A few things to keep in mind when overwintering Bananas indoors.
Firstly, watering, humidity, and light. I would keep them in as bright a light possible, south/ west facing window being the best where they can get the max amount of direct sun possible. Water them sparingly during the winter months, espeically if the temps inside are in the mid 60s or lower. Keep in mind soil is often a good bit cooler than the ambient air temp inside. Bananas go dormant when soil temps hit around 60 or lower I think, and thus they plants take up/ use less water, and leave more water/ moisture in the soil which can lead to root and corm rot. I would water them ALOT less in winter than you do any other time of the year. I allow the soil on mine get very dry between waterings. I would also keep the indoor humidity as high as possible without causing any problems for yourself or your home. You should be able to get it up to around 50-60% indoors and not experience any adverse affects. Also, keep them AWAY from any source of heat such as air ducts / radiators.
Another thing you will most likely battle, are spider mites. You will need to keep a close eye on your plants, and take measures to knock back the populations of mites. They almost assuredly will attack. I battle them every year, and its almost solely on bananas that I overwinter indoors. Misting, higher humidity, and taking them out on warm days and hosing them off good can help. You probably will not completely get rid of them, but controling their numbers is almost as good.
When you go to set/ plant them out in the spring, make absolutely sure that you gradually expose them to higher light intensities/ sun over a period of time. Take them out for maybe 30 mins to an hour a day for a week, and increase the time they are exposed to direct full sun over a couple weeks. I made the mistake once of setting mine directly out from overwintering, and sure enough all the leaves burned off. The plant lived, a nd fully recovered rather quickly, but its still not something you want to do to your plant or go thorugh. Its pretty unsightly and will give you a good scare for sure.
No need to pot them up for the winter, as most bananas kept indoors (unless in a greenhouse / hothouse) will slow their growth down considerably.
These are the things I have done and had great success with over wintering. Granted, the larger your plants the harder it is to overwinter, and more work it is. I now am overwintering two pretty large bananas indoors and it is a bit of work for sure.
I have 1 Musa acuminata 'Sumatrana' (Rojo) Banana in a large clay pot, and 1 Ice Cream Banana in a smaller plastic pot.
The Rojo is about 5 and 1/2 feet tall, and the ice cream is about 4 and 1/2 feet tall.
When spring comes and you plant them out get ready to enjoy them and enjoy the fantastic show they put on because they grow EXTREMELY FAST!!!! Ive had Basjoos put on 15-20 feet in one single growing season. Its amazing to watch them grow almost right before your eyes!
Happy Banana-ing!
