Trees are in the news again for 2023! This past April we talked about how trees were in the news on a daily basis as the atmospheric river of storms battered California and caused many trees, weakened by the drought to fail. News stories about failing trees finally subsided once the storms stopped in May. Which makes sense. Then in mid-July trees were in the news again. A large limb from an oak tree fell on a picnic table at a local park. No one was hurt but it certainly scared the living daylights out of the family that was there. As the camera panned the scene, a local arborist’s voice-over explained that the failure was partly due to the drought, which weakened the tree, and to the hot summer temperatures which was a factor in the tree’s failure. The tree had suffered from what is known in the industry as Summer Limb Drop or Sudden Limb Drop.
What is Sudden Limb Drop?
Just like the name suggests, Sudden Limb Drop (or SLD) is when a tree sheds a limb, large or small, for no apparent reason most often in the summer months usually after the temperature has spiked to extreme highs.
What causes Sudden Limb Drop?
Research over the years has led to lots of speculations but no definite cause. There is enough consensus among arborists to believe water is a factor.
One theory suggests that on hot days, branches become lighter because the tree is evaporating water quicker than it can pull it up. Many SLD branches fall in the late afternoon, early evening, or early morning when there is a change in temperature. If a fog rolls in as the sun sets, or the overnight temperature reaches its lowest point in the early morning hours, the tree is able to pull up more water, weighting the lifted branch and causing it to fall back down and snap off. Or so the theory goes.
We also know that Sudden Limb Drop is more frequent in trees that have not been pruned in a number of years (or not at all!) and in trees with defects. An overabundance of rain, as we had in the winter of 2023, can encourage trees to flush out with lots of new growth and produce more end weight on already elongated branches and predispose them to failure. However, many sudden limb failures have occurred in trees with no defects at all, which makes it really difficult for arborists to predict.
Can Summer Limb Drop Be Prevented?
Education is your first line of defense. Being knowledgeable about Sudden Limb Drop will help you understand and value the need for periodic inspection of your trees by a professional arborist. Our arborists look for long branches that extend outside the canopy and that contain a lot of foliage at the ends, creating the perfect situation for SLD to occur. Minimize this by thinning the heavy foliage and if possible reduce the length of the branch back to a suitable lateral.
Sudden Limb Drop List of Common Observations
1. SLD is more likely in summer or early fall on extremely hot, dry days with little or no wind
2. SLD is more likely in afternoons and early evenings
3. Deciduous trees appear to be more susceptible than conifers
4. Older oaks, maples, ashes, beeches, and elms are common victims of sudden branch drop
5. Mature trees with larger limbs are more susceptible than younger trees with small limbs
6. Horizontal limbs that extend outside the canopy are more susceptible than upright limbs
7. Affected limbs may appear healthy and show no signs of defects
8. Trees experiencing SLD on one occasion may be more likely to drop limbs in the future
If you are concerned about your tree or it has been several years since it has been pruned, please call and arrange an appointment with an ISA Certified arborist.