how deep to make flower bed
I'm making a flower bed that is at the corner of the front lawn beside the sidewalk. There is a maple tree in the strip between the sidewalk and the street and it shades that corner. Grass won't grow. I've made a bed and tried some hosta and ferns. They died, I suspect because the tree sucks up the moisture in the soil. So I'm going to put down a layer of cardboard as a barrier to the tree roots and add a mixture of topsoil and compost on top. I want to plant some hardy geraniums (I've ordered Nimbus), a dwarf plumbago, a hosta, a fern, and some blue-eyed grass. How deep should the soil be on top of the cardboard? We got some lower limbs removed from the tree so other is more light now, though with the canopy still over the bed, it is primarily shady. I know the cardboard is not a permanent barrier but I'm hoping the flowers will have a chance to develop good roots before it disintegrates.