Bananas are in a group of about seventy species in the genus of Musa. They are indigenous to India, SE Asia and Northern Australia. They are not going to be used to the type of winters that we get in the UK!
The first thing to do though is to cut off the leaves. Do this after the temperature has gone low enough to blacken the edges of the leaves. This triggers the plant to go in to a state of dormancy.
Then wrap the stem in a few layers of horticultural fleece. This will have to be tied to keep it in place. If you live in a fairly mild winter area this might well be enough but for harder areas then you are going to need further protection. Hesian sacking can be wrapped around the fleece. This is quite awkward stuff to get in place and will have to be tied like the fleece. make sure that the ties have a bit of play and are not strangling the stem.
For the really cold areas you are going to need a further layer. The most effective insulator for this is straw. the problem is getting it to cover the stem. One method is to make a chicken wire cage around the stem, held in place by a couple of stakes. This can then be filled with straw.
The problem with any wrapping is that the stem can easily start to rot if it gets wet or has no air circulation for extended periods of time. To help to alleviate this, the stem needs to have a final outer layer that is waterproof. Old washing line covers are ideal. The stem should also only be covered when there is a real risk of very low temperatures.
The roots should be covered with a thick layer of mulch.
Can you leave ensete bananas outside in the winter?
Ensete ventricosum ‘Maurelii’ are not frost hardy and must be taken in if low temperatures are expected. To over winter the plant you must first let them dry out. Do not water them at all over the winter as they are prone to rot. To over winter first cut the leaves off near the stem. next turn the plant upside down and hang up for a week. This will drain off any internal liquid. The plant can then be stores in perlite in a dark place between 5 and 15 degrees centigrade
Cavendish bananas must be treated as an indoor plant. They can go outside in the summer but as soon as the temperature goes below 10c they must be brought indoors. They cannot be dry stored like ensete and must be kept as a houseplant in full light with temperature 10c +