Retaining walls can create a nice flat area for beautiful landscaping. However, tree roots can reek havoc on any concrete or stone structure. As we have discussed previously, large trees act as a surcharge on the wall. This means they require special attention.
How Far To Place Trees from the Wall
Place shrubs a minimum of 2 times the wall height away from the wall. In addition, place trees at least as far away from the wall as the expected height of the tree when fully grown. So, for a Butterfly Japanese maple that reaches a height of 15-ft, plant the tree a minimum of 15-ft from the retaining wall. The same goes for shrubs.
If you are intent on planting a tree near a retaining wall, place the tree in front of the wall, not behind the wall. Only plant small annual plants within several feet of the wall. Pansies or creeping jenny make for a nice cover on top of the retaining wall.
Trees & Plants to Avoid
Avoid water lovers. Water is a major cause of retaining wall failures. That is why we spend so much time talking about retaining wall drainage. Surface water is diverted away from a retaining wall. The wall drainage system then removes any excess water from behind the wall. This creates a dry environment behind the wall. Plants and trees that require moist soil or constant watering should be avoided behind a retaining wall.
Avoid trees with expansive roots. These trees defy the rules suggested above. Here is a quick list of trees with invasive root systems.
Trees Behind Existing Retaining Walls
Existing trees located behind a retaining wall require close investigation. Cutting tree roots is not recommended by arborists, but there are other ways to prevent root migration. Pruning your trees is a great way to limit how far the roots extend. Generally speaking, tree roots extend about twice as far from the trunk as the branches do. A well pruned tree does not require as extensive of roots to prevent it from toppling over.
Don’t discount removing a tree to avoid problems in the future. The tree will continue to grow in size, creating a bigger potential hazard.