It is necessary for me to provide cold protection of my bananas, since we are guaranteed a few nights of below freezing temperatures during the winter.  It is very very rare that the temperatures where I live will be low enough to actually kill a banana plant.  These methods are attempting to protect the leaves of banana plants so I can get fruit.

Method 1

My bananas planted under tree cover have survived 30°F, but they grow slow. 

Method 2

Draping blankets over a banana with a light placed at the base has protected my bananas from 25°F.  The problem with this method is that by the time spring arrives all of the leaves are broken or mangled.

A variation to this method would require two poles longer than the width of the plant and one pole taller than the plant.  The tall pole is tied vertically to the tallest banana stalk.  The two equal length poles are lashed together to form an X.  The X should be lashed to the tall pole above the plant.  This provides a framework to drape blankets over and prevents mechanical damage to the leaves.  I have no experience with this cross framework yet, but I will definitely try next winter.

Method 3

This method was told to me by a gentleman at a flea market.  My Ice Cream Banana is the guinea pig for this method.  Plant a banana in a huge pot, I am using a 30 gallon storage container.  When freezing temperatures are predicted, I tip the banana on it's side.  I have found a quick controlled descent does not break any leaves.  Heavy blankets can be draped over the entire plant from pot to leaves.  The blankets will need to be weighted down so they don't blow away.  The radiant heat from the soil eliminates the need to provide any supplemental heating.

My Ice Cream Banana showed no signs of cold damage at 25°F using this method.

The banana will require much more water and fertilizer since it is confined to a pot.  My Ice Cream Banana has grown so tall and top heavy, I have a brace to hold it upright.  The brace is made of two 1"x4"x10' screwed together with one screw at 1.5" from the end for form an X.  The stem of the banana is allowed to rest in the small part of the X.  The brace is tolerated since most fruiting bananas need to be braced so the stem doesn't break.  Tipping a full grown banana in a 30 gallon pot is not difficult, but moving the pot it is very difficult.

Method 4

Of course dwarf bananas can be protected in a greenhouse as long as the temperature in the greenhouse never drops below freezing.