HOW TO PROTECT AND OVER-WINTER BANANAS
There are only a couple of banana species that are hardy enough to over-winter in this country, but you would be foolish to leave them outside without any protection in all but the mildest parts of the country. Even so, while it is true that the plant may not die, you may well lose the highly ornamental single stem, and instead, be left with numerous small stems growing back through in the spring. Although this may still sound like an attractive prospect, in reality your prized banana will end up looking nothing more than a regular canna lily - minus any flower.
While the plants are still reasonable small you will be able to get away with having them planted in the ground while still in their pots. This will make it much easier - come late autumn/early winter - to lift and store in a frost free area. However there will be a time when its size makes this is no longer practical and the plant will need to be protected where it stands.
A tried and tested method of protection is to lag it with straw. To begin with, you will need to remove any dying or frost damaged leaves, as these can be a point of entry for fungal rots once the stem has been wrapped. The next thing to do is to create a large ring of chicken wire or heavy duty mesh around the stem - with the stem acting as the centre. Securely join the ends of the mesh together, then back fill with heaps of dry straw - compacting it well. Make sure that the entire plant is protected and leave no gaps with can allow the cold and wet to get in. Secure the mesh/wire to the ground and to the plant (making sure the plant isn't damaged), so that your good work isn't destroyed by unfavourable weather.
This protection can be removed come April or May, making sure that the threat of frosts are over before you do so.
If you only suffer from mild winters, you may be able to get away with a simple wrapping of large-bubble bubble wrap. You can bubble wrap the trunks in late October, and this can be left on until the end of March. Don't worry about protecting the leaves, because even if they get knocked back by a hard frost they will still grow back once the weather improves.
Try not over-winter in water-logged conditions as the root system can be prone to rot, and stop feeding at the same time you stop watering. One more thing, Musa basjoo is considerably hardier than it cousin Ensete ventricosa so protect accordingly.