It’s late afternoon on a hot summer day with the temperature hovering above 95 degrees and you’re outside on the porch enjoying a cold drink. Suddenly you hear a popping sound like firecrackers in the distance, then a loud crack and a branch from a tree in your yard comes crashing down.
What’s so unnerving about this branch failure is there was no wind, it was calm day, and the limb that just fell is green and full of foliage.
If you have lived in Sacramento for any period of time, you are familiar with this phenomenon arborists describe as summer branch drop. This is not just a local problem but occurs throughout California and has been reported from New York to Texas, in addition to Australia, England, and South Africa.
Why does it occur and what are the causes?
Temperatures above 95 degrees are a key factor. Branches that drop are usually more horizontal than vertical and extend to the edge of the tree crown. The break occurs most often out on the limb some distance from its attachment. They can be as small as five inches in diameter and 15 feet long or as large as 30 inches in diameter and 45 feet long.
Branch failure patterns are species specific. For example, summer branch drop for American liquidambars is linked with how many of the spiky seed balls the tree produces. When there are no defects in the wood caused by decay, weak branch attachment or cracks, the main cause for failure is excessive weight due to the spiky balls. This excessive amount of fruit is directly related to the weather.
During spring, liquidambars bud out and produce a small inconspicuous flower. If there is no wind or rain to knock off the flowers, then every flower develops into a fruit.
Dead or decayed limbs may also fail during the summer due to the high temperatures drying out the wood fibers. Dry wood has less bending capacity and decay fungi reduce wood strength.
Some of these large majestic oaks are removed because a large summer limb failure has compromised the tree’s structure.
Healthy branches that fail usually have internal cracks that may not be visible where the branch broke off. But in dissecting the branch, the crack will be revealed further down the branch.
Cracks near the outside of the branch develop a rib that is a longitudinal bulge of response wood growth. These cracks usually develop during winter storms. Then the summer heat dries out the wood along the crack and the limb fails.
Many species have such crack defects, but the main concern is with mature native oaks. The valley oak is the most prevalent native oak in the Curtis Park neighborhood. Some of these large majestic oaks are removed because a large summer limb failure has compromised the tree’s structure.
Internal cracks and other hidden defects limit an arborist’s ability to predict summer branch drop. Support systems, either cables or Cobra® on mature valley oaks, are recommended preventative measures. These systems modify the wind load on the limbs, which prevents internal cracks.
However, branches could still fail in extreme weather conditions, but the risk is greatly reduced having a professionally installed support system.
Homeowners should periodically look at the crown for any dead, broken or split limbs that may pose a problem. If you have any concerns, call an ISA-certified arborist.