
If you are very, cold wrap the outside of the cage with another insulating material
Cover the project to keep it dry
Open the top to allow air to circulate during mild periods
The pups won't survive outdoors if your winter weather is really cold. If so, I'd sever them from the mother plant and pot them up, then take them indoors for the winter. If they have bright light and lots of moisture, they should be happy campers indoors until next spring.
Best wishes with your banana tree!
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