that's my tree bed hexagon -- it turned out pretty well considering a certain amount of warp in the boards. certainly much cheaper than any other method of construction. simpson strong-tie for the win!
i might just leave it at this one layer (8" high, 7" of soil). the boards look pretty junky, but they're solid. i think i'll ask the landlord whether he has any paint left from painting the houses, because that would look better than to just let them weather. we have topsoil, so tomorrow i'll finally get to plant the tree.
i wish i knew some place to pick up leftover lumber, but driving around looking at construction sites will use up in gas what i would save. i'll probably check the restore and if they have nothing then just pick up some 2x6's at slegg's (building some square-foot boxes for more veggies).