Growing things like bananas in a hole/shaft (like a well or a round mini walipini)?

Ok. I have a Walipini style greenhouse (probably better termed: passive solar greenhouse away from the equator). It's been through its first winter in the Alps. It never froze inside. It bore the heavy snow on the north side and its solawrap shed it well on the south side. Citrus, Pomegranate, Feioja, Figs... all doing well. The young grape growing from a cutting did not even realize it was winter and never shed its leaves. Tomatoes suffered a little. Chilies are alive. I was happy. As were the mice :-/
It only has one flaw: it is of course too small.

I while back I saw this video:
Now, while a completely different climate I was wondering.... I have no room for bananas in the greenhouse.....

Has anyone in a temperate climate dug a hole supported by dry stone walls like an old well and planted, say at a level 5 feet underground in a hole with say 6 foot diameter? The advantages that I maybe naively have in mind:
+ in the cold season the rhizome is way underground and well protected from cold. AS LONG as I can avoid the hole becoming a cold sink. The hole would have to be protected with a mini greenhouse on top, which is then removed in the warm season. It would be like a mini walipini.
+ The bananas in the hole would have no height limit in summer as they would in the walipini
+ the ground in which the bananas grow would remain moist because the sun does not reach the bottom.

Note:
I'm on a slope and have no issue with water draining. The hole would never become a pool
I assume the banana (like in a jungle) will strive for the light and be fine with shade at the bottom (and cooler soil temps?)

Please let me know:
Am I in lala land or behind the times because everybody is already doing this in a temperate climate for subtropical plants?

Cheers


p.s. here is a winter shot of the my greenhouse just if someone was interested:

I assume the banana (like in a jungle) will strive for the light and be fine with shade at the bottom (and cooler soil temps?)

Bananas do poorly without at least some amount of direct sunlight. Even "in the jungle" bananas typically form large clonal clumps where the old pseudostems fall and rot after fruiting, making openings in the canopy for sunlight to reach the new pups. They do not grow in deep shade and "strive for" the sunlight.

However, if you start your banana in a pot, and grow it for a season in a good sunny spot, then plant it in a covered tube like you describe before first frost, it might work very well to protect it for a winter. It will likely stop growing over the winter, and as long as the height of the tube/well is only a meter or less above the top of the plant, it should get enough sun in spring when the sun angle improves to grow out into the full sun quickly the next season.

The trick will be ensuring that it's large enough when you plant it that it can flower and fruit during the very next season, since it will be hard to protect once it grows above ground level. In your reduced growing season compared to the tropics, it may take 3+ years to fruit a banana. You may need to grow it in a pot, put it indoors the first winter (horizontally under a house in the crawl space?), then grow it outdoors for a second season before planting in the banana tube.

I have been thinking of ideas to grow orange trees in Wisconsin.  We have long winters and our lowest yearly temp is somewhere near -35 to -40.

One idea I keep ending up at is to dig a hole that is a upside-down truncated pyramid.  The bottom would be about 4feet by 4 feet.  The top would be about 7 feet x 7 feet.  The hole would be about 6 feet(once I get to 8 feet I hit ground water)  Some of the dirt from the hole would be banked up around the rim of the hole to make the hole 7 to 8 feet deep.  

My subsoil could probably doesn't need any reinforcement but I would shore the sides of the hole up by lining them with sawmill slabs and small logs.

On top of the hole I would put a passive solar green house.  The south side would be 4 layers of glass from doubling up a second hand sliding glass door. The south wall could be plastic but glass is reflective to IR so will keep more heat in at night.  Another option could be one layer of glass and a layer or two of plastic(with air gap between layers) that way you get the IR reflection from the glass but the cheapness and lightweightness of plastic. The south side would be angled for best solar gain.  The other 4 sides and roof would be earth sheltered using the remaining dirt from the hole.  I could also insulate on top of the earth sheltering dirt with straw or Styrofoam.

The front glass panels would be removable for the spring, summer and fall.

I dug a test hole last fall it is 6 foot deep,2 foot wide and 4 foot long.  It was covered but not insulated.  It never froze.  I had a temp logging thermostat in it and the lowest I saw was high 40's.   But this was a very unusual winter.  We had 60 degree days in December, lots of above freezing in January and February. I think the lowest temp I saw was -11.

I don't know if this idea would work for bananas.  Orange trees come in dwarf varieties and can pruned heavily to keep them short.  I don't think bananas can be.  Also orange trees can survive down to around 18 degrees before you risk killing the tree.

I have no idea if my idea will work but I plan to try it this summer.

I think I like this tree in a hole idea.
I will warn that I have a yellowhorn tree that is the 3' space between two raised beds and I think it has suffered from lack of sun.

J's idea avoids this by using a wide hole and sloped sides .
I have a lot of squirrel pressure,  so the idea of an enclosed tree is very attractive to me.
I wonder, would it help to lay mylar faced insulation on the slope of the hole?

bananas.org is really slow to load sometimes and doesnt load at all at other times lately but if you give it a min to load this thread is similar to what you are describing

http://www.bananas.org/f15/banana-sphere-pit-greenhouse-22664.html

also here is another one which you might like as well

http://www.bananas.org/f2/my-semi-pit-banana-greenhouse-18518.html

Thanks everyone for the input and the caution about the light. That is a fair point. I was a bit naive, I guess - or "optimistic". The reverse-pyramid idea might be a solution.
I'll have to finish some projects first and then see what I could give a try in my location and will update when I'm there.