Palo Alto arborists must follow criteria to determine if a tree is hazardous. Municipal Code Chapter 8.10.020 defines "hazardous" as an imminent hazard or threat to the safety of persons or property. If a tree has a structural defect that may cause the tree or its parts to fall on someone or something of value and the risk is imminent, the tree is considered hazardous.
Palo Alto's Tree Technical Manual includes a checklist of potential hazards and suggests actions to promote safety. Not every condition on the checklist would result in removal of a tree. In many cases, pruning might be needed. But if a property owner answers "yes" to one of the items on the checklist, a certified arborist should inspect the tree, according to city arborists.
Target: If the tree or branch falls, will it hit cars, houses, structures, power lines or people? If so, immediate action might 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.
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 winds that could cause it to blow over?
Living branches: Do live branches bend abruptly upward or downward where tips of large branches were cut off? These might 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 might 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? Added fertilizer may stimulate fungi growth that will rot the supporting roots while the top gets heavier. A moderate storm could cause the tree to fall.
B Guying of trees:== Staking and guying of small to medium trees may benefit from the additional support. But discretion must be exercised that guying doesn't hide weaknesses, such as toppling over, that result from poor-quality nursery stock or girdling roots.