Answer: It really is not a good idea to construct a planter over your tree's root system. Changing the soil grade can actually suffocate the roots - even adding more than 2-3 inches of soil can cause damage. And, tree roots will seek out moisture in any raised bed that is even close to the dripline of the canopy of the tree. You will be forever trying to rid the planter of tree roots. From experience I can tell you that it is a never ending job and the tree roots always win. Instead of a raised bed, how about planting in small planters or large containers and setting them under your tree? Tree roots won't be able to grow into the soil in the planters and since they will be raised up on feet or even large stones, they should not hurt the roots of your trees.
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