Root protection: Ensuring healthy growth for your plants

Trees are vital to urban landscapes, providing shade, improving air quality, and enhancing biodiversity. However, their root systems are often at risk due to construction, paving, and soil compaction. In this post we want to talk about Root protection and the role of geosynthetics in it.

What is a root protection area?

A Root Protection Area (RPA) is the minimum area around a tree that must be safeguarded to ensure its long-term health and stability. This area is crucial because a tree’s roots extend underground to absorb water, nutrients, and provide structural support. Disrupting this zone through excavation, soil compaction, or construction can lead to root damage, poor tree health, or even tree failure. The RPA is usually determined based on the tree’s diameter at breast height (DBH) and is calculated using standard formulas, such as those outlined in the British Standard BS 5837:2012. Larger, mature trees typically require a wider protection zone to accommodate their extensive root systems.

To protect the RPA, barriers are often installed to prevent machinery, vehicles, and heavy foot traffic from disturbing the soil. If work must occur within the RPA, specialized methods like no-dig construction, permeable surfaces, or air spades for careful excavation may be used. Many local regulations and planning authorities enforce RPA guidelines, especially for trees protected under Tree Preservation Orders (TPOs) or those in conservation areas. By maintaining the integrity of the RPA, trees can continue to thrive, benefiting the environment, biodiversity, and landscape aesthetics.

The calculation of a root protection area

The Root Protection Area is typically calculated based on the tree’s diameter at breast height (DBH), which is measured at 1.5 meters above ground level. The standard method used in the British Standard BS 5837:2012 for tree protection in construction is:

Formula for RPA Calculation

1. Radius of RPA =DBH (mm) × 12 (in millimeters, converted to meters)
2. RPA (Area in m²) = π × (Radius)²

Example Calculation

• Suppose a tree has a DBH of 50 cm (500 mm)
• Radius of RPA = 500 mm × 12 = 6 meters
• RPA (Area) = π × (6²) = 113.1 m²

Is it possible to construct on a tree root protection area?

Yes, it is possible to construct within a Root Protection Area but it must be done with extreme care to avoid damaging the tree’s roots and compromising its health and stability. Any construction within an RPA should follow strict guidelines to minimize soil disturbance, root severance, and compaction.

How to Build Safely in an RPA

1. Use No-Dig Construction Methods – Traditional excavation can harm roots, so techniques like raised boardwalks, pile foundations, and floating structures are used instead.
2. Employ Permeable Surfaces – Hard surfaces can compact soil and limit water and oxygen flow. Instead, materials like gravel, permeable paving, or cellular confinement systems help protect the roots.
3. Limit Excavation – If excavation is unavoidable, tools like air spades can expose roots gently without cutting or damaging them.
4. Supervision by an Arborist – A professional should assess root distribution and recommend protective measures.
5. Retain Natural Drainage – Avoid altering water flow, as changes in moisture levels can stress the tree.

To what extent do roots spread?

The extent to which tree roots spread depends on the species, soil conditions, water availability, and environment. However, in general, tree roots spread much wider than the tree’s canopy.

Typical Root Spread Patterns:

1. Lateral Spread – Most tree roots extend horizontally rather than deeply. In many cases, roots spread 2 to 3 times the tree’s height. For example, a 10-meter-tall tree may have roots extending 20 to 30 meters outward.
2. Depth – The majority of tree roots are found in the top 60 cm (2 feet) of soil,as this is where oxygen, water, and nutrients are most available.
3. Structural vs. Fine Roots – Large structural roots are closer to the trunk, while fine feeder roots, responsible for water and nutrient absorption, can spread far beyond the canopy drip line.

Example of Root Spread by Tree Type:

• Oak (Quercus spp.) – Can spread up to 3 times the canopy width.
• Pine (Pinus spp.) – Typically has deep taproots but still spreads widely.
• Willow (Salix spp.) – Has an aggressive, wide-spreading root system, often extending well beyond the tree’s height.

