California’s walnut production keeps growing and expanding north of the Central Valley, with crops reaching 400,000 acres in 2022, and total production volumes of 752,000 tons. 

This year’s record rainfall in the state has been very positive for walnut trees, leading to historical projections for the crop which is expected to reach 790,000 tons. 

“Starting in January, rain hit California pretty much constantly, reaching upwards of 200% from normal precipitation in some places,” Tapan Pathak, a specialist in climate adaptation in agriculture from UC Merced shared during a presentation at the 2023 Exponut which took place last week in Santiago, Chile. 

Pathak explains that precipitation helped to build the salt profile in trees and that the cold winter, without many heat events, created an “ideal situation” for walnuts to grow. 

The El Nino phenomenon, which California is going through right now, is unpredictable, there are very wet years and others that are not, therefore Pathak insists that Californians will have to wait and see how El Nino affects the state. 

Changes in California's climate 

Most of California's agriculture production is in the Central Valley, and it supplies about 25% of the nation's fruits, nuts, and other food products.

“Minimum temperatures have increased more than maximum temperatures, which is causing a lot of issues to agriculture in California, consistent throughout different regions of the state and with different commodities,” says Pathak. 

High minimum temperatures cause snow packs in the California mountains to melt earlier, and water reserves are losing a lot of their volume. According to Pathak, by 2070, the state will see a 50% loss of water reserves, and by the end of the century, close to 65% or more. 

“California weather has been so inconsistent, that it's really hard to predict whether it's going to be wetter or dryer in the future. We can be confident, however, of the fact that we will see more and more extremes from both sides (hot and cold),” indicates Pathak. 

“Californians have to be prepared for extreme drought as well as flooding in the future,” he adds. 

Pathek told attendees that extreme heat episodes will not only be more prevalent, but that they will start much earlier in the season, and will last longer. 

“Plants are going to be stressed for a longer period of time. The agricultural workforce will also have to experience these extreme conditions, which is also a big issue,” he said. 

Climate change impact on walnuts

As weather conditions become more uncertain, different crops, including walnuts, are projecting production declines due to a lack of water. 

“For walnuts, we project about a 15% decline, which does not mean that they will be out of business, but the industry will have to start looking at alternative varieties for this region,” says Pathak. 

Chill hours are a critical component for walnuts because without enough “chill” trees can not break their dormancy in an accurate time frame, and it's also essential for bud break and flowering. 

“In the 1950’s there were 70 to 80 chill hours per year, that has come down to about 40 to 50 hours only, so it is a critical issue for walnut crops, which is why we have seen a migration of crops towards the north of the state,” says Pathak. “If there are low-chill varieties available in the market, it's good to consider those options.”

Pathak announced that his team at UC Merced is working on a forecast so farmers can know, even before the season starts, whether or not they will have enough chill hours in the season, to allow better management.  

Pest related impacts

Nuts are being “significantly impacted by the navel orangeworm pest in California. 

“What we estimated is that by 2050, almost the entirety of the walnut crop in the Central Valley will struggle with this pest. In some seasons already, orangeworms have affected 30% of the total harvest in California. Pest pressure, due to the increased population of the worm, is affecting a variety of crops, creating visible issues on the fruit,” says Pathak. 

To deal with this pest, Pateck suggests that farmers should implement integrated pest management practices and be proactive because pests also react to warmer weather. 

“Pests are increasing their life cycle much more rapidly and breaking their dormancy earlier, damaging crops for a longer period of time,” he says. 

Pathak suggests preventive measures such as sanitizing orchards and harvesting crops as soon as they are ready to harvest, rather than waiting and risking possible pest damage. 

How heat affects walnuts

Heat is critically affecting walnuts, and it matters at what stage of the growing cycle the heat events occur. 

“If it is early in the season, the size of the fruit is affected, but if it is later in the season, it damages the nut itself,” notes Pathak. 

One good thing about rising temperatures has been the reduction of frost risk, allowing farmers to grow their crops in places that were once risky.  

Resources for farmers

A recent assessment showed that farmers are very concerned about weather conditions in the state, and many fear for their livelihood as they have invested all their lives in their crops. Considering alternative crops or reducing the size of their crop led to hard decisions.

“Water comes as a top priority for California farmers, and they have been very impacted in the last ten years. Technical service providers are working on sharing information with farmers that is easy to digest and use for adaptation. They need more extension in providing this information to them in various forms: workshops or other sources, but they need to know this information,” says Pathak. 

Climate-smart agriculture is a “more holistic approach” to address sustainability, increase production, adapt to climate, and mitigate greenhouse gas emissions. So Pathak assures that any practices that California growers are implementing should have a trade-off with at least one of those three pillars.

Government initiatives and incentives reward farmers who implement climate-smart agriculture practices through subsidies, which is something to keep an eye out for.