4 Vital Observations to Assess the Health of Blue Gum Eucalyptus Trees

Tasmanian blue gum eucalyptus (eucalyptus globulus) are ubiquitous in California. The species was imported as seeds from Australia during the Gold Rush during the mid-1800s, in the hopes that the fast-growing trees’ wood could be used for construction, furniture making, and railroad ties. They were unsuitable for that use due to the propensity of the planks to warp, but the blue gum continued to be planted as windbreaks for agriculture and to be harvested for firewood. Now, rotting blue gum eucalyptus can present a significant risk to people and property. Today’s urbanized groves and windrows (rows of trees planted to deflect wind) 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, which weaken trees over time by defoliating, interrupting vascular flow, or introducing fungal infections. In 2011, California municipalities were tragically reminded of the public risk presented by blue gum eucalyptus trees when a 10-ton tree in Orange County toppled onto a roadway and killed a motorist.

Whether these long-lived trees should be preserved or removed in general is the subject of debate, but data-backed tree risk assessments can help inform the best course of action for individual trees or specific groves within a municipality or on a project site. Dudek arborists have evaluated thousands of blue gum eucalyptus trees over 30 years to understand how visible, external signs can indicate internal wood rot and tree instability. One Dudek study of 5,000 windrow trees showed that whether a tree has significant internal wood rot can be determined by observing four key areas on and around the tree, namely, the ground, trunk, crown, and foliage.

How to Perform a Blue Gum Eucalyptus Assessment

Dudek Urban Forestry Practice Director Michael Huff, RCA said cities and property owners with blue gum eucalyptus trees, especially those in windrows, should assess them annually with a 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 a 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, crossing, or hanging branches, poor attachment angles, cracks, or splits. These issues can lead to branch failure.
  • Examining the foliage for the presence of pests, unusual color, or declining conditions. Pest infestations may not represent an imminent threat but should be addressed early to avoid further issues that are harder to manage.

When to Inspect Trees

Blue gum eucalyptus inspections should be performed annually, as well as after wind and/or rainstorms. Our arborists recommend more thorough exams if people or property are at risk should a tree topple. In these cases, a professional arborist like those at Dudek can internally probe the tree, establish a baseline condition, prepare a hazard assessment and develop an internal decay inspection protocol, such as sonic tomography. Sonic tomography is a non-invasive assessment method, which measures the velocity of sound waves in wood to detect decay.

Dudek employed sonic tomography to help the City of Irvine evaluate the risk of its blue gum eucalyptus trees. Our arborists completed a tree risk analysis on the City’s 1,970 blue gum eucalyptus trees across 24 windrows. Dudek arborists recorded detailed external attributes for each tree and then performed sonic tomography, evaluating each tree at two vertical positions on the trunk. Following the assessment, we prepared arborist reports for each of the 24 windrows. Reports included findings, recommendations, and photographic logs of all trees. Our experts identified hundreds of at-risk trees with extensive internal decay that were likely to fail. As a result of our assessment, the City was able to prioritize unhealthy trees for removal and identify healthy trees suited for long-term preservation and maintenance.

Graphic explaining the 4-step process to assess the health of blue gum eucalyptus trees. 1. Examine the ground around the trunk to determine root health
2. Observe the trunk to detect lean, conks, cracks, splits, or cavities
3. Evaluate the crown for dead, crossing, or hanging branches
4. Examine the foliage for the presence of pests or unusual colors

For more information about how Dudek’s arborists can help assess the risk of your blue gum eucalyptus or other tree species, contact us.