Walnut Trees Can Handle Water ‘Stress’ And Still Thrive, Study Finds

Sometimes a sustainable farming practice is as simple as not doing something.

That’s the takeaway from a recent project conducted by UC Davis researchers, who found that delaying irrigation of walnuts led to similar crop yields and healthier trees in comparison to those that received water sooner.

UC Davis plant scientist Ken Shackel says the research could lead the way to more efficient water use in the tree nut industry.

For at least 50 years, it was standard practice for California growers to start irrigating walnut trees in mid-May. The approach was based on the notion that one needed to replenish a tree as soon as it used up the water in the soil, says Shackel. So, the protocol was like a water investment — a way for trees to store enough water in their deep roots to survive the shock of fall, when irrigation gets turned off so walnuts can be harvested.

Shackel says the “watchword” for walnut growers was: “Don’t let the bottom dry out.”

He and his colleagues — Bruce Lampinen, UC Davis Extension orchard management specialist and Allan Fulton, UC Extension irrigation adviser — decided to test that practice by delaying irrigation until trees showed a little bit of stress. “The thinking was, ‘Look, let’s wait till the tree says it’s at least a little bit thirsty, and then start irrigating then.’”

The researchers conducted the study over a five-year period during which they applied different water treatments to five blocks of walnut trees in a Tehama County orchard.

Turns out, the walnut trees handled the stress just fine. Those trees did not get stressed later on at harvest, says Shackel, and were actually healthier at that point than other trees. And the researchers found trees that were part of this experiment produced crop yields comparable to trees that received early irrigation.

“‘Ag’ uses a lot of water, and it has to use a lot of water to be productive,” argues Shackel. “But what we’re doing is helping growers use their water more effectively to get the tree to do what we want it to to do. So, ultimately, it [will] lead to a higher efficiency of water use.”

Funding for the research came from the California Walnut Board. Walnuts are one of California's top 10 agricultural products. The value of the 2017 crop was $1.5 billion according to the California Department of Food and Agriculture.

Now, the project moves on to an orchard in the Modesto area, to see if the results hold for walnut trees planted in areas where the soil contains more clay.