About roots and soil health

When tree protection fails

You may have seen this happen: A construction project wraps up, the tree protection zones (TPZs) are dismantled, and everything looks great. The protected trees appear healthy, but a few years later, property owners notice signs of stress or decline. What went wrong?

The sad truth is that many street trees only live about 20 years. That’s a short life, considering many tree species can live decades or centuries in the natural environment. A 2019 Boston University study confirmed that “urban trees live fast and die young” compared to those in rural forests.

A major culprit in urban tree mortality is often traced to unseen construction damage, particularly to the root zone. While the TPZs may have been properly marked on paper, as with all the best-laid plans, success or failure is determined by execution. Soil compaction, root disturbance, and trunk damage may go unnoticed at first, but they can lead to long-term decline. If TPZs are not properly enforced, the entire investment in tree preservation is wasted.

Getting it right

Construction and trees don’t have to be at odds, though. Tree protection zones (TPZs) are the front-line defence against construction-related soil damage. 

Done right, TPZs can be seamlessly integrated into your project to protect the trees—without sacrificing operational efficiency or design flexibility.

This guide dives into the science behind tree protection zones. Later in the article, we’ll highlight best practices for TPZ design and implementation and practical tips to help integrate tree preservation into the development process.

Soil and roots:  the living foundation for a tree

Preserving healthy soil structure is a primary concern within the TPZ and one that is often misunderstood. 

Soil is not just dirt—a medium that holds trees up. Soil is a complex ecosystem composed of minerals, organic matter, water, air, and a diverse community of microorganisms, fungi, and invertebrates that work together to support healthy plant growth. 

Tree root systems serve as supply lines for water and minerals, carbohydrate storage, and hormonal signalling systems while physically anchoring trees in the ground.  These roots range in size from large anchor roots to tiny, almost microscopic root hairs. Disturbances to this system can lead to physiological stress, reduced growth, and increased susceptibility to disease. 

Compromising the root zone sets off a chain reaction of decline that may take years to become evident but is ultimately irreversible.

Construction activities can disrupt this underground ecosystem in several ways:

Understanding these biological relationships explains why construction-phase soil management is so important for long-term tree preservation.

 Preventing compaction through proper site management during construction helps support the health and stability of the protected trees over time.

The first step is to show the area that needs to be protected by the TPZ. This is often bigger than one might expect. That’s because most tree species produce horizontally spreading roots(see image below left) rather than vertically to penetrate deeply into the earth (below right).

Illustration 1: Roots Grow like Plates, not Mirrors

root growth

Image source:  New York State Urban Forestry Council

Understanding Tree Protection Zones (TPZs)

A TPZ is a designated area surrounding a tree that must remain undisturbed during construction activities. The main purpose of the TPZ is to ensure the structural integrity and long-term health of trees by protecting the soil, roots, trunk, and canopy from construction-related damage.

In many cities, TPZs are mandated by law and are part of the construction permit application for projects where trees are present. For example, the City of Toronto’s TPZ regulations are set out in three main documents: 

The TPZ provides a physical boundary that protects the essential anchor and feeder roots and shields the trunk and branches from physical impacts caused by machinery or construction materials.

Most TPZ requirements are based on a multiple of the trunk diameter at breast height (dbh). The dbh measurement is a common industry method for estimating root spread to ensure adequate protection of the critical root zone (CRZ) for water, nutrient uptake, and structural stability.  The dbh multiple may be adjusted to account for the species, age, and site conditions.  

Minimum protection distances are measured from the outside edge of the tree base. The distance will be either the tree’s drip line, or the specified distance based on trunk diameter, depending on municipal requirements. For instance, a tree on city or private property with a DBH of 40 cm requires a minimum protection distance of 2.4 meters from its base. That distance increases to 4.8 meters for trees protected by Toronto’s Ravine and Natural Feature Protection By-law. (See page 6 of the Toronto Tree Protection Policy and Specifications document.)

Illustration 2: Minimum tree protection zone determination method used by the City of Toronto 

TPZ requirements

Image source: City of Toronto

 Other cities and municipalities within the Greater Toronto Area (GTA) have similar regulations, and it is important to consult the local rules governing your project site.

Best Practices to Design and Implement TPZs

The rest of this article delves into the best practices in tree protection as described in the ISA-BMP Managing Trees During Site Development and Construction. Best Management Practices: 3rd Edition.

Note: A summary of these TPZ best practices for the 6 phases of a construction project is provided at the end of this article. 

Pre-Construction Planning

Establishing TPZ requirements for your construction permit application

If you work in an area with tree protection regulations—which includes almost everywhere in the GTA and several other cities across the country—you will need an Arborist Report as part of the construction permit application. That report includes a tree inventory and tree protection plan (TPP). 

The TPP can have a significant impact on the total project design, including the placement of structures, access points, and driveway and parking lot locations. It details the location of each tree, its TPZ boundaries and specific protection measures such as fencing and barriers, and adjustments to access points or infrastructure needed to avoid root zone conflicts. 

The TPP must also account for soil preservation strategies, which are often overlooked yet necessary for maintaining tree health. Soil aeration, organic amendments, and avoiding material storage within the TPZ are considerations in the planning phase.

That’s why it’s so important to engage the arborist and integrate the TPP into the overall site plan from the very beginning of the project, long before any construction work begins. 

The arborist first inventories the trees within and next to the construction zone, compiles species, size, and condition data, and plots the tree location and TPZ requirements on the construction site plan. The inventory also includes trees on neighbouring properties within a boundary zone.

