Choose which landscaping material you want to use for the border. Consider using the same materials in the rest of your yard for continuity, or try something new to make the trees stand out more. For instance, you can surround the base of the tree with gravel and edge the perimeter with stones, or spread mulch around the base and lay bricks around the outside for a uniform look.
Use a hose or wooden stakes and a string to outline the border. Make sure the outline is wide enough to cover any above-ground roots.
Excavate the area inside of the border. Remove grass and dig a trench 6 inches deep. Use a hoe to smooth a 2-inch layer of sand over the trench.
Place the first brick or stone against the outside of the trench. Add more sand to raise the level of the border if desired.
Set the bricks 1/4 inch apart around the outside perimeter of the trench. Place a level on top of the bricks to ensure they are even. Adjust them as necessary. Continue setting bricks until you reach the end of the trench.
Use a hand shovel to pour sand in the spaces between the bricks. Brush off the excess sand and moisten the border so the sand will cling to the bricks when it dries.
Fill in the remaining trench between the brick border and the tree. Use mulch, lava rocks or gravel to prevent weeds and erosion. Mound the rocks or mulch around the base of the tree so water will run off in the opposite direction.