City of Palo Alto Tree Technical Manual 1

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City of Palo Alto Tree Technical Manual 1-2 Section 1.00 Definitions

16. Hazardous Tree refers to a tree that possesses a structural defect which poses an iminent risk if the tree or part of the tree that would fall on

someone or something of value (target) (see Determining if a tree is Hazardous, Section 4.00).

Structural defect means any structural weakness or deformity of a tree or its parts. A tree with a structural defect can be verified to be hazardous by a certified arborist and confirmed as such by the City Arborist.

For the purpose of tree removal information required by the City, the tree report shall include a completed ISA-TREE HAZARD EVALUATION FORM, or an approved equivalent.

The City Arborist retains discretionary right to approve or amend a hazardous rating, in writing, and recommend any action that may reduce the condition to a less-than significant level of hazard. If the tree has been determined to be hazardous, removal of the tree is permitted under Section 8.10.050 of the Municipal Code.

notes: A ‘target’ may mean people, vehicles, structures or property, such as other trees or landscape improvements. A tree may not be a

hazard if a ‘target’ is absent within the falling distance of the tree or it’s parts (e.g., a substandard tree in a non-populated area away from pedestrian pathways may not be considered a hazard).

17. Injury means a wound resulting from any activity, including but not limited to ‘excessive pruning’, cutting, trenching, excavating, altering the grade,

paving or compaction within the tree protection zone of a tree. Injury shall include bruising, scarring, tearing or breaking of roots, bark, trunk,

branches or foliage, herbicide or poisoning, or any other action foreseeably leading to the death or permanent damage to tree health.

City of Palo Alto Tree Technical Manual 4-2 Section 4.00 Hazardous Tree Determination

4.25 DETERMINING A TREE’S HAZARD RATING

For the purpose of removal, if a tree is declared a hazard it must be rated for the level of hazard to persons or property by using the Hazard Rating Formula, or other professional methodology acceptable to the City of Palo Alto (see Hazard rating formula Table 4-1 and Appendix C):

ISA - HAZARD RATING FORMULA International Society of Arboriculture TABLE 4-1

Hazard Rating Formula

1 = low

4 = severe

1 = low

4 = severe

1 = low

4 = severe

3 =

12 =

Note: The above factors are combined to quantify a hazard rating. For example, a minimum rating of 3 is the safest (a low predicable hazard), and the maximum rating of 12 is an imminent hazard (a high predictable hazard). Further details regarding this formula can be found in the ISA- HAZARD EVALUATION FORM (see Appendix C) and the ISA publication * Evaluation of Hazard Trees in Urban Areas, most current edition.

Failure Potential + Target + Additional Factors/Size of Part = Hazard Rating + + = low severe

Required Practices

City of Palo Alto Tree Technical Manual 4-3 Hazardous Tree Determination Section 4.00 notes:

A. Failure Potential Rating

Failures do not occur at random, but are the result of a combination of defects and aggravating conditions. The scope of the professional evaluation

will include structural defects in the tree ( including branches, trunk and roots; and if necessary, shall employ the use of the most current methods of

internal decay inspection available); soil/slope and/or creek bank stability; individual species susceptibility to failure; pruning; history; decay weaknesses

and any other compromising or pertinent factors considered by the consultant.

B. Target Rating

Evaluation of potential targets shall include people, structures or property use and occupancy that are imminently threatened. Property use shall

consider what structures or activities are under or around the tree (e.g. building, parking, pedestrian, recreational, utility lines, hardscape, etc.).

Occupancy shall consider frequency of the use (occasional, intermittent, frequent or constant), and whether the target will be present when failure

occurs.

Consideration shall be given as to whether the target can be reasonably removed or isolated to reduce the hazard rating to a

less than significant level. A target means people or property (public or private).

A tree may be a potential hazard if it is: (a) a tree with the potential to fail; (b) in an environment that increases the likelihood of failure and; (c) a tree that would strike a target.

C. Additional Factors

Evaluation of other factors that contribute to aggravating conditions shall be considered, such as: size of the affected defect (i.e. a small branch vs. the

entire tree uprooting); significant potential of fire, utility line contact or catastrophic effects, etc.

4.30 TREE EVALUATION CHECKLIST

This part is intended to further help the property owner understand tree defects and how they may be interpreted by an arborist. Many tree defects are not readily apparent because decay or structural damage may be internal. Also, poor tree health may not reflect poor tree structure. Hazardous trees must be carefully evaluated.

The following checklist of criteria that is typically used by professionals may indicate potential or current tree hazards. The checklist is not meant to be a comprehensive guide, however, it is an outline of indicators that may alert a property owner to potential hazards and suggest action to avert a tree failure and liability. If you answer ‘yes’ to one or more of the checklist items, you should contact an arborist to discuss how to reduce the potential hazard.

