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Preserving Trees during Landscaping and Construction

Landscaping and Construction Work can Impact Trees in Numerous Ways

  • Root cutting that increases the likelihood of a tree falling over and causing severe damage.
  • Root cutting, soil compaction, and other soil changes that reduce a tree’s overall health.
  • Causing open wounds on a tree’s trunk, roots, or branches and that become vectors for disease and insect attacks.
  • Excessive pruning that reduces a tree’s ability to get sun energy it needs and causing its decline or death.

Tree root damage is the type of impact most often overlooked and is especially important to keep at front of mind. Note that a tree’s roots can extend beyond its canopy drip line and the critical root zone may sometimes be considered to extend up to 1.5 feet from the tree’s trunk for every inch of trunk diameter. For a 20-inch trunk that would be 30 feet in every direction.

Read on to learn about the specific ways your construction or landscaping project may pose risks to a tree’s health and structure. And, please be advised that Takoma Park regulates all construction and landscaping activities near trees through the Tree Impact Assessment and Tree Protection Plan Permit processes. Don’t hesitate to contact the Urban Forest Manager if you have any questions.

 

Construction and Landscaping Activities of Concern

Excavation

 

What is the potential tree impact?

Excavation can cause root loss and wounds that lead to infections and decay. If excessive structural roots are lost, the tree can topple over and cause severe damage. If a contractor or property owner causes structural root damage that leads to a tree failing, they may be held liable for the damages. Excessive loss of non-structural fine feeder roots can destroy a tree’s ability to absorb water and nutrients and cause it to decline and die.

What types of activities might involve this impact?

  • Digging for a footer for a foundation of a new structure
  • Digging to install a new paved surface, such as a patio or driveway
  • Digging behind a new retaining wall to adequately secure it
  • Trenching for new underground utility lines or for utility repairs
  • Digging for a drywell or other stormwater infrastructure
  • Digging to lower and/or level the grade of the site
  • Digging to create terraces and stormwater management swales
  • Digging during the installation of new shrubs and trees with sizable root balls
  • Digging during the installation of irrigation systems and outdoor electric circuits

What can be done to minimize or avoid this tree impact?

  • Design your project such that any excavation is sufficiently far from any trees
  • Design your project such that the depth of excavation is reduced. Ideally, no excavation deeper than three inches will occur within a tree’s root zone. Surfaces like patios, walkways, driveways, and slabs for light structures like sheds can sometimes be raise above grade to preserve roots underneath.
  • Consider if your project may allow for the preservation of roots during excavation. This means finding a way to carefully dig without damaging the roots.
  • Consider trenchless methods for utility installation, such as using a “bullet” or “mole”, directional drilling, pipe bursting, or other techniques.

 

Soil Compaction from Construction Traffic and Materials Storage

 

What is the potential tree impact?

Vehicle traffic, heavy repeated foot traffic, and the storage of heavy materials can all lead to soil compaction and root damage.  Soil that becomes excessively compacted can no longer hold or transfer sufficient water, nutrients, or air to the fine feeder roots that support the tree.  This leads to root death and tree death.

What types of activities might involve this impact?

  • Equipment traffic over soil, including excavators, trucks, and others.
  • Heavy foot traffic during an extended project that last longer than two weeks. The ground pressure of a human foot is surprisingly high.
  • Storage of heavy materials, such as pallets of concrete bags, dumpsters, piles of lumber, piles of excavated soil, etc.

What can be done to minimize or avoid the tree impact?

  • Define a tree protection zone by installing tree protection fence to exclude construction traffic and storage from the root zone of a tree.
  • Lay down root protection matting, such as wood chips or woodchips plus plywood, to absorb and spread the load.
  • Use equipment with tracks and a low ground pressure instead of wheeled equipment.
  • Use existing paved or compacted gravel surfaces for any construction traffic and materials storage.

 

 

Tree Wounding from Equipment Traffic

 

What is the potential tree impact?

Wounding a tree’s roots, trunk, or branches can lead to infections and decay, causing a tree to decline or become structurally compromised and fall over in time.

What types of activities might involve this impact?

  • Equipment traffic that comes within a few feet of an exposed tree part is at risk of causing wounds.
  • Laying down a dumpster can cause wounds.
  • Vehicles traveling over soil that is loose or saturated with water can sink in and wound roots. This is especially true when vehicles repeatedly make turns in the soil.  This is true for tracked equipment as well.

What can be done to minimize or avoid the tree impact?

  • Define a tree protection zone by installing tree protection fence sufficiently far from any tree parts.
  • Install trunk protection to armor the tree from potential wounding. This can be plywood, lumber, or sturdy geogrid material strapped securely to the tree.
  • If the tree branch is small enough that the tree’s health would not be impacted, you may consider pruning it away preemptively to avoid wounding or branch breakage during construction.

 

Paving or Raising the Soil Grade

 

What is the potential tree impact?

Paving and raising the grade can severely change the soil environment for a tree even if roots are not cut.  Both pavement and excessive soil placed on top of the existing soil where tree roots are growing can starve the soil for water, air, and natural organic matter additions, leading to root decline and death and ultimately tree death.

What types of activities might involve this impact?

  • Paving for driveways, patios, walkways, slabs, etc.
  • Leveling the soil on a sloped site
  • Spreading excess soil from excavation over the root zone of a tree

What can be done to minimize or avoid the tree impact?

  • Design your paved areas to be sufficiently far from a tree’s root zone
  • Consider porous options for paving that maintain air and water flow to the soil.
  • Consider root aeration matting systems that maintain air flow to the area under the pavement or added soil. (You will want to work with an experience arborist to determine if this option is appropriate and what the recommended specification for root aeration matting is.)
  • Reduce the depth of soil additions to three inches or less.

Pruning

 

What is the potential tree impact?

Excessive or improper pruning of a tree can lead to reduced ability of the tree to photosynthesize the nutrients it needs and can cause wounds that become vectors for insect and disease problems.

What types of activities might involve this impact?

  • Pruning for clearance over a new structure
  • Pruning for clearance over a street, driveway, walkway, patio, etc.
  • Pruning for clearance for equipment access
  • Pruning for aesthetic preference

What can be done to minimize or avoid the tree impact?

  • Design the location and height of your structure to minimize required pruning.
  • Plan for smaller equipment or no equipment to accomplish the needed work.
  • Reconsider your aesthetic preferences and keep in mind that a tree almost always prefers to keep the leaves and branches it has grown to maximize its health.
  • Pruning of structurally compromised branches is acceptable

 

Tree Education Sections