Hazard Tree Prevention


Site Development

Existing Trees, New Construction

It is becoming more common that existing trees are left, to the extent possible, when new developments, homes or other buildings are constructed. Unfortunately, this is often not done well. Here is what often happens:

  • Stem wounds: Construction equipment bangs into the trees. The trees are used to hold tools, to tie cables onto, etc. This creates infection courts and weakens trees.
  • Root wounds: Excavation too close to the tree severs and damages major roots, reducing tolerance to stress and creating infection courts for root-rot fungi.
  • Fill: To follow the design of the site, fill is often placed over the soil and even up against the base of the tree. This is much more damaging than many people, even some tree professionals, seem to realize: insufficient oxygen, water molds, root and butt rot can result.

Here is what should happen:

  • A competent person should select trees on the site that are in the best condition and have the greatest likelihood of survival and safety for some time. Sometimes it is best to forget the mature trees and save the 4-6″ trees.
  • A heavily guarded buffer should be established that prevents equipment from approaching trees. At a minimum, this should extend to the drip line, but more is better.
  • Design and construction should be carefully planned to avoid tree damage. Utilities should be routed to avoid trees.
  • Install bumpers to protect trees during construction and, if necessary, afterward. One of the big problems with trees in developed areas is wounding and excavation. Sooner or later, someone runs into the tree with a plow or something, or excavates for a sewer. Install bumpers to control vehicles if possible and do what can be done about planning for utilities and sidewalks.
  • Roof runoff should be planned so that it does not hit rooting soil. This tends to greatly increase compaction.


How to Prune Trees

At right is a link to an excellent primer from the U.S. Forest Service on why and exactly how to prune trees. Please read it before you go near a tree with a pruning saw!

Pruning is best done when trees are young to develop good form. The goals are to:

  • Eliminate/prevent forking stems (“codominant stems”).
  • Eliminate crossing branches.
  • Minimize future infection courts for decay fungi.
  • Reduce interior growth.
  • Develop strong structure.

If there are no other concerns, winter, especially late winter, (or dormant season) is usually chosen for pruning for several reasons:

  • The crown of hardwoods can be easily seen at that time to assess crown form.
  • There are no leaves to deal with in hauling the branches.
  • Sap and resin flow is usually minimal.
  • It takes advantage of the spring, when healing is fastest, for rapid wound closure.
  • In winter there are minimal chances of transmitting disease and attracting insects that vector pathogens or are themselves damaging. When certain diseases are a concern, one must know about the pathogens and insects involved to choose the best pruning time.
  • Practical scheduling of work makes winter the most convenient time for commercial operations.


  1. Trees and shrubs that flower in early spring, and whose flowers are desired, should be pruned immediately after flowering; flower buds will then form on new growth.
  2. Maples and birches bleed a lot in the spring, so late summer is a better time for pruning. The wounds will harden off so that they won’t bleed in the following spring.
  3. Christmas trees are pruned in summer after shoots extend but before buds are formed. This stimulates lateral buds to form, making the foliage denser.
  4. Dead branches can be pruned at any time, but you still need to follow proper pruning procedures to avoid tree damage

Vegetation Management

We know that the trees that make our developed landscapes so beautiful will, at some point in the future, either die, fail, or be removed because we can no longer tolerate the risk that they present. There is no better time than now to plan for replacement of those trees. If woody vegetation and the benefits that it provides are to be maintained indefinitely, it is important to manage the vegetation, preferably based on a vegetation management plan.

Vegetation management, broader in scope than hazard tree management, helps to ensure that we will enjoy the many benefits of trees and reduced hazard problems far into the future. Ensuring the establishment of young trees to eventually replace the old, planning for placement and species of trees, protection of trees from injury and long-term disease and insect problems are some of the issues considered in vegetation management planning. A vegetation management plan should take into account current conditions of the vegetation (including canopy, regeneration, shrubs, etc.), management objectives such as aesthetics and screening, potential pathways of stand development in the future, current and potential development of insects and diseases that may affect those pathways, and alternative approaches to meeting the objectives in the future.

Tree Failure Reporting

Reporting of tree failures and analysis of the resulting data can provide useful information on species, types of failures, defects and indicators associated with failure, etc. It is part of a good hazard tree management program.

An organization at some level must provide the service of receiving reports, incorporating them into a database, and analyzing the data. Until 2005, the best known and most successful reporting system was the California Tree Failure Report Program. Building on this model, the U.S. Forest Service, in cooperation with some leaders of the arboricultural community, initiated the International Tree Failure Database.

Unfortunately, funding for maintenance of the database has been discontinued.  Still, agencies and organizational units should keep their own records of failures.  They are an outstanding learning tool.