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A Nutty Idea: A Little Stress Could Be Good for Walnuts

Updated: Feb 7, 2019

By Diane Nelson on December 4, 2018 in Food & Agriculture

California produces 99 percent of the walnuts grown in the United States, which is the second-largest walnut producing country in the world behind China. In 2018, California farmers harvested about 690,000 tons of walnuts, up 10 percent from 2017.

Hal Crain is a second-generation farmer whose family has been growing the nut in California’s northern Sacramento Valley for 55 years. Like most walnut growers, he starts irrigating his orchards in early-to-mid May, when the days grow warmer and the trees sprout leaves.

“That’s standard practice for probably 90 percent of California’s walnut growers,” said Crain, standing beneath a canopy of lush, green trees. Crain wears a flannel shirt and an easy smile as he walks through his orchard on a recent sunny afternoon.

Most growers believe you need to water early to keep trees healthy and productive through the long, hot summer. But a long-term experiment with researchers at the University of California, Davis, shows growers can improve crop production if they hold off irrigation until later in the season and directly measure their trees’ water needs.

The findings may help farmers optimize water use.

“It’s a game changer,” said Crain, who welcomed researchers on to his orchard to test irrigation optimization. “It’s clear to me you can improve nut quality and yield by applying water based on what the tree wants and needs, rather than just watering when it’s hot outside and the soil is dry. That’s a big deal for walnut growers and for the entire agricultural industry.”

Recently harvested walnuts at the Crain Ranch in Los Molinos, California, where Cooperative Extension and UC Davis professors are conducting a long-term experiment to study irrigation optimization. (Karin Higgins/UC Davis)

Changing the paradigm

Walnut growers tend to start irrigating earlier than farmers who grow other nut crops like almonds and pistachios. “The theory is that when you irrigate early, you preserve the deep moisture in the soil that trees need to survive the heat of summer,” said Crain.

But that’s not how it works, the research shows. Instead, trees that grow in saturated soil early in the season don’t develop the deep roots they need to thrive.

“With all the water right there at the surface, the lower roots suffer,” explained Bruce Lampinen, Cooperative Extension orchard management specialist with the UC Davis Department of Plant Sciences. “Trees end up with a very shallow root system, which doesn’t serve them well as they try to extract moisture from the soil later on.”

Lampinen has long suspected that walnuts were getting too much water in the spring.

“A lot of the symptoms we see like yellowing leaves and various diseases can all be explained by overwatering,” Lampinen said.

So Lampinen did what scientists do: He set up an experiment. Five years ago, he joined forces with Ken Shackel, a UC Davis plant sciences professor and Allan Fulton, a UC Cooperative Extension irrigation advisor. Together, they led a team of scientists testing irrigation on Crain’s ranch.

Professors Ken Shackel (left) and Bruce Lampinen in a walnut grove at UC Davis measure plant water stress with a pressure chamber, pressure bomb. Pressure chambers let farmers get a more accurate read when their trees need water. Their work is increasing walnut quality and yield. (Karin Higgins/UC Davis)

Tough nut to crack

When is the best time to irrigate? Researchers say the trees hold the answer. Scientists use pressure chambers, which are air-pressure devices that measure a leaf or small shoot to gauge how hard the plant is working to pull moisture from the soil.

“Just because the soil looks dry doesn’t mean the plant is suffering,” said Shackel, whose research expanded the use of pressure chambers to measure plant water stress in the early 1990s. “Pressure chambers let you ask the tree how it’s feeling — sort of like taking a human’s blood pressure — which is a much more accurate way to measure whether a plant needs water.”

For the last five years, the team has been applying different water treatments to five blocks of trees. One block is getting standard, early irrigation. Crain’s orchard managers begin irrigating the other blocks when the trees reach different levels of water stress based on pressure-chamber readings.

The trees that experience moderate stress are doing the best. Their irrigation usually starts in mid-to-late June, several weeks later than when standard watering begins.

“You can tell just by looking at that block that the trees are healthier,” said Crain. “And, we’re starting to see greater yields and better nut quality.”

Recently harvested walnuts at the Crain Ranch in Los Molinos, California, where Cooperative Extension and UC Davis professors are conducting a long-term experiment to study irrigation optimization. (Karin Higgins/UC Davis)

Translating the research

Researchers will continue to test the effects of delayed irrigation on the health and productivity of fruit and nut trees. Lampinen said more research is needed to know if the practice will increase yields on a broad scale or reduce water use. But Lampinen said one thing is clear: “My biggest take-away is that knowing when to start watering is really important to the health of your trees.”

Pressure chambers — sometimes called pressure bombs — can cost more than $3,000, and high-tech versions are under development.  They can be used to measure water stress in virtually any plant. 

“I tell growers a pressure bomb would pay for itself even if you just used it once a year to determine when to start watering,” Lampinen said.

Crain is certainly convinced.

“When you irrigate based on your trees’ needs, you optimize water,” Crain said. “I’m not using less water overall, but the water I do use is producing more food. That’s good news for everyone.”

Media contacts:

Diane Nelson, UC Davis College of Agricultural and Environmental Sciences, 530-752-1969,

Ken Shackel, UC Davis Department of Plant Sciences, 530,752-0298,

Bruce Lampinen, UC Davis Department of Plant Sciences, 530-752-2588,


Diane Nelson. “A Nutty Idea: A Little Stress Could Be Good for Walnuts.” UC Davis, Accessed 2 February 2018.

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