The pruning standards described here are consistent with the Pruning Standards adopted by the International Society of Arboriculture and the ANSI A300 Part 1: Tree Shrub and other Woody Plant Maintenance – Standard Practices, Pruning and the International Society of Arboriculture Best Management Practices. Given that these guidelines are generic in nature and do not apply to specific tree species, the standards in this Management Plan have been tailored to meet the criteria for tree care in Sierra Madre.
In permitting any trimming or removal of trees, the Public Works Department will follow the standards of the latest version of International Society of Arboriculture Pruning Guidelines and the American National Standards Institute A300 standards for tree maintenance. The aforementioned standards, as they may be amended from time to time, should be kept on file and available for public reference at City Hall and the City Library.
Time of Year
Tree pruning activities optimally should be timed to minimize stress to the trees and refrain from subjecting them to infestation and disease or during periods when insects are most active. The times to prune some species may vary in a given year depending on weather and other conditions. The following general guidelines should be followed:
- Hazardous trees may be pruned at any time to mitigate risks.
- Trees with thin bark should not be pruned in the summer.
- If the tree has known root damage or disturbance, pruning may be delayed until the deadwood is apparent, usually within one to three years after injury. Crown cleaning is the recommended pruning treatment.
- Neglected trees may need specific limbs removed or reduced in length, crown thinning, or crown restoration.
- Eucalyptus and pine species should only be pruned between November and April, when wood boring insects are not active.
- Oak species should only be pruned between July and September to avoid fungal and other infections and pest infestation.
Trees located in the public right of way require pruning both to maintain their health and form and for public safety. The objective of pruning is to improve health and structural stability for the tree. Reasons to prune a tree include, but are not limited to:
- Structural pruning for young trees to avoid future problems for the tree.
- Clearance pruning for vehicles, bicycles and public transportation.
- Removal of dead and dying branches from palm, broadleaf or conifer trees (crown cleaning).
- Pruning to restore the form of a tree after improper pruning, storm damage or limb failure (crown restoration).
- Selective removal of branches and foliage to reduce “wind-sail” (crown thinning).
- Reducing the size of trees to limit size or reduce end-weight on heavy branches (crown reduction).
- Pruning after a natural disaster or storm event to restore tree structure.
- Utility line clearance.
Trees with known pathogens that can be spread with pruning tools should be pruned using additional caution. The following cautionary steps should be followed when working on City trees:
- Avoid pruning on windy days in order to reduce the transmission of spores.
- To avoid the spread of pathogens to other trees, pruning tools should be sterilized before pruning another tree. Acceptable sterilization methods include fifty percent (50%) bleach solution for ten (10) minutes or handheld butane torch heating for fifteen (15) seconds per side.
- Wood with known wood boring insect infestations should be chipped into pieces smaller than four inches (4”) and spread at the site.
- Wood that is infected with disease should be handled and disposed of in a manner that minimizes the possibility of transmission of disease. This may include transporting green waste in covered containers.
Palms are pruned using the following measures to avoid disease and decay:
- Sterilized handsaws are used to prune all palms of the Phoenix genus.
- Trunks may not be skinned unless directed by the Public Works Department.
- Chainsaws are not used on any palms in the Phoenix genus.
- Chainsaws are not used on any palm species susceptible to pathogens that can be spread by chainsaw use.
- Typically, only the dead fronds are removed. Leaves should be pruned to 180 degrees.
- Trunk balls should not be shaped to avoid creating large wounds.
Clearance pruning consists of the selective shortening or removal of limbs to provide vertical clearance for bicycles, pedestrians, vehicles and structures. Selected branches may be subordinated by shortening them so that they can be removed later. The ratio of live crown to clear trunk remaining after pruning should be no less than fifty percent (50%) to preserve the health of the tree. Only those branches that need to be removed to achieve the established height clearance standard are typically pruned. All pruning cuts should be taken to the nearest lateral found above the set minimum height standard.
Whenever possible, young or developing trees should be pruned so that at least one-half (1/2) of the foliage is on branches that originate in the lower two-thirds (2/3) of the tree. Branches should have even distribution of foliage along their lengths for a well-formed, tapered structure and even weight distribution throughout the tree.
- Trees that line sidewalks or park pathways should have limbs and branches raised to a minimum of eight feet (8’) above finish grade. Limbs may be retained below this minimum elevation as long as they do not interfere with pedestrian or vehicle traffic, do not otherwise create a safety risk, and conform to the natural shape of the species.
- Street trees that line streets should have branches and limbs raised gradually from eight feet (8’) at the curb to fourteen feet (14’) over travel lanes. The branch structure should create an arch over the street when completed. Major arterial streets may require a higher maximum over central traffic lanes to accommodate existing limbs that contribute to a mature canopy.
- Trees should be maintained for streetlight and/or signage clearance by removing selected limbs to create an open canopy that will allow increased light and visibility. Only those branches that need to be removed to attain visibility clearance desired should be pruned. All pruning cuts should be made at the nearest lateral found away from the structure that is to be cleared. Severe pruning treatments are not allowed.
This pruning treatment consists of the removal of dead, dying, crossed or hanging branches. Crowded, weakly attached branches are also removed during this process. Up to twenty-five percent (25%) of the live foliage may be removed.
