Constructing Tree Wells

Tree wells are constructions built to save an existing tree when the soil grade on the site is being raised or lowered.  Most who may have encountered a tree well likely saw one built around the tree when the grade was raised.  Tree wells have several factors involved that determine whether they are worth installing.

Purpose of a Tree Well
The primary purpose of a tree well is to save an existing tree from certain death when the soil around it is raised or lowered.  If a construction site requires the ground level be raised or lowered, the existing trees are almost certainly likely to die a slow death unless something is done for them.  In cases where the tree cannot be moved (dug up and raised/lowered), a tree well is the option turned to.

Evaluating the Tree for a Tree Well
The first step is to determine whether or not a tree well is feasible.  Several factors should be weighed before the decision is made.  First, is the tree well-established, too old to be transplanted, and of a variety and age that makes it worth keeping?  The expense of a tree well cannot be justified if the tree is a fast-growing variety, is old and doesn’t have a long expected lifespan, or if the tree is severely injured (has a large trunk opening, split by lightning, etc.).  Finally, if the property has many established trees and only a few are at risk due to the construction, it’s probably not worth the expense to save them. If the tree is a good, healthy specimen with plenty of years left and one that cannot be easily replaced, then a tree well is a good option.

How To Build a Tree Well
Once it’s been determined that building a tree well is worth the effort, then the type of tree well to be used should be chosen.  Trees require water,  oxygen, and nutrients to their roots, so completely enclosing the trunk and surrounding soil where the roots are is not going to work.

The most common type of tree well when the soil is being raised is the dry well with vents. This is constructed around the tree’s trunk as a well (usually made of ornamental stone or bricks so air and water flow freely and are not trapped).  At the base of this well, which will be buried, are laid a radius of perforated pipes that have small up-shoots that reach the top of the soil or very near it.  These allow air and water to get into the soil where the roots of the tree are and can be buried under the new topsoil being added to raise the grade.  The top of the dry well can be covered with a grate made of wood or metal that closely hugs, but doesn’t quite touch, the trunk of the tree.  This keeps children and animals from falling into the well.

Another type of tree well involves adding a “breathing layer” to the soil above the tree’s roots before adding the new grading.  This is accomplished by laying down coarse gravel in a layer on top of the existing grade and then placing heavy straw or landscaper’s matting down before adding the new soil on top.  The gravel is run into a “chimney” to the trunk leaving a bare, rocky area around the trunk of the tree.  For shallow additions to soil grade (less than a foot or so), this is a low-cost, easier option.

In either case, the extent of the area being created so the tree can breathe should be about the same radius around the tree as the canopy reaches (also called the “drip edge”).

For Further Reading on Tree Wells:
West Virginia University – Tree Wells [PDF]
Texas A&M Unviersity – Protecting Existing Landscape Trees from Construction Damage Due to Grade Changes