The Basics on Safely Building Around Trees

We’ve all seen it. A beautiful oak tree surrounded by a sea of new asphalt parking spaces.  A developer has decided to put a new parking lot around the 100-year-old tree.  The majestic oak hangs on for a couple more years, until branches finally die and limbs fall.  Sadly, the once thriving tree must be cut down to protect cars and people. Building around trees is a common practice. If done correctly and responsibly, trees can survive and thrive.

Existing trees are a valuable resource for any site and should be protected and preserved to the greatest extent possible. They provide many benefits. Trees cool the air, provide oxygen, help prevent soil erosion, provide wildlife food and habitat, as well as clean the air. Sites that have healthy trees are desirable locations.  Many developers want to take advantage of these benefits, so they construct buildings and projects near them.

Building around trees in a safe manner is possible. Appropriate planning and steps must be taken early to ensure existing trees are protected properly.  The first step should be to involve a professional arborist or landscape architect with knowledge of construction impacts on trees and what strategies can be used to protect trees.

Topics we will explore today include:

• Tree Damage from Construction
  • Root Injuries
  • Soil Damage and Changing Soil Levels
  • Physical Injury
• Preservation and Protecting Trees During Construction
  • Barriers
  • Limited Access
  • Reducing Compaction
  • Grade Changes

So how close to a tree can I build? Well, it depends. The tree species and size will tell you a lot about how close you should build near trees. This article outlines the answer to that question.  Feel free to reach out to us with specific questions on our Contact page.

Tree Damage from Construction

When building around trees, most damage occurs from physical injury to the tree, damage to the surrounding soil, adding or taking away soil from the root area, and exposing the tree to new environmental factors. Construction damage can kill trees. Oftentimes, the damage resulting from construction leads to a decline in tree health over several years. Eventually, the declining tree becomes hazardous to people and property and must be removed. This chain of events began with poor planning before construction.

Root Injuries

Damages to roots are the most dangerous injuries to trees. Roots are critical for tree survival. They anchor the tree to the ground, store water and nutrients, and transport water, oxygen, and nutrients to other parts of the tree.  Without roots, trees cannot survive.

A surprising fact about tree roots is that they can extend outwards from the trunk to distances much greater than the height of the tree.  Additionally, a majority of a tree’s roots are located in the upper six inches of the soil.  Due to this, construction activity can easily damage and kill roots.

When roots are injured, common symptoms seen include yellowing of leaves, branch tip dieback, small new leaves, lots of water sprouts (new small branches) growing out of the main trunk, and dead branches.

On a more serious note, if anchor roots are cut or a large portion of the root system is removed, the tree can be at risk of falling over. Trenching and digging within the root zone will cut and injure roots.  While these are necessary construction activities to install underground utility lines, like electric and water lines, extreme care must be taken to prevent injuring roots.

Soil Damage and Changing Soil Levels

Now that you understand the importance of roots, protecting the soil and roots is important.

Compaction

Soil compaction damages the soil by compressing the soil particles close together. This reduces open pore spaces which leads to less water and oxygen in the soil. Compaction can also reduce the ability of water to drain into the soil.  Roots may struggle to grow and expand in heavily compacted soil.

When construction equipment drives over the soil, the weight of the equipment compresses the soil. To make matters worse, wet soils compact even more.

Adding Soil

Changing the level of the soil in the root area of trees is harmful.  Adding as little as a few inches of soil on top of the existing grade is enough to smother and kill tree roots.  As mentioned earlier, a majority of the absorbing roots occur within the top six inches of the soil where they have the best access to water, oxygen, and nutrients. If these roots are buried, they can die.

Removing Soil

Removing soil usually results in the cutting and removal of the absorbing roots.  As you can imagine, this will harm the tree. 

Physical Injury to Trees

Equipment moving around the trees during construction can cause physical injuries to the trunk, bark, branches, and more importantly the root system. When injuries occur, open wounds are created. These wounds create opportunities for insects and diseases to attack the tree.  Many times, the injuries create wounds that trees struggle to heal by using their natural healing processes and become permanent.

