Lean in trees: what the standards and agencies describe

This page describes what published sources say about one structural condition in trees. It gives no verdict on any tree and is not an assessment. The sources come from the United States and the United Kingdom. The parent page, structural defects in trees, compares the conditions that have pages here.

What the sources describe

The US Forest Service brochure lists poor architecture, which includes lean, among its defect types. It says a leaning tree may be a hazard but that not all leaning trees are dangerous, so any leaning tree of concern should be examined by a professional arborist. Its list of reasons to act names a tree that leans excessively and a lean with fresh root exposure, soil movement or soil mounding near the base, and one of its pictures shows a mound at the base as a sign that a tree has recently begun to lean.

The University of Florida extension publication says a lean often forms over time as a tree grows away from an object in search of light, and that a leaning tree may have structural issues with its roots. It says trees leaning on steep slopes or in saturated soils have an increased likelihood of failure, and that if a tree can produce extra reaction wood to account for the imbalance, the risk can be significantly reduced. It says a lean that formed relatively rapidly, for example after a severe storm, is an indication of partial root failure that requires immediate attention. It lists signs of a less severe, longer-persisting lean: an enlarged lower trunk and branches in full sun that sweep upward over time. A lean that keeps increasing over a short period, or that comes with soil upheaving, is called high risk to be inspected immediately by an arborist.

The University of Kentucky leaflet says trees that lean are at a greater risk of falling and that trees with abnormal shapes, such as more branches on one side, could be hazard trees. The ISA Trees Are Good leaflet has no item named lean. It says cracks or separations in the soil may indicate soil heaving, especially if the tree is leaning, and that asymmetric shapes may be caused by reaction wood and may indicate an internal problem.

Where the sources add a caution

The National Tree Safety Group guidance says deformities are not necessarily weaknesses and can be a normal adaptive response that compensates for stresses that might otherwise cause breakage. It defines a defect as a condition that predisposes a tree or part of it to structural failure, so a lean is a description of the tree, not a finding about how it will behave. The Florida publication treats reaction wood and a rapid lean as separate topics within its lean section. The US Forest Service brochure says a defect matters where a target, such as a person or property, is within striking distance of the tree.

Who looks at this

For the methods and the credentials, see tree risk assessment methods and consulting arborist credentials. Related conditions are on root damage and root plate lifting.

Lean in trees: what the standards and agencies describe

What the sources describe

Where the sources add a caution

Who looks at this

Sources