Six years of UC research has shown that walnut growers may be able to delay starting irrigation by up to two months without significantly affecting yield. And one year of research into delaying irrigation in almonds is showing similarly positive results.

The research could have profound effects on water usage, pumping costs and plant health, according to researchers.

The results in walnuts were surprising, UC Davis Plant Sciences Department Professor Ken Shackel said, in part because of the belief that walnut trees rely on stored soil moisture to avoid defoliation and other negative occurrences at harvest. Researchers, however, found no supporting evidence that preserving a deep soil-moisture profile with early season irrigation helped walnut trees at harvest.

“The concern is if you don’t irrigate early and keep a full profile from the get-go, you will get behind and never be able to catch up in both almonds and walnuts,” said Luke Milliron, UCCE orchard advisor for Butte, Glenn and Tehama counties. “I can see how growers would have that worry, but so far, we have not seen that bear out in our experiments.”

Over the years, growers have used an assortment of tools to determine when to start watering walnut and almond trees, Milliron said, including soil moisture probes, evapotranspiration measurements or simply waiting for that first dry spell, all inexact sciences that aren’t looking at the primary element: the tree itself. The biggest mistake growers make is starting too early, he added.

“My concern is that growers start irrigating too early and that is a risk,” Milliron said. “The early season is so important in terms of root growth, and for root growth, you need good aeration. And if you are over-watering early on, you could really be compromising root health for the whole season.

“Roots need oxygen,” Milliron added, “and if you are filling up more of those soil pores with water, you could be starving the roots.”

Milliron advocates letting the tree determine when to begin irrigating by measuring stem water potential with a pressure chamber. The pressure chamber determines the water stress experienced by trees by gauging the amount of water tension in a leaf, a more direct gauge of a tree’s needs than soil moisture or other factors.

Initial Skepticism

Researchers started looking into the effects of delaying irrigation in walnuts in 2014 using funding from the California Walnut Board.

“I don’t mind telling you, the people on the Walnut Board’s research committee were very skeptical that this would work at all, because they knew that their trees needed water and they had to give them water when they leafed out,” Shackel said.

“It was very surprising to everybody what we found,” he added. “They thought we were going to hurt those trees, and, if anything, the trees looked better in the delayed trial than they did in the control. It was very counter-intuitive for the growers to think that less is better.”

Using stem-water-potential readings, which express water tension through bars on a graph, the researchers started irrigating at five different points, with the most delayed irrigation taking place four bars below the baseline, or nearly two months after growers in the testing area started irrigating, or typically about 30 days after leaf-out.

“We expected that one or two bar triggers might cause mild water stress with minimal effect on the trees, but that the three or four bar triggers would show some detrimental effects,” Shackel wrote in a report. However, researchers found that yield was only about 10% below the control plot when delaying the start of irrigation until four bars below the baseline, or nearly two months after the control plot. And, Shackel reported, the trees that weren’t watered until the four-bar trigger had a healthier looking canopy.

“The first year’s results were kind of a jaw dropper in terms of ‘holy moly,’ we waited a month and it didn’t do anything,” he said.

Researchers also found that trees with an early start of irrigation showed more water stress at harvest than trees given a delayed start. This occurred despite the fact that the trees under a delayed start received about 10 inches less water, about 28 inches over the course of the growing season than trees under an early-start, which received about 38 inches over the course of the season.

“After three or four years, walnut growers decided it worked on really deep, well-drained soil, but now we have to try it on clay soil,” Shackel said. “So, we went down to Patterson three or four years ago, and that is pretty much showing the same thing as the first trial, but with even more clear results. If anything, it looks like you can wait a long time to start irrigating for trees in a clay soil.”

Researchers initially proposed the project to the Walnut Board because Bruce Lampinen, UC ANR integrated orchard management specialist, started thinking that maybe overwatering was causing poor root health and triggering several in-season tree health symptoms, including chlorosis and potassium deficiencies and boron excesses. Symptoms included distorted leaf margins, purple veins, yellowing leaves and leaf-tip burn.

“They were just very unusual and interesting leaf symptoms that were all over the board,” Shackel said. “I think that led Bruce to thinking, ‘maybe this isn’t a nutrient availability problem, maybe it is a root physiology problem, where the roots are having trouble taking up the nutrients that the plant needs.”

Whether the delayed watering solved those issues is hard to say, Shackel said, given that the researchers have had a hard time reproducing those symptoms in their plots. But the idea that early watering may be drowning out a tree’s deepest roots has risen to the surface with merit, he said.

“Those deep roots are very important later in the season,” he said, “and it looks like it is better to let that bottom soil dry out a little bit to keep the roots healthy, even though it involves a little bit of stress for the tree.”

Almonds Responding Similarly

Researchers launched two almond orchard trials in 2020, and thus far are encountering similarly promising results.

“The (almond) trees did what we thought they would do,” Shackel said of the first year’s results. “They get a little bit of stress before they get their first drink, and that takes a couple of weeks to wear off, just like it did in walnuts. Then, at the end of the season, there was no difference in yields (between the control plot and the plot under delayed irrigation).”

Despite the encouraging initial findings, Shackel doubts that almond growers will find the same type of water saving opportunities that are apparent in walnuts.

“My guess is that we won’t save as much water as we did with the walnuts,” Shackel said. “But I think we are going to find there are a lot more efficiencies possible to manage almonds in terms of the yields we get for the same amounts of water.”

“I think there is a lot to learn yet about what the tree is experiencing,” Shackel said. “I think we’ve just seen the tip of the iceberg. I don’t think we have a very good idea about how much stress is too much stress at any one time. We have some idea, but, really, the whole field of irrigation has never really been based on plants. It is based on ET (evapotranspiration) or soil, so I think we have to rewrite the book when it comes to irrigation when it is based on plants. And I suspect there are going to be a lot of surprises that we are going to find here in the next ten years.”

As for more immediate applications, Milliron said he knows of at least one grower who has started using the pressure chamber to determine when to start irrigating walnuts in the Butte and Glenn county area. The results? “He said the trees are looking healthier and he is saving a lot on his pumping costs,” Milliron said.