Inner ring MUST have a layer of large river stones for air +water to get to trunk, otherwise it will suffocate + kill the tree. The soil will support small shallow rooted plants like Allyssum or Primrose.
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three levels of cement block.bottom level is underground. The second level has the vents (blocks turned sideways) and then decorative paver bricks to cover the tops of the cement blocks. use Liquid Nails to cement to stick everything together.