Severe water stress can hurt almond yields for several years

Issue Date: October 15, 2008
Kathy Coatney

Allan Fulton, UC irrigation and water resources farm advisor for Tehama, Glenn, Colusa and Shasta counties shows the weather station at his almond research site where he collects data for his deficit irrigation project. Fulton is doing research to see if withholding water during hull split (top photo) has a negative impact on yields.

When it comes to causing almond trees to have water stress, there's a fine line between moderate stress and severe stress, but the difference in adverse impact between the two can be felt for several years.

Ken Shackel, professor of plant sciences at University of California, Davis, has been conducting almond irrigation research for years. One area of focus was to see if water could be withheld from trees during hull split to improve hull split while not having a negative impact on yield.

"Basically what we're doing is forcing the trees to use the stored rain," Shackel said.

The research was conducted in an almond orchard where the trees are in very shallow, very gravelly unforgiving soil. Because of the type of soil, the trees can go into stress quickly and the effects of the stress can be very severe, Shackel said. The key, Shackel said, is to adjust the irrigation so that the trees are under moderate stress, but not severe stress. "Then you keep from having any really negative effects."

"What we were doing is restricting the water stress just to the period of hull split. But of course, you also have to cut back the water for harvest. And since different varieties harvest at different times, there's always going to be water stress associated with that period of time," Shackel said.

There haven't been any negative effects seen, the yields have been going up during all five years of the study, and the water savings was about five to 10 percent, Shackel said.

"That's all very good news. I don't know whether this is going to make a big splash or not, but we're doing this on a very, very difficult soil to manage water," Shackel said. "I think if you can do it on that kind of soil, you can do it pretty much anywhere."

Shackel's recommendation is to put the trees under moderate stress during hull split (late June/July), then following this period go back to normal levels of irrigation.

Allan Fulton, UC Cooperative Extension irrigation and water resources farm advisor for Tehama, Glenn, Colusa and Shasta counties, has been conducting a deficit irrigation project on almonds.

"We had a comparison between orchards that we thought were irrigated without much deficit and with a deficit," Fulton said.

Fulton used a method called the midday stem water potential to measure the water levels in the orchard to determine when the trees needed water.

"Our intention was to stress from the beginning of hull split, which in these varieties, in these orchards, on these soils, occurs somewhere around the first week of July," Fulton said, adding this was done up to harvest. "There's about a four to six week window where we purposely stressed. The net effect was that over that four-year period, we didn't see any negative impact of deficit irrigating in that orchard.

"The main thing we worked on was validating the concept that we could purposely impose stress, but from the onset of hull split up to harvest. That has been the main working recommendation that the universities have, and we were continuing to work to validate that and make sure we weren't going down the wrong path," Fulton said.

Bruce Lampinen, UC Davis integrated orchard management/walnut and almond specialist, said, "We have a trial looking at the interactions of fumigation with irrigation in almonds with Greg Brown and Brent Holtz in Madera County. What we found is when you don't fumigate, then the irrigation management is really, really critical. If you put on either too much water or too little, the trees just stop growing."

The trees in the first trial got behind on water, and they were moderately to severely stressed, Lampinen said.

"We were measuring water potential, so we know that they were stressed. And after they recovered a week or so later, the fumigated trees took off and kept growing perfectly fine, but the non-fumigated ones just never grew again for the rest of the year, no matter what we did. So basically that one stress event just shut them down.

In another trial, Lampinen put in different irrigation levels in fumigated versus non-fumigated orchards. With the fumigated trees, the irrigation levels had very little effect, but there was a dramatic effect on the non-fumigated trees whether they were over or under irrigated, Lampinen said.

At this point, it is not known why fumigation is playing such an important role, Lampinen said, but any time a grower doesn't fumigate, it is really important to pay more attention to irrigation management. Fumigation, Lampinen said, is an insurance policy.

"If you don't fumigate, you really have to probably be measuring soil moisture and plant water potential in order to manage the water," he said. "Different soils are also more or less efficient in terms of the fumigation, depending on how wet they are when you fumigate."

Lampinen has also been involved in a large irrigation and nitrogen study in almond trees that is just concluding in Kern County. The purpose of the project was to look at how long spurs live under different conditions of nitrogen and water management.

The almond orchard where the project was conducted was still filling in at the time that they started the treatments, Lampinen said. What he found was that if a grower is going to be short on water, and he has to choose between reduced water on a mature or a growing orchard, he should short the mature orchard.

"If you knock your production back or canopy development back (on a growing orchard), that's going to effect the next five years of production," Lampinen said. "Cutting back on water in a mature orchard will have an effect this year and maybe next year, but after that you'll probably be back to normal."

(Kathy Coatney is a reporter in Corning. She may be contacted at

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