I recently built a 2-foot tall boulder retaining wall in my
front yard. I have a small landscape bed along the top of the wall. I
want to plant a small evergreen tree in the landscape bed to provide
privacy from a busy intersection at the corner of my property. I’m pretty much settled on a Magnolia grandiflora ‘Little Gem.’ Do you think the roots of this tree will interfere with/knock-down my boulder wall, if the tree is installed in a location where the trunk of the tree is approximately 2 feet behind the wall?
Two feet from the edge does not sound like enough space to me. Although
it grows relatively slowly, Magnolia grandiflora ‘Little Gem’ will reach
at least 20 feet, and dislikes root disturbance. From the tree’s point of
view, the boulders might be a problem. Below is a link to general
information about the tree, from Urban Forest Ecosystems Institute.
The following, from University of Florida Extension, describes the root
system of this tree:
“The root system is wider spreading than most other trees, extending from the trunk a distance equal to about four times the canopy width. This
makes it very difficult to save existing Magnolia trees on construction
sites.”
It is possible your tree might coexist peacefully with the retaining
wall, but my recommendation would be to plant it as far away from the
wall as you can, and leave the bed at the edge for perennials and small
shrubs. Here is general information on the needs of tree roots, from
Brooklyn Botanic Garden’s Tree Care Primer by Christopher Roddick (2007):
“Out of view, and usually out of mind, roots make up almost a third of a
tree’s mass. Trees need a serious amount of underground real estate.
Unfettered by subterranean obstacles, their root zones easily spread far
beyond the tree’s dripline, the perimeter of the tree’s branches. If
roots are curtailed by obstacles that inhibit their spread, the amount of
water, nutrients, and oxygen to which they have access will be limited.”