Hosta terrace around maple tree
Building miniature raised hosta bed off the ground around a maple tree. Trying to keep roots out and allow water to drain. Mesh floors or something should allow drainage. How deep of soil would be needed to ensure winter survival? Would gapping and placing insulation between tiers do anything? Straw it all? Thanks for any response.
Your concern seems to be with the health of the hostas you've not yet planted. It should be with the maple tree whose roots you are considering burying.
A high percentage of a trees roots are in the top few inches of soil. These roots provide the tree with oxygen, nutrients and water. Burying them with more than 3 or 4 inches of added soil might kill the tree. Read here:
As for your hosta bed, why not simply plant the hosta around the tree at the current grade level?