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Sunday, June 26, 2022

Summer oak branch falls | real dirt

At this time of year, it is not uncommon for large branches to suddenly fall from oak trees without warning.

While this phenomenon is not limited to oaks, the sheer size of their branches makes summer branch drop (also known as “sudden branch failure”) in oak trees much more dangerous to life and property than branch drop in other tree species.

“Real Mud” is a column of various local Master Gardeners affiliated with UC Master Gardeners of Butte County.

Richard Harris, a professor of landscape horticulture at the University of California, Davis, published an article on the subject in the Journal of Arboriculture in April 1983, describing the condition and its possible causes, and suggesting steps that tree owners can take to reduce the danger of summer falling branches.

The summer decline of branches is not related to the wind and most often occurs in the afternoon on hot windless days. Unlike most wind fractures, which occur where the branch attaches to the trunk, branch fall fracture in summer typically occurs 3 to 12 feet from the trunk along the entire length of the branch. The branches that break are usually long and horizontal rather than vertical, often reaching to or beyond the middle canopy of the tree. While some of the falling limbs show signs of injury or decay, many of these broken limbs appear to be in good health. Older, less vigorous trees seem to be more susceptible to this problem. Once a tree has lost one branch due to summer branch fall, it is more likely to lose another.

Drought stress may somehow contribute to summer branch drop, but there is currently no generally accepted hypothesis to explain this phenomenon. In California, this type of branch failure occurs in both native and planted trees, and in irrigated and non-irrigated landscapes. One possibility is that drought stress on a hot, windless afternoon reduces water flow in the branch, causing the branch temperature and ethylene concentration to rise. Ethylene, a gaseous hormone found in all plants, is known to contribute to the aging process of cells. Elevated ethylene levels can weaken cell wall cementation, which, combined with reduced transpiration and increased root pressure, increases internal sap pressure, branch moisture, and limb weight, leading to branch failure. Old wounds and rot hidden inside the branch (perhaps as a result of improper pruning) sometimes contribute to falling branches, but this does not explain most cases of falling branches in the summer. Pruning that promotes uneven growth at the end of a branch can put enormous stress on the branch due to the extra weight of the new growth.

While there is no guaranteed way to prevent falling branches in the summer, there are a few things you can do to reduce this danger for oaks and other commonly affected tree species such as eucalyptus, elm, and ash. On mature trees, shorten and lighten long horizontal branches and open the tree by thinning to healthy side branches to reduce the weight of the branch. Examine the tree for externally visible defects and cut off damaged or diseased undersized branches with rot or cavities. While watering is necessary to keep most ornamental trees healthy in our hot summer climate, keep in mind that summer moisture can encourage the growth of oak root fungus and other oak pathogens that can kill oaks when the summer watered area is within 10 feet of the trunk. Finally, do not park cars or place playgrounds, benches, or picnic tables under old, vulnerable trees. Falling branches cannot harm people (or property) unless they are under the tree.

The UC Butte County Master Gardeners are part of the UC Cooperative Expansion System, serving our community in a variety of ways, including 4-H, farm advisors, and nutrition and physical activity programs. To learn more about Butte County UCCE Master Gardeners and get help with gardening in our area, visit https://ucanr.edu/sites/bcmg/. If you have questions or concerns about gardening, call the hotline at 538-7201 or email mgbutte@ucanr.edu.

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