It’s storm season. Healthy, well-maintained trees do well to weather many storms, however, unique circumstances of storm events can still bring down trees and limbs. The incidence and severity of tree failure during storms will vary depending on the interaction between tree- site-, and storm-specific factors. Consider the following if you have concerns about trees where you live:

Tree-specific factors:

Tree defects. Tree defects may be any tree part that is dead, dying, diseased, or decayed. Other defects include but are not limited to cracks, splits, or other evidence of physical injury; branch unions with included bark; uncorrected leans; and low live crown ratios on limbs, leaders or entire trees.

Improper pruning. Improper pruning can create tree defects that might not have otherwise occurred. Examples of improper pruning may be referred to as topping, lion’s-tailing, or wind-sailing. Such detrimental practices can compromise tree health and structure, predisposing them to storm damage.

Pest and/or disease issues. Trees under stress from drought, heat, compacted soils, or other adverse conditions are those most vulnerable to pests and diseases. Nonetheless, pests and diseases can decrease overall tree health, increase tree stress, and weaken or kill branches, leaders, or entire trees if attacks are severe or prolonged.

Landscape trees planted too deep. Trees without a visible root flare where the trunk meets the soil grade may have been planted too deeply. Such trees may have root problems, decay, or other structural weaknesses. It is not uncommon to see storm-related tree failures associated with the consequences of improper planting.

Root structure. Shallow root systems often take the blame, however, poor soil conditions may have more to do with trees’ uprooting. Compacted soils or soils with a high water table can force trees to develop shallow root systems that are more prone to failure in storms.

Site-specific factors:

Saturated soils: The force of friction between roots and soil particles is what helps a tree remain vertical and resist uprooting in heavy winds. In saturated soils, however, water acts as a lubricant that decreases friction between soil and tree roots and can lead to trees uprooting in strong winds.

Land disturbance: Sites that have experienced excavation, grade changes, compaction from heavy equipment, or other forms of disturbance may have damaged soils and adjacent trees, making them more susceptible to storm damage or wind throw.

Exposure. If trees are removed from a naturally forested setting, such as occurs in new developments, remaining trees will be exposed to new wind patterns. These remaining, newly exposed trees may be more susceptible to wind damage.

Storm-specific factors:

Wind Speed. Even healthy, defect-free trees can still be damaged if wind speeds are great enough. Severe thunderstorm warnings are issued once wind speeds first exceed 58 mph; anecdotally, this is when tree damage tends to increase during storm events.

Wind Direction. Trees adapt to ‘typical’ conditions of their environment as they grow, allowing most healthy trees to tolerate moderate to heavier winds coming from the prevailing wind direction. Winds that occasionally come from other directions may cause greater damage since trees are less adapted to those winds.

Rain, snow, and ice. Precipitation sticks to leaves, twigs and branches, adding additional weight that can burden leaves, twigs branches, and entire trees. Ice is particularly damaging to trees, especially when coupled with high winds.

This list is not comprehensive but covers many factors that can contribute to tree damage in storms, depending on the circumstances. If you are concerned about specific trees where you live, consider enlisting the services of an ISA Certified Arborist to assess their health and condition.


This article was co-authored by Tree Link Editor Ben Thompson and Cindy Maitland-Deffé, environmental sciences instructor at Spokane Community College and member of the Washington Community Forestry Council.