Sudden limb drop

Colusa trees experience early bout of summer branch-shedding phenomenon 

Colusa residents may want to think twice before seeking respite from the sweltering summer heat under the canopy of one of the city’s many large shade trees.

Since the start of the year, 18 limbs have fallen around the City of Colusa, Streets and Parks Superintendent John Balderrama said on Monday. Ten of those limbs fell in the month of May.

The culprit? For many of the branches that fell last month, the cause was sudden limb drop, City Manager Jesse Cain said. Also known as “summer limb drop,” “high temperature limb drop” or “sudden branch drop,” sudden limb drop is a somewhat mysterious and potentially dangerous phenomenon that affects many different species of trees. Often without warning, a large and seemingly sound and healthy limb suddenly comes crashing down to the ground. It most often affects big, mature trees with large, long limbs, and occurs in the afternoons or early evenings on hot, calm days. Typically, the limbs susceptible to sudden limb drop are long, heavy “laterals” – branches that run sideways rather than up and down – that haven’t been properly maintained, said Joseph Graham, a certified arborist with Richard’s Tree Service, a Yuba City-based company that does a lot of work in Colusa County.

“We see a lot of reports of trees dropping limbs in the summer time. It’s real common,” Graham said.

Why Summer Limb Drop Happens

While the exact science behind the phenomenon is unknown, there are a number of theories as to why sudden limb drop occurs.

One of those theories is that the limbs are weighed down by excess water and ultimately break under the stress.

During the course of the day, trees pull up moisture from the surrounding soil through their roots and disperse the water out to their branches, where they slowly release the moisture in the form of water vapor through small pores on the undersides of leaves. The process, known as transpiration, serves two primary purposes: delivering nutrients to the leaves for photosynthesis and cooling the tree down.

In terms of the latter, transpiration is effectively the process of trees sweating. On very hot days, trees can draw up a huge amount of water from its roots and trunk into its limbs, intended to cool the tree, as the moisture is released through the leaves. Limb drop occurs when trees draw up large amounts of water into their limbs but are unable to release enough of the moisture through the transpiration process. The weight of the water bearing down on the limb in the heat of the day causes it to suddenly break off. A lot of times, water can be seen shooting out from the break immediately after it happens, Graham said.

Sometimes, there is some sort of damage to the limb that results in a break. Frequently, however, the limb appears to be completely healthy, making predicting sudden limb drop difficult.

“That’s just the basics: just can’t wick off enough of moisture to reduce the weight,” Graham said. “You don’t necessarily have to have any decay… (but) “If there’s any weak attachment on a crotch, or any decay in the limb, that causes them to usually fail a lot quicker.”

Transpiration rates are affected by atmospheric factors including temperature, relative humidity, wind and air movement, and soil-moisture availability. Sudden limb drop typically occurs during calm, hot summer days, Graham said. As temperatures rise, so do transpiration rates; however, transpiration rates decrease as the relative humidity increases, and also fall as wind and air movement decreases.

“When you have a breeze, the moisture will be pulled from the leaves and evaporate. But usually, on a nice calm day, that’s not going to happen as much,” he said.

The arborist noted that after years of drought, this year’s heavy rainfall would likely result in more incidences of sudden limb drop in Colusa and throughout the valley.

“We’ve definitely got more chance of it happening this year. The trees are kind of stressed out from all of the drought for so many years, so they’re pulling a lot of water up right now. All of a sudden, you get a whole lot more water intake, and you’ve got a good potential for failure now,” Graham said.

Sudden Limb Drop gets early start in Colusa

City Manager Jesse Cain said that after a limb falls, the city investigates the tree to determine the cause, and what action they should take (if any).

“If we come across anything that looks like it is diseased, we cut them down… Once we know it’s damaged and has the potential of doing it again, we remove it,” Cain said.

While Colusa has yet to experience prolonged bouts of triple-digit temperatures, Cain said that almost all of the fallen limbs in Colusa last month were caused by summer limb drop, and the trees they fell from were otherwise healthy.

“Most of it is due to the water, too much moisture being sucked up into the limbs,”

Cain said. “The hotter it is, the worse it becomes.”

While Balderrama said that summer limb drop occurs fairly regularly in Colusa during the warm months, he noted the number of cases in May was atypical.

“It was because of all the water. It gets into the 90s and the trees, especially the elms, start sucking up the water really bad,” he said. “It’ll probably get a little worse – hopefully not – but it likely will as we get into July, August and September. It happens every year.”

Maintain trees, use caution

According to Graham, oaks, elms and some walnut trees are the species most commonly affected by sudden limb drop in Colusa.

Graham said that properly trimming and maintaining trees is the best course of action to take to prevent potential problems with falling limbs, and that people should keep an eye on trees with lateral limbs and heavy foliage.

“A lot of the large elm trees are the ones you have to be concerned with,. A lot of them have been trimmed, but still, I try to stay out from under them directly. You can enjoy the shade off to the side,” Graham said.

He added that it would be wise to avoid setting up shop under the canopy of larger trees, particularly at those times when the shade seems most appealing: during a calm, hot summer afternoon.

“I generally look up and position myself so that I’m not directly under the tree. I teach my children to do that too. Make sure that you’re not directly under any limbs,” Graham said. “Don’t park your car directly underneath them, or keep your insurance paid up. When it falls on your car, the insurance companies generally say it was ‘an act of god,’ so generally it’s not covered by your homeowners, so it falls on your auto insurance.”

Brian Pearson
Brian Pearson is the Managing Editor & Reporter for the Williams Pioneer Review. Brian joined the Williams Pioneer Review in June 2016 and is committed to bringing hyperlocal news to its readers. A few of his projects include reporting on local government and the newly feature sports page. To contact Brian about this article, or for future articles, please email him at