Oak Tree Retaining-Wall Requirements

Sizing

• The size of the retaining wall for your oak tree depends on the size of the tree and the amount of soil you are adding or removing from around the tree. Oak trees have broad root systems that are designed to anchor the tree and absorb moisture and nutrients from a large area. The root system of oak trees typically reaches 1.5 times the length of the tree's drip line, which is equal to the diameter of its foliage; building your retaining wall inside the drip line can fatally damage your oak tree's root system. Build your retaining wall as far outside the perimeter of the drip line as possible to limit damage to the root system.

Drainage

• Oak trees are sensitive to changes in the drainage patterns of the surrounding soil. Install drainage lines, or weep holes, throughout the structure of your retaining wall to ensure that the retaining wall does not divert or trap water. Add a weep hole for every two feet of retaining wall to ensure that the soil around the tree is adequately drained.

Alternatives

• Building a retaining wall that will adequately protect an oak tree is often a cost-intensive process. Instead, change the grade of the soil around your tree gradually. Limiting the changes in soil depth to less than three inches of soil annually will allow your oak tree to adapt to the changing conditions in the soil naturally. In yards with mature oak trees it is often more cost-effective to remove the tree and plant a replacement than building a wall, since the extensive root system of these trees can require a retaining wall large enough to cover an area in excess of 50 feet in diameter.

Considerations

• If you are decreasing the grade of your yard downhill from a young oak tree, the root system of the tree may comprise the integrity of your wall as it expands, unless the wall is built outside the diameter of the tree's mature root system. If you expose the edges of the tree's root system when you decrease the grade of your yard, cut the roots where they leave the soil. This will encourage them to regrow beneath the soil and protect them from further damage.