Risk Assessment of Aging, Non-Native Eucalyptus Trees

Rotted blue gum eucalyptus present significant risk to people and property. Dudek arborists have evaluated thousands of blue gums to understand how external conditions can indicate internal wood rot.

California municipalities were reminded recently of the risk presented by blue gum eucalyptus trees when a 10-ton tree in Orange County toppled on to a roadway and killed a motorist. One hundred trees were eventually removed from the roadway area.

Blue gums were imported from Australia to California in the mid 1800s in failed experiment to use them as railroad ties. The fast-growing blue gums continued to be planted as windrows for agriculture and for firewood.

Today’s groves are significantly weakened due to years of injuries and stress from aggressive pruning, root severing, soil compaction and high soil moistures. Blue gums also are susceptible to attacks by pests that have no natural predators. The devastating long-horned borer kills many tree trees despite the release of predator wasps.

Dudek arborists have evaluated thousands of blue gum eucalyptus trees over 20 years to understand how visible, external signs can indicate internal wood rot.

A Dudek study of 5,000 windrow trees showed that determining whether a tree has significant internal decay can be indicated by the following conditions: basal conks, cavities, foliage pests, basal wounds and raised soil. The study confirmed that flush cuts in the crown or on the trunk from old pruning activities typically resulted in decay columns extending downward from the wound.

Inspection and Assessment
Michael Huff, Dudek’s urban forestry practice manager, said cities and private property owners with blue gum eucalyptus trees, especially those in windrows, should assess them annually with bottom-up examination that includes:

  • Examining the ground around the trunk to determine root conditions. Raised soil and cracks may indicate tree movement or “tipping”.
  • Observing the trunk for noticeable lean, presence of “conks” (fungal fruiting bodies), cracks, splits, or cavities. These may indicate structural issues.
  • Evaluating the crown’s branches looking for dead branches, crossing branches, poor attachment angles, cracks or splits, or hanging branches (also known as “widow makers”). These issues can lead to branch failure.
  • Examining the foliage for presence of insect pests, unusual color or declining conditions. Pest infestations may not represent imminent issues, but should be addressed in a short time frame.
  • Perform inspections following wind and/or rain storms in addition to the annual inspection.

More thorough exams are recommended if people or property are at risk should a tree topple. In these cases, a professional arborist can internally probe the tree, establish a baseline condition, prepare a hazard assessment and develop an internal decay inspection protocol.