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Canopy leaf-out in walnut orchards investigatedCanopy leaf-out in walnut orchards investigated

Struggling upper canopies have researchers seeking answers.

Logan Hawkes 1

May 15, 2019

5 Min Read
Walnuts grow in an orchard near Red Bluff, Calif. Some growers in Northern and Central California report that their upper canopies are struggling to leaf out.Tim Hearden

Sporadic reports from walnut farmers in the northern San Joaquin Valley and across parts of the Sacramento Valley recently indicate walnut trees in some orchards are struggling to leaf-out in their upper canopies, leaving researchers to struggle for answers to the problem.

“At this stage I would call this development a bit of a mystery,” reports University of California Cooperative Extension (UCCE) Advisor Luke Milliron for Butte, Tehama, and Glen Counties. “UC farm advisors, specialists and professors are currently working with farmers and crop consultants to determine the cause and extent of the problem.”

Milliron said so far researchers are leaning toward blaming the development on frost damage. Some orchards experienced freezing temperatures in November when nighttime temperatures dropped to or below 28 degrees (F) in the eastern corridors of the valleys where most of the damage has been reported. He noted that on the western slopes of both valleys, temperatures were generally a little higher.

“Overall this damage we are seeing is something rare, at least as far as recent memory is concerned. It is unusual to see such severe damages in mature orchards from fall freeze events. We are familiar with early winter or late autumn freezes that cause damage in young trees, but widespread damage on mature blocks is unusual,” he told Western Farm Press.

Researchers say freeze damage in tree nut orchards is “incredibly variable and sporadic,” and it is concerning to see this type of damage in some blocks that are surrounded by other blocks that show little or no signs of freeze damage.

Milliron says while it is easy to look at freeze events as the possible cause for the damages researchers are seeing in some orchards, there remains inconsistencies from orchard to orchard and block to block that leave unanswered questions that “are surprising.”

“We are also wondering what orchard practices may be at play. There is still much we do not understand about this incident, other factors that need to be addressed, like carbohydrate levels for example, as well as the stage of dormancy trees may had reached before those autumn freeze events, and even other contributing factors.”

Areas where temperatures were recorded at or below 28 degrees (F) and where reports of damages in walnut orchards have been reported include around Durham and Gridley in Butte County, around Verona and Yuba City, and in the Farmington area in the San Joaquin Valley just to name a few.


Damages generally indicated leaf out in the lower branches of affected trees and in some cases demonstrated early bud break, while the upper canopies have failed to produce few if any leaf activity.

Milliron said upon close inspection, twigs on the symptomatic wood on affected trees has often had a shriveled and desiccated appearance and still had green/living tissue when cut.

Dr. Maciej Zwieniecki, a plant physiologist in the Department of Plant Sciences at the College of Agricultural and Environmental Sciences at the University of California at Davis, who provided an in-depth written response to our inquiry about possible causes for the damages in walnut orchards, offered multiple possible explanations.

“The pattern of damage (often death of the upper part of horizontal thicker branches while lower parts survived) suggests frost damage related to long wave radiation cooling,” he noted in his email.

Zwieniecki also suggested the day time/night time extremes in temperature changes may have contributed to damages in some trees.

“The days preceding the cold nights were very hot – above 70-degrees F. (At that temperature most of sugars (in the branches) are translated to starch (energy storage – providing zero protection from frost capacity). Slow cooling will promote starch degradation and accumulation of sugars and protection from ice formation in cells.  Fast cooling, (as was the case in the last fall i.e. more than 5-degrees per hour) might interfere with starch degradation and effectively reduce the natural antifreeze formation.”

He also noted that survival of any species in any climatic region does not depend on long time averages but on extreme events. Zwieniecki notes that extreme events shape any species’ range and natural selection.

“It is in fact very difficult to manage orchards for extreme events or conduct studies, as extreme events by nature are not frequent. Preparing orchards to dormancy is thus a difficult task and may not always be a success when confronted with unusual weather pattern(s).”


Researchers say they are also working with farmers to determine if post-harvest irrigation was administered in the orchards where damages are being reported, which could also be another factor involved in the mystery.

Milliron said Yuba and Sutter County Farm Advisor Janine Hasey first noticed there may be a possible pattern in damaged blocks that had not been irrigated for a longer period of time before the early November freezes occurred, and that may be one clue to the current mystery. But he warned that is only one factor researchers are looking at as they continue to sort through the mystery.

“We know that moisture levels in soil can help radiate heat during freeze events, so if an orchard were irrigated shortly before those November freezes it should have offered some protection,” he noted.

Milliron said other possible factors could also include the timing of the early freezes that coincided with Butte County’s Camp Fire, the worst in recorded state history. But he said generally it is believed that smoke from wildfires is generally a benefit to orchards rather than a risk.

Researchers say so far, as of this writing, their research has focused on walnut orchards that seem to be heavily impacted in some areas. But Milliron said he has heard rumors that pistachio trees in some orchards may be showing similar signs of damage. He said he has not yet heard of any similar damages in almond orchards so far.

In terms of mitigation efforts, researchers say it’s too early and not enough information is known to recommend action. In extreme frost-damaged trees, pruning out dead wood may be a possible option to save trees if conducted in mid-summer months, but the circumstances and the lack of a definitive cause of this most recent development has left researchers looking for best answers and advice for growers.

Study of the problem is expected to gain momentum as more researchers and industry officials get involved in investigation efforts.

For more news on tree nuts as reported by growers and farm advisors, subscribe to the Tree Nut Farm Press e-newsletter.

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