Cutting tree roots is possible without killing the tree, but it requires a precise, informed, and cautious approach. People often need to cut roots because they interfere with foundations, sidewalks, driveways, or utility lines, creating trip hazards or structural damage. However, the root system is the lifeline of the tree, meaning any removal must be carefully calculated to avoid compromising the tree’s stability and long-term health. The success of root pruning depends entirely on understanding the biology of the tree and adhering to strict limits on how much and how close to the trunk the cuts are made.
Understanding Tree Root Function and Structure
Tree root systems are composed of two distinct types of roots, each with a specific function. Structural roots are the larger, woody roots that radiate horizontally from the trunk, providing the physical anchorage that keeps the tree upright against wind and gravity. These roots also serve as pipelines, transporting water and nutrients absorbed by the finer roots back toward the trunk and canopy.
The second type are the fine feeder roots, typically less than one-sixteenth of an inch in diameter, responsible for the vast majority of water and nutrient absorption. These delicate roots spread out extensively, often reaching two to five times the diameter of the tree’s crown. They are predominantly located in the top six to twenty-four inches of the soil where oxygen and minerals are most abundant. This shallow spread explains why construction far from the trunk can still harm the tree. While the tree can regenerate fine feeder roots, cutting the larger structural roots can permanently impair its stability and health.
Defining the Critical Root Zone
The Critical Root Zone (CRZ), sometimes called the Root Protection Zone, is the area immediately surrounding the tree that must be protected from disturbance. Disturbing this zone, especially by cutting large roots, can cause tree decline or catastrophic failure. The CRZ is a calculated radius extending outward from the trunk, serving as a boundary for all construction and excavation activities.
A widely accepted formula for estimating the CRZ is based on the tree’s diameter at breast height (DBH), measured four and a half feet above the ground. A common guideline uses a radius of one to one and a half feet for every one inch of DBH. For example, a tree with a 20-inch DBH might have a CRZ radius of 20 to 30 feet, and root cutting should occur outside this boundary.
Cutting any root larger than two inches in diameter within the CRZ is risky, as these roots provide essential stability. If a structural root must be cut, it should be done as far from the trunk as possible, ideally three to five times the trunk diameter away. Cutting a large structural root too close to the trunk compromises the tree’s ability to anchor itself, making it susceptible to falling in high winds.
Safe Techniques for Root Pruning
When root removal is necessary outside the Critical Root Zone, the technique used is important. The goal is to make a clean incision that promotes rapid wound closure, allowing the tree to compartmentalize the injury and prevent decay. Roots should never be torn or crushed; avoid tools like backhoes or chain trenchers, which leave ragged wounds difficult for the tree to heal.
The proper method involves digging a clean trench to expose the roots intended for removal, allowing for a precise cut. Use a sharp tool, such as a bypass pruner, specialized root saw, or sharp shovel, to make a smooth, perpendicular cut across the root. This creates the smallest possible wound surface area. To minimize stress, limit severance to no more than 25 percent of the total root system circumference in a single season, and only cut roots on one side of the tree.
Monitoring the Tree After Root Removal
After root pruning, the tree requires sustained monitoring to help it recover from the stress of losing a portion of its root system. The most important post-pruning action is ensuring the tree receives adequate water, especially during the first year, since its ability to absorb moisture is reduced. Deep, infrequent watering within the remaining root zone is better than shallow, frequent watering, as it encourages new root growth deeper in the soil.
Applying a three-inch layer of organic mulch over the remaining root zone helps retain soil moisture and regulate soil temperature. Keep the mulch several inches away from the trunk flare to prevent moisture buildup and decay. Watch for signs of stress, including wilting leaves, premature yellowing, early leaf drop, or dieback of smaller branches. If the tree shows a noticeable lean or significant dieback, consult a certified arborist immediately.