Geosynthetics’ application in root protection zones

Geosynthetic products play a crucial role in protecting tree root zones, particularly in urban and construction environments where root systems are vulnerable to soil compaction, erosion, and mechanical damage. These synthetic materials, including geotextiles, geogrids, and geomembranes, help create stable and permeable surfaces that allow for construction while preserving the health of trees. Geotextiles, for instance, act as a protective barrier between soil and hard surfaces, preventing soil compaction and root suffocation. This is particularly useful in pedestrian pathways, driveways, and roadways that need to be built near trees without harming their root systems.

Another key application of geosynthetics in root protection zones is load distribution and reinforcement. Geogrids and geocells are used to create reinforced surfaces that spread the weight of vehicles or structures, reducing the pressure exerted on the soil and underlying roots. By incorporating these materials into no-dig construction methods, such as permeable pavements or floating foundations, developers can maintain natural root growth while allowing infrastructure development. Additionally, geomembranes can be strategically used to redirect root growth, preventing roots from damaging foundations or underground utilities while still allowing water and nutrients to reach the tree.

Geosynthetics also contribute to moisture retention and erosion control in root protection areas. In areas prone to soil erosion, geotextiles and erosion control blankets stabilize the soil, ensuring that roots remain anchored and receive sufficient nutrients. Moreover, permeable geosynthetic layers allow for proper water infiltration, preventing excessive runoff and maintaining optimal soil conditions for tree health. By integrating geosynthetics into root protection strategies, urban planners and landscapers can successfully balance environmental conservation with infrastructure development, ensuring that trees coexist with urban growth.

Tree root protection standards

Tree root protection is governed by various international and national standards that provide guidelines for preserving tree health, particularly in construction and urban development. These standards define Root Protection Areas and outline best practices for minimizing root damage.

Key Tree Root Protection Standards:

1. British Standard BS 5837:2012 (UK)
   • Defines how to protect trees during construction.
   • Specifies that the Root Protection Area should be calculated as 12× the tree’s diameter at breast height (DBH).
   • Recommends the use of protective barriers, no-dig construction techniques, and permeable surfaces within the RPA.
2. ANSI A300 (USA)
   • Provides arboricultural best management practices, including tree protection during construction.
   • Advises against excavation within the critical root zone (CRZ), which is typically equal to 1 foot of radius per inch of trunk diameter.
   • Encourages the use of mulching, root pruning, and aeration techniques when working near tree roots.
3. AS 4970-2009 (Australia)
   • Focuses on the protection of trees on development sites.
   • Defines the Tree Protection Zone (TPZ), calculated as 12× DBH for most trees, with a minimum radius of 2 meters.
   • Specifies that construction activities, including excavation and soil compaction, should be avoided within the TPZ.

Differences between root protection & root guidance

Root Protection and Root Guidance are both strategies used in urban planning and construction to manage tree roots, but they serve different purposes and are applied in different situations.

1. Root Protection

Root protection focuses on preserving existing roots and ensuring the long-term health of trees, particularly during construction or landscaping. This involves preventing root damage from excavation, soil compaction, or changes in drainage patterns. Common methods include Root Protection Areas (RPAs), tree protection barriers, permeable surfaces, and no-dig construction techniques. Standards like BS 5837:2012 (UK), ANSI A300 (USA), and AS 4970-2009 (Australia) define best practices for safeguarding tree roots in development projects.

2. Root Guidance

Root guidance, on the other hand, is a proactive approach designed to direct the growth of new roots away from structures, utilities, or sensitive areas. This is typically done using root barriers, geomembranes, or deflectors that encourage roots to grow downward or laterally in a controlled manner. Root guidance is commonly used near sidewalks, foundations, underground pipes, and roadways to prevent root-related damage while allowing trees to thrive in urban environments.

Conclusion

Root protection is essential for maintaining tree health, especially in urban and construction environments where roots are vulnerable to damage. It involves safeguarding the Root Protection Area through methods like protective barriers, no-dig construction, and permeable surfaces.