Debbie Arrington

Watch where you park on hot days; shady spot may be dangerous

A duck forages near a fallen tree branch in Land Park in Sacramento. Trees can drop limbs unexpectedly during summer heat.
A duck forages near a fallen tree branch in Land Park in Sacramento. Trees can drop limbs unexpectedly during summer heat.

Thump! Out of nowhere on a quiet but sweltering afternoon, an oak dropped a great big branch on a sidewalk in midtown Sacramento. Fortunately, no one was under that branch when it came down. But the incident was still unnerving – and made me think twice about where I parked my car.

On windless triple-digit days last week, branches suddenly fell in several Sacramento neighborhoods. Was it too much heat? Too little water? Another aftermath of drought?

“The short answer to why it happens? Unfortunately, there isn’t one really,” said arborist Pamela Sanchez of the Sacramento Tree Foundation. “It affects both trees in watered landscapes and native trees that aren’t irrigated. Some species in our area that are susceptible are sycamores, oaks and elms.”

Eucalyptus and ash trees often drop limbs in summer, too.

“The phenomenon is called variously ‘summer branch drop,’ ‘summer limb drop’ and ‘sudden limb drop,’ ” explained Ellen Zagory, director of public horticulture at the UC Davis Arboretum. “It tends to occur when we get these really high temperatures.

“From what I can tell, no one knows exactly what causes it, but there are various theories,” Zagory said. “There are various conditions that seem to predispose trees, like being older and over-mature. Certain species are more prone (to dropping branches), and long horizontal branches are more susceptible. It happens on hot, still days. Perhaps drought stress is involved.”

Wind usually plays a factor in most broken branches, but this phenomenon is not wind related.

According to the University of California Cooperative Extension, research found that summer branch drop most likely happens in the afternoon on hot, calm days. How the branch breaks is different, too. Instead of snapping off near the trunk as it does with wind-related breaks, large diameter limbs will break three to 12 feet from the trunk.

Upright branches rarely are part of this problem, but watch out for those long branches that run parallel to the ground and stretch out past the edges of the tree’s canopy, the outer fringe of its foliage. Those are the most likely to drop.

In addition, a tree that has dropped one branch in summer is more likely to drop another during its lifetime.

Older, less vigorous trees are more likely to be affected, but otherwise, any big tree may be a candidate for limb drop. Both native and non-native trees can drop limbs. Trees that receive adequate irrigation as well as drought-stressed trees drop branches, too.

“The best explanation I know relates to the slower transpiration – how trees take up water through roots and give off water vapor through pores in leaves – due to high humidity in tree canopies on a hot day, which can result in higher pressure, and then higher moisture content inside the branch, which makes it heavier,” said Sanchez.

“Explanations are hypothetical and have not been proven,” Zagory added. “It is proposed that perhaps pressure builds up inside the stems. ... As far as I know though, it’s a mystery still.”

That inability to pinpoint a cause makes summer branch drop hard to prevent. Experts suggest the best way to mitigate any potential damage is careful preventative pruning. Shorten and lighten long horizontal branches. Trim out any sickly branches or ones that show signs of decay.

“You can’t prevent this occurrence totally, but you can have a certified arborist regularly inspect your trees to make sure their structure is strong,” Sanchez said, “and also to specifically reduce the weight of long, large, horizontal-growing branches.”

Find a certified arborist at

And remember: Don’t park under a big tree on a hot day.