Recently, I received an update from a favorite how-to site, which included directions for building a raised bed around a tree. This unfortunate practice leads to many dead trees, often years down the road when the tree owner no longer mentally connects the tree dying with the creation of the raised bed.
Case in point, it was apparent this tree was and is having some problems. (Notice the dead branches on the right side of the tree.) A closer inspection reveals a raised bed and a new sidewalk. So not only was the trunk flare trimmed to make room for the new sidewalk, but the tree’s remaining root system was put in peril with a load of soil placed over the roots!
A better understanding of why this kills trees starts with knowing where tree roots are and how they function. Every tree will have a few deep anchoring roots, but the vast majority spread parallel to the soil surface, in the top 18 inches or so of soil. That they occupy this area is no accident—tree roots need oxygen to survive and oxygen is readily available in the top 18 inches of soil. When soil is piled over a root system, it immediately puts the lower roots out of range of their oxygen source. Roots begin to die, starting the tree on a long slow decline and, eventually, death.
In the best of all worlds, a tree owner will never put a raised bed around their tree. If you’ve already done this, taking away the raised bed and the excess soil are the next best things to do. Bear in mind a tree in decline may not rally from the removal of a raised bed if the bed has been in place for 5 or more years.