Field stone is a great versatile material for use in the home garden and yard. Field stone is basically stones that retain their natural edges, and are used as edging for homeowners that want to convey a natural appearance to their garden. Place field stone in a ring around your tree, securing with stakes and plastic paver borders, which are plastic tracks that can be bought at any home improvement store. You can lay the plastic border material in a ring around your tree prior to placing the stones.
Landscaping timbers are effective for trees with large exposed roots. These pieces of wood resemble railroad ties, and are typically pressure-treated to prevent rotting. They are flat on two sides, and thus can be stacked one on top of the other to form a border or garden edge. Surround your tree with landscaping timbers nailed together to form a large square. Drill holes completely through the timbers and hammer railroad spikes through the wood to secure them to the ground, being careful to avoid hitting roots.
Many types of wall stones are commercially available. These are concrete blocks with decorative edges and flat sides enabling them to be stacked and mortared together. A foot or two of wall stones forming a ring around your tree is one way to avoid digging in to the ground and potentially damaging roots. The area can then be back filled with top soil or mulch, and plants and flowers added.
Soft Borders with Top Soil Bags
To retain a softer look to the border surrounding your mature tree's base, place bags of top soil, in the bag, around the base of the tree. Make small holes in the bags to enable planting ornamental plants such as begonias and marigolds, and cover the whole area in mulch or pine bark nuggets until the bags are no longer visible. The plants will grow inside of the soil bags, and will not damage or interfere with the tree roots. Each year simply replace the soil bags and recover the area in mulch to retain a softer border to the base of your tree.