When building around trees, having a Certified Arborist evaluate injured trees is highly recommended.  Arborists will provide proper pruning techniques and strategies for helping trees recover.

Visit the International Society of Arboriculture website to learn more about proper tree care or to find a local Certified Arborist in your area.

Preservation and Protection of Trees During Construction

When building around trees, it’s critical to involve a professional arborist or landscape architect to effectively save and protect trees.  Their involvement needs to begin during the planning and design phase, not after construction has begun.  Furthermore, it’s important to focus on protecting trees from the beginning since the ability to heal construction damages is limited.

Professionals can provide recommendations for modifying the site plan in ways that help the trees, but also still achieve your project goals.  They also make recommendations on safety measures that can be taken to help the trees survive. For example, if a trench is needed under a tree for a water line, professional arborists and landscape architects can use tunneling methods to go under the roots instead of cutting them.  Below we will discuss some other techniques.

Barriers and Tree Protection Fences

The single greatest tool for protecting trees during construction is putting up barriers and fencing around trees you want to keep safe. The barrier fences should be located as far from the trunk as possible.  However, a general guideline is to place the fence so that for every one inch of trunk diameter, there is one foot of distance from the trunk.  For example, a 12-inch diameter tree would have a fence located 12 feet from the trunk.  This allows for the protection of the trunk and branches, but more importantly the roots.

Another important strategy is to inform construction workers and display signage on the tree protection fencing stating that no activities are permitted within the fence.  This includes storage of materials or equipment, cleaning or washouts of equipment, digging, trenching, adding or removing soil, and trash storage.

This can be difficult to enforce because trees provide valuable shady spots.  Construction workers naturally gravitate to them for relief from the heat and sun.  Can you blame them for trying?

Preventing Soil Compaction

Previously, we discussed the harmful effects of soil compaction on tree roots.  There are two primary methods to reduce compaction caused by construction equipment.  Temporarily placing a 6-12 inch thick layer of organic mulch, such as wood chips is one option.  Alternatively, sheets of plywood can be placed on top of the mulch layer or the existing ground.  These two methods disperse the weight of the construction equipment as it drives over it.

These are temporary measures and must be promptly removed once the equipment has completed its tasks and finished driving over the areas being protected. Leaving the mulch at that thickness for long periods can harm the trees.

Grade Changes

Altering the elevation or height of the soil, also known as a grade change, can be extremely harmful to trees. The percentage of roots impacted by the grade change will determine the extent of the damage.

The goal is to maintain the original, existing grade as far out from the trunk as possible. If there is no way to maintain the grade that far out, there are some techniques that can be used.  Survival of the tree will depend on several factors, including tree species, how much the grade changes, how much of the root system is affected, and how healthy the tree was before the impact.

Terracing

Lowering the soil level requires cutting away roots. Aim to maintain the existing grade as far from the trunk as possible.  Then lower the grade in terraces or steps.  When a step down is made, cut roots with a sharp, sterile tool so they can heal properly.  Keeping the soil and root zone moist can also help the tree deal with the stress on the roots.

Tree Islands

When the grade must be lowered completely around the tree, a tree island can be created.  The more of the root system that is protected, the greater the chance of survival.  Work with a professional to determine if the conditions are favorable for this approach.

Tree Wells

If the grade must be raised around a tree, the roots are at risk of being suffocated.  This can happen with as little as four inches of soil. One approach to preserving a tree in this scenario is to create a tree well around it. For this to be effective, the tree well must be constructed as far from the trunk as possible.  This will protect a large percentage of the roots at the natural grade levels.

These are often expensive solutions and have questionable levels of success.  Small diameter wells have little success.   

Closing Thoughts on Building Around Trees

Trees are valuable assets to our natural and built environments. They provide many benefits to people, animals, and the planet.  In our experience, it is so much easier to protect healthy, well-established tree canopies than to invest the time and resources to plant replacement trees. It may be 50 years before replacement trees match the size of the existing trees.

Through early planning measures, property owners, designers, and developers can use some of the techniques outlined here when building around trees.

Let us know your thoughts and questions!