Answer: Most perennials will need 12 to 18 inches of soil for good root growth and if you are purchasing topsoil, another foot of soil won't cost that much. An alternative might be to set stones in the bottom and then add at least 18 inches of soil. Building a raised bed, especially one under 3' in height, should not be any different than building one only 12 inches in height. Just dig a trench to lay your first row of block so they are well secured. If you are using wood, just set it on top of the ground - you won't need a trench. For wood, set short posts into the ground 8-10 inches, every 6' in length and use screws rather than nails to attache the boards to the posts. Good luck with your garden!
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