A. Hazard Evaluation Questionnaire

Target: If the tree or branch falls will it hit cars, houses, structures, power lines or people? If so, immediate action may be necessary. Dead Branches: Are there dead tops or branches? Is the tree dead?

Cracks: Are there deep, open cracks in the trunk or branches? These are major starting points for trunk and branch failure.

Recommended Practices City of Palo Alto Tree Technical Manual 4-4 Section 4.00 Hazardous Tree Determination notes:

Crotch Cracks: Are there deep, open cracks below joining trunks or stems?

Tree Architecture: Has the tree grown beyond its species specific shape into a hazardous form? Is the tree leaning?

History: Has the tree recently lost large branches?

Edge Tree: Were neighboring trees recently removed, leaving tall trees exposed at the edge that may be subject to unexpected wind dynamics and blow-over?

Living Branches: Do live branches bend abruptly upward or downward where tips of large branches were cut off?

These may pull out of trunks that are weakened by rot or cracks. Beware of large branches on rotten or cracked trunks.

Topping: Are large branches growing rapidly from topping cuts?

These sprouts have weak attachments and may weaken further as they grow. Is there decay below topping cuts?

Storm injury: Are there broken branches, split trunks, or injured roots? Are branches close to power lines?

Root Rot: Are there fungus fruit bodies (mushrooms) on roots or near the trunk? Were roots injured by construction?

Rots and Cankers: Are there hollows or cankers (dead spots) in the trunk or major branches, some with fungus fruit bodies?

Construction injury: Have roots, trunk, or branches been injured? Is there a new lawn or garden over injured roots? The added fertilizer may stimulate the growth of fungi that will rot the supporting roots while the top gets heavier. A moderate storm could cause the tree to fall.

Guying of trees. Staking and guying of small to medium size trees may benefit from the additional support. Discretion must be exercised that the guying does not hide weaknesses, such as toppling over, that result from poor quality nursery stock or girdling roots.

4.40 HAZARD REDUCTION AND PREVENTION

Review the following list to reduce hazardous conditions. Plant trees that are not problematic and that fit the site The International Society of Arboriculture (ISA) has developed a list to assist you to avoid planting a tree that may become a problem (see Inherent Failure Patterns for Selected Species, Appendix D).

A healthy, vigorous tree that receives regular care is less likely to become hazardous than one that is ignored. Prevention is the best solution to the tree hazard problem.

The risk of a hazard tree may be reduced by removing dead and broken branches, reducing branch end weights, by mechanically supporting weak branches from below, or by cabling and bracing.

Recommended Practices notes: In some cases, targets may be removed such as by moving picnic tables or other items beneath a precarious tree, fencing to prevent access to such trees, or rerouting pedestrian or vehicular traffic.

If there are no other options to abate the hazard, the tree may need to be removed entirely (see Removing a Hazardous Tree, Section 4.10). Steps outlined in the Tree Removal Procedure (see Section 3.05) should be submitted as soon as possible for review by the City.

The following checklist will help property owners avoid future problems: Inspect your trees carefully at least once each season every year. Annually, have a Certified Arborist inspect your trees and provide you with a written report.

Avoid planting brittle species where falling limbs could injure people or property (see Inherent Failure Patterns for Selected Species,Appendix D).

Prune trees when they are young (see Pruning Young Trees, Section 5.30) and regularly thereafter.

Use correct pruning methods, always making the pruning cut outside the branch collar. This will allow only the minimum of decay infection. Do not allow topping (see Definition, Section 1.32).

Always plant the right tree in the right place. Select trees based upon their mature height and shape, and make sure the species selected matches the soil

and other site characteristics.

For example, avoid planting tall-growing trees such as redwoods near power lines or too close to your house (see Inherent Failure Patterns for Selected Species, Appendix D).

Water thoroughly (generally, until saturation is reached) during dry periods, slowly applying at least 2-inches of water per week (see Watering, Section 5.45).

Erect barriers around or slightly beyond the root protection zone of trees during construction. Insist that these root protection zones be honored by construction workers.

Consider cabling or bracing weak forks of branches in larger trees of high value. Do not plant trees with a narrowly-forked stem v-crotch, imbedded bark or girdling root ball. Where a valuable specimen tree may be suspected of developing into a hazardous tree, use landscaping to keep people at a safe distance.

This may require techniques such as rerouting walks, moving patio furniture, or planting shrubs and hedges to function as barriers to keep foot traffic at a safe distance (see Determining if a Tree is Hazardous, Section 4.20).

END OF SECTION