The arborist will quickly find conflicts between the initial plan and the tree protection requirements and offer remedies to address issues that may cause a permit application to be delayed or denied. 

**Best practice: Bring the ISA Certified Arborist into the project during the early planning phase—well before any site work begins.  Their guidance will speed up the permitting process and ensure the tree protection requirements are met with the least disruption to the project plan.

Establishing Physical Barriers

Once the permit is approved, it’s time to install the protective fencing around the TPZs. The City of Toronto mandates specific fencing requirements, including a minimum fence height of 2.4m (8ft) plywood hoarding or equivalent, or 1.2 meters (4 ft) orange plastic web snow fencing along roadsides where visibility is a concern. These fences must be placed before any site work begins and remain intact throughout the project to ensure compliance.

Clear signage is usually required on all sides of the barrier, showing the presence of a TPZ and outlining prohibited activities within this zone.

Prohibited Activities Within TPZs:

Where space limitations prevent full TPZ barriers, horizontal hoarding may be used. Horizontal hoarding consists of protective ground coverings of thick mulch layers under three cross-laminated plywood sheets to be installed where access requirements encroach on a TPZ. These methods help distribute weight and minimize pressure on the root system.

If excavation is unavoidable, lower impact no-dig techniques such as air-spade, hydro vac, or hand digging can minimize the damage. These methods allow for carefully showing and protecting major root structures, reducing the impact on the tree’s stability and nutrient uptake.

If root pruning is unavoidable, it must be performed by a qualified arborist following proper procedures.

Monitoring During Construction

Well-planned protection measures can be undermined if site personnel do not adhere to restrictions, so ongoing monitoring and education are needed throughout the project. 

Any breaches of the TPZ should be documented and addressed at once, ensuring accountability among all stakeholders.

You should also expect periodic inspections by a city arborist to ensure that TPZ barriers are still intact and that soil conditions are not compromised.

Post-Construction Recovery & Long-Term Monitoring

Even with the best precautions, construction activity places stress on trees. Once the project concludes, additional remediation steps are often recommended to ensure tree health. These may include: 

Many post-construction failures occur due to unaddressed root damage that manifests over time. It’s important to monitor tree health for an extended time to detect the symptoms and address them promptly. For this reason, annual arborist inspections are recommended for at least three growing seasons post-construction to ensure long-term tree health. These assessments offer an opportunity to detect delayed stress responses and apply remedial treatments such as targeted pruning, fertilization, and pest management.

Why TPZ Compliance Matters

Not adhering to TPZ regulations carries significant financial and legal consequences. 

In Toronto, unauthorized damage to protected trees can result in fines exceeding $100,000 per tree, with added penalties for not implementing remediation measures. Beyond regulatory repercussions, trees that suffer root damage become structurally unstable, posing hazards to people and property. Developers may also have to fund expensive tree replacement work, significantly affecting project budgets.

From an environmental perspective, every mature tree lost to poor construction planning is a significant setback to urban canopy goals. Trees provide important ecosystem services, including carbon sequestration, stormwater management, and temperature regulation. Preserving these assets through proper TPZ enforcement helps urban environments stay resilient and ecologically sustainable.

Tree loss can also reduce property values as trees are known to make property more attractive to potential buyers and tenants.

TPZ Best Practices by Construction Phase

Tree Protection Zones (TPZs) offer a straightforward method for safeguarding trees against soil damage during construction, making it easier to plan and execute projects effectively.

This guide highlights the ISA-recommended best practices for TPZ management across the six construction phases, detailing the responsibilities of arborists, landscape architects, and construction managers. 

This list is drawn from the official ISA document titled Managing Trees During Site Development and Construction. Best Management Practices: 3rd Edition.

Phase 1: Planning – Establishing TPZ Requirements

Goal: Identify and define TPZs early to integrate them into the project scope.

Phase 2: Design – Finalizing TPZ Implementation

Goal: Ensure TPZs are incorporated into final construction and landscaping plans.

Phase 3: Preconstruction – Implementing TPZ Barriers

Goal: Install and enforce physical TPZ protections before construction begins.

Phase 4: Construction – Protecting Trees During Work

Goal: Enforce TPZ protections to prevent root damage, compaction, or soil contamination.

 Phase 5: Landscaping – Ensuring TPZ Stability

Goal: Prevent tree damage from final grading, irrigation, and plant installations.

Phase 6: Postconstruction – Long-Term TPZ Management

Goal: Ensure long-term tree health after construction is completed.

TPZ Best Practices Checklist

Final Thoughts

As we’ve explored, TPZs are powerful tools that allow designers and builders to weave the natural beauty and benefits of trees into our cities without compromising the financial and functional priorities of the project. 

The science we’ve discussed highlights the importance of careful planning, enforcement, and monitoring at every step of the construction journey.

When we prioritize tree protection, we’re not just ticking boxes–we’re creating projects where infrastructure and nature dance in harmony. The result is developments that look fantastic, boost property values, and contribute to a more sustainable urban environment.

At Gray Matter Forestry Consulting, we’re all about helping you achieve this balance. By bringing us on board early in your planning process, you’re ensuring that your Tree Protection Zones are designed and implemented with expertise and care. Together, we can craft projects that meet today’s needs and nurture Toronto’s urban forest for generations to come. Let’s build a greener, more vibrant city–one protected tree at a time.