Roots will only grow in soil that contains oxygen. For this reason the majority of roots are in the first 12-18". Many many of them are right under the ground surface.

That's a good idea rigid isulation. The roots go a couple feet down in loose soil, not to mention the corms which can get big on sikkimensis, probably 12" below soil. It's gonna be pressed for space, but you could divide it when it gets cramped.

Thanks for all of the advice, much appreciated.

A quick follow-up question - I've got the planter all done and the musa installed. I'd like to mulch with crushed filbert shells (abundant around here and attractive) - any thoughts as to whether or not they will hold too much water come winter? It's not super cold here but certainly wet so rot will be my biggest challenge, I susupect.

Thanks -