Looking for a little advice here. I'm going to be building a stone planter around a business sign for a client. The sign sits underneath a mature maple tree. The small bed that was around the old sign (only about 8" tall) was completely filled with roots from the tree, preventing anything from growing there. The new planter will be about 18". The planter is not right next to the tree but most of it is within the trees dripline. I am concerned about the tree roots invading the new planter as well and I'm looking for some advice about how to prevent this. Some thoughts that we've thrown around include a layer of crushed stone covered with weed fabric (Lumite Ground Cover), plywood (with drainage holes?), and nothing at all - the roots wont grow up that much as to fill the bed. Any thoughts on the success of these methods or other thoughts?