Root pruning is undertaken when a tree’s root system threatens hardscapes such as foundations, sidewalks, or underground utilities. While necessary to prevent structural damage, severing roots always risks the tree’s health and stability. Success depends on diagnosing the root type, calculating a safe cutting distance, and executing the cut with the correct tools and aftercare.
Determining Which Roots Pose a Threat
The first step is identifying which roots are causing the problem versus those that are structurally significant. Tree root systems consist of two primary types: large, woody anchorage roots and fine, fibrous feeder roots. The large roots near the trunk provide stability and transport water and nutrients.
Feeder roots, typically less than one-tenth of an inch in diameter, absorb water and minerals. These smaller roots extend far from the trunk, often well past the tree’s canopy, and their removal is generally less damaging than the loss of major anchor roots. Roots actively lifting concrete or pavement are usually the ones of concern, but they must first be excavated carefully to assess their size and proximity to the trunk.
A general rule is to avoid cutting any root two inches or more in diameter, as these are almost always structural and their removal can compromise the tree’s stability and long-term survival. Probing and excavation confirm the root’s size and path, ensuring that only the specific root causing the conflict is targeted. Removing a large structural root can drastically reduce the tree’s resistance to wind and may lead to a dangerous failure years later.
Calculating the Safe Cutting Zone
The proximity of a cut to the trunk is the most important factor in determining the tree’s survival and stability. Arborists use the Structural Root Zone (SRZ) to define the area around the trunk that must remain undisturbed to maintain stability. Disturbing roots within this zone significantly increases the risk of the tree falling over in high winds.
A practical rule of thumb for establishing the safe cutting radius involves measuring the tree’s trunk diameter at 4.5 feet above the ground, known as Diameter at Breast Height (DBH). The safe cutting line should be no closer to the trunk than four to five times this DBH measurement, converted to feet for the radius. For example, a tree with a 10-inch DBH should not have roots cut closer than 40 to 50 inches (about 3.3 to 4.2 feet) from the trunk.
It is important to limit the overall percentage of the root system removed during any single pruning event. Removing more than 25% of the tree’s root mass at one time is considered a severe risk that can lead to decline or death. If a large section of roots must be cut, “quadrant cutting” can be employed, where roots are removed from only one or two opposing quadrants, allowing the tree two to three years to recover before the next quadrant is addressed.
Proper Techniques and Tools for Severing Roots
The cut must be clean to promote proper wound closure and minimize the risk of decay or pathogen entry. The best tools are sharp, purpose-built cutting instruments: bypass pruners for smaller roots, root loppers for mid-sized roots, and specialized root saws for roots over two inches in diameter. Using dull tools like axes or shovels that tear and crush the tissue should be avoided, as jagged cuts create larger wounds that the tree struggles to compartmentalize.
Before cutting begins, the problematic root should be fully exposed by digging a trench or using an air spade to loosen the soil without damaging surrounding roots. The key is to make a single, precise cut that is perpendicular to the direction of the root. This clean, straight cut allows the tree to form a callus over the wound more effectively than a torn or angled cut.
For situations requiring a permanent barrier, a trench should be dug to the desired depth and the roots cut cleanly along the trench line, which is often lined with a root barrier material. Maintaining tool hygiene is important; cleaning and sharpening tools prevents the transmission of diseases between different plants.
Mitigating Tree Stress After Root Pruning
Immediate post-pruning care helps the tree recover from the shock of root loss. The primary focus of aftercare is ensuring the tree has adequate moisture, as water uptake capacity has been temporarily reduced. Deep, slow watering of the remaining root zone immediately after the cut and consistently in the following weeks is essential for hydration and fine root regrowth.
Applying a layer of organic mulch, such as wood chips, over the disturbed soil helps retain moisture and moderate soil temperature fluctuations. The mulch should be spread in a wide, shallow ring, maintaining a depth of two to four inches, but kept several inches away from the trunk flare to prevent moisture buildup and decay.
It is best to avoid applying fertilizer in the season immediately following root pruning, as this can force new growth and add unnecessary stress to the compromised root system. Instead, the tree should be monitored closely for signs of stress, such as wilting, yellowing leaves, or branch dieback, especially during dry periods. Recovery may take several seasons, and additional canopy pruning should be avoided to allow the tree to dedicate energy to root regeneration.