Restoration pruning is remedial pruning for a tree that has been topped, vandalized, over pruned or broken in a storm. Restoration pruning should involve several pruning treatments over a period of many years.
Thinning is utilizing conservative pruning techniques to minimize the necessary amount of foliage removed. It is generally used to reduce the density of canopy by as much as twenty-five percent (25%) to increase air movement through the crown and increase light under the canopy, but still retain the wind dampening effects of branch massing. It must be performed carefully, leaving interior foliage and concentrating on the outer two-thirds of the canopy. An even distribution of foliage should be maintained throughout the canopy. Removal of all sucker growth may not be necessary.
Crown reduction is the selective removal of branches to reduce the height or spread of a tree. The work is accomplished using reduction cuts and not heading cuts that result in stubbed limbs. Crown reduction may be used to reduce end-weight on trees that are unbalanced, utility line clearance and other types of clearance pruning. Older, stressed mature trees may require careful evaluation by Public Works staff prior to this pruning treatment.
Restoring Damaged Trees
Pruning after a storm requires an evaluation by the Public Works staff to determine the best course of action. Trees that experience storm damage may require restoration pruning under the following guidelines:
- Trees that can be restored that are free from cracks or large wounds on the main trunk or structural roots.
- Clean broken and cracked branches. Trees need to access stored energy in the limbs to recover. Leaving the tree unbalanced is acceptable to avoid removing too much live canopy.
- Heading cuts, or cuts that leave a stub, may be necessary to preserve canopy after a storm event. These cuts should not be used on healthy, undamaged trees.
- Storm damaged trees may require reconstructive pruning over several years in order to recover.
Pruning Young Trees
Properly pruned young trees can grow to have strong structure and greater longevity. They are also more cost-effective to maintain because they will require less corrective pruning as they mature. Cultivating the branching structure is an ongoing process during the first three to seven (3-7) years for most tree species. The following measures should be used for cultivating the branching structure of young trees:
- Prune newly planted trees to one central leader by shortening competing stems. All branches and stems should be shorter than the central leader after pruning.
- Retain and shorten temporary lower branches.
- Select the lowest branch that will become the first permanent branch. These branches should have a wide angle of attachment and be smaller in diameter than the trunk.
- Shorten branches that are below the permanent crown.
- Space main branches along the central stem.
- Reduce upright stems and branches back to lateral branches.
Utility Clearance Pruning
New construction, modification or replacement of existing utility facilities, and the replacement or repair of utility lines and structures, are governed by the SMMC except that utility company employees need not be Contractors to accept payment to remove or substantially trim any tree in the City. Utilities are responsible for damage to any protected tree, street tree, or public tree. Utilities are required to notify the City five working days prior to any maintenance activity that might affect any protected tree, street tree, or public tree. “Maintenance activities” as used in this section should include Public Utilities Commission-mandated line clearing.
As an alternative to individual prior notifications for each maintenance activity, the utility may submit an annual notification of maintenance activities to the City. This notification should include, but is not limited to, the following:
- List of facilities.
- Schedule of work.
- Extent of maintenance activities.
- List of protected trees, street trees, or public trees that might be affected.
Utilities may take emergency action with respect to any protected tree, street tree, or public tree without giving advance notice when immediate action is required to protect the public or the utility’s employees, prevent damage or destruction of facilities and property, or to effect expeditious reinstatement of utility service following an interruption. Any utility taking emergency action under this paragraph has to notify the City of such action within 72 hours of commencement of the action.
The California Public Utilities Commission regulates utility line clearance and standards are based on the type of hardware affixed to the lines. Regulations include General Order 95 Rule 35: Tree Pruning. Additionally, the following should be considered when pruning street trees for utility line clearance:
- Each species of tree has a slightly different branch collar where a proper cut can be made. Utility line clearance contractors are required to know the species and understand where to cut to encourage the formation of callus tissue.
- Flush cuts and stub cuts are not allowed.
- The amount of wood taken in a season should not exceed twenty-five percent (25%). This percentage may be adjusted based on the age, health and tree species. Older, stressed trees may perform better if pruning occurs over several years rather than all in one year.
- To avoid damage from sunburn injury, pruning should not expose the bark tissue of the interior trunk and branches.
Line-clearance tree workers should be trained to work around high voltage conductors. The United States Occupational Safety and Health Act (OSHA) and the American National Standards Institute (ANSI) have established minimum distances to be maintained by tree workers from electrical conductors. All line-clearance work involving City trees should adhere to these standards as well as the most recent version of the utility pruning standards established by the International Society of Arboriculture (ISA) and the Utility Arborists Association (UAA).
The following guidelines are designed to maintain required clearance of City trees from high voltage distribution and transmission lines with a minimum of re-sprouting and fewer pruning cycles. These guidelines are based on the biological response of trees to pruning techniques and should only be used when combined with safe work practices.
- Tree growth adjacent to utility lines should be managed with lateral or directional pruning (thinning cuts). Directional pruning removes a branch from the trunk or large lateral branch growing away from the conductor.
- Heading cuts are prohibited.
- Pruning cuts should be determined by structure and branching habit of the species. Branches should not be arbitrarily cut to a pre-established clearance limit.
- All trees should be examined for hazards prior to line clearance work. Hanging branches and dead wood should be removed first.
- Climbing spikes on live trees is prohibited. Only dead trees may be climbed with spikes.