Answer: If you are using them for a patio surface, the best way is to set them loosely laid with only sand between the joints. This allows air and water to soak into the soil and help to minimize the effect on the tree.
If you are using them as an edger or border, set them on the soil surface. Avoid damaging any roots when you set them in place. Also keep in mind that any mulch should be kept about six inches from the trunk and spread out in a flat later about two to three inches thick. Avoid that "volcano" look with mulch piled against the trunk -- that can cause damage to the bark and can eventually kill the tree.
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