Root pruning is an injurious practice that can cause significant damage to trees. Both the health and structural stability of a tree can be greatly affected by root pruning. Cutting one major root can cause the loss of 5 to 20 percent of a tree's root system.

Tree roots are found mostly within the upper 12-18 inches of soil, but will extend far from the trunk. In a healthy mature tree, roots typically grow outward to a distance of two to three times the radius of the crown. When roots are cut, trees can become unstable and the risk of failure can greatly increase, especially in areas given to high winds.

For these reasons root pruning is avoided. If there is no alternative, root pruning must be performed minimally and as far from the trunk as possible.

Root Pruning Guidelines

In order to prevent the increased risk associated with improper root pruning, the City has established these basic guidelines:

• Root Protection Zone – this is the area of land immediately surrounding a tree that is left undisturbed and protected. The dimensions of the root protection zone are determined by measuring the diameter of the trunk and applying one foot of land area for every inch of trunk diameter (trunk diameter is measured at 4½ feet above grade). For example, a tree with a diameter of 12 inches will have a 12 foot radius as a root protection zone.
• No person is permitted to cut roots within the root protection zone without permission from the Public Works Services Department.
• The City of Rancho Cucamonga may perform minimal root pruning within the root protection zone in order to:
  1. Relieve the pressure of a root on an underground utility, provided the root is not too large.
  2. To facilitate the placement of a new sidewalk or curb & gutter, provided the tree is small, the root pruning is minor, and there would be no loss in stability.
• The City of Rancho Cucamonga does not root prune in order to repair private irrigation systems.
• Property owners may root prune outside the root protection zone subject to the following conditions:
  1. The maximum amount of root pruning allowable outside the root protection zone is 10% of the tree's roots, on one side of the tree only, and no more than once every three years.
  2. The maximum size root that may be cut outside the root protection zone is 2” in diameter.
  3. Roots shall be cut cleanly with a sharp cutting tool and may not be torn, ripped or pulled.
  4. Trees with an un-balanced root system shall have no roots cut on the weaker side.
  5. In some situations, especially when larger trees are involved, a property owner may be required to cap off an irrigation line in order to prevent overpruning of roots.