YOU DON'T need a container in the ground. The tree has to push roots THROUGH the container and into the ground--for an appreciable amount of time--to get any benefit from ground growing. Planting it directly into the ground will produce faster and better results.
The only reason to plant a container in the ground is to speed its re-collection. If you only allow the tree to grow so that snipping small roots off outside the container is enough to re-collect it, it's not been in the ground long enough to produce and significant trunk development.
Since BC can be root pruned dramatically, like 99 percent at re-collection, so trying to keep a neat compact root mass in an underground container isn't necessary. You can saw through heavy roots at re-collection, redeveloping a neater root base as the tree re-adapts to a container.
Poink,
The research done by Louisiana State University surveyed thousands of trees, logging histories, location surveys and other data. If you're interested in BC, get the book "The Tree Unique, The Wood Enternal"--it is pretty much a definitive work on BC and its habits. Can't recommend it enough for those growing BC as bonsai.
From the research and personal observation, BC structures vary tremendously from site to site. Bottle-shaped trees in deep water swamps, flat-surfaced rooting in shallow water or mucky sites or a combination of both. In some sites, there are floating roots below the bottle-shape, extending to another narrower trunk that shoots straight into the swamp bottom with no flare...There are other trees with deep ridged fluting growing on dry sites that's flooded only a few times in a century...
I spent alot of time in Gulf Coast Texas near Orange growing up. The BC down there are spectacular and made a lasting impression.
