Actually, raising the grade around any tree is setting the stage for doing damage to the tree, if not killing it. Tree roots grow where they do, in the layers of soil below the surface, because of the amount of air and water available at that level beneath the surface, and the level of compaction within the soil. Raising the grade decreases the amount of air and water within the soil, and increases compaction, thereby decreasing the ability of the tree to get what it needs to thrive. It's one thing to create a flower bed around trees at the same grade that the trees are used to, but it's not advisable to do so by raising the grade.

I have 3 large maple trees in my yard near the street and they all have slightly raised flower beds around them. There are medium sized rocks placed around the tree with about 5-6" of soil and planted perennials inside the circe. All three trees are in great shape!