Plan Ahead to Protect Walnut Trees Against Heat Stress

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Excessive heat in California’s walnut growing regions lends to sunburn damage of crops (all photos by J.R. Johnson.)

When winter and spring temperatures are at hand, it is hard to think about the inevitable heat waves of the coming summer.

But thinking and planning ahead is what walnut growers need to do if they want to protect their trees and crops from heat stress and sunburn caused from excessive temperatures and ultraviolet (UV) light, both of which can significantly reduce marketable walnuts yields.

The Heat Is On
Temperatures in California and other areas of the western U.S. have risen almost 3 degrees F since the beginning of the 20th century, according to the NOAA National Centers for Environmental Information, which reported the six warmest years on record have all occurred since 2014.

“We’re at the warm end of the range where walnuts are grown,” said Pat Brown during the UCCE Virtual Walnut Series. “We’re already losing our chilling opportunities and we know it’s going to happen further. We’ve got to face the problem head-on.”

He even suggests that it may come to the point where climate change results in the Chandler variety, the most widely grown variety in the state, not being the good choice for the San Joaquin and Sacramento valleys.

How to face that problem head-on can come in several forms, such as newly developed walnut varieties, irrigation practices, application of sunscreen, cover crops and looking at mineral levels.

Be it young or established trees, when they experience extreme heat stress, photosynthetic activity can shut down, limiting overall productivity and maximum potential.

Irrigation
“When they’re calling for a heat wave,” said DC Felciano, JJB Farms ranch manager in Tehama County, “I try to irrigate a couple days ahead of the heat wave to help relieve some the stress I know the trees will suffer. When the temperatures are 114 degrees [F], I just leave the water running in the hopes of cooling the orchards down. Even then the orchards suffer damage. They just simply shut down as a matter of survival.”

In addition, excessive levels of heat can deform nuts during critical development stages. This damage often goes unnoticed until harvest.

Cover Crop
Clayton Handy, grower and independent agronomist/PCA, believes there are ways to help curtail that damage by looking at two sources of heat that can impact trees.

“Trees deal with heat sources from two areas: direct heat from the sun and indirect heat as the soil absorbs heat from the sun,” he said.

When excessive temperatures roll in, soil temperatures also rise.

“Soil temperatures can reach over 100 degrees [F] in the top few inches when the ambient temperatures are excessive, which greatly decreases the efficiency of uptaking water and nutrients,” Handy said. “Studies have shown that when you can cover the ground with a living mulch/cover crop, it significantly lessens the soil temperature. We’ve seen as much as 20-degree [F] differences.”

Diversity is the key for cover crops, he added.

“While we don’t often hear about summer cover crops, if planted they can help moderate the extremes when it comes to soil temperatures,” Handy said. “Growers need to plant something that offers the greatest amount of cover on the orchard floor, yet not too much biomass that makes it difficult for harvest in the fall. You want diversity, grasses and broadleafs.”

Seed suppliers, such as Lockwood seed, can put together a seed mix for growers (Chiquita mix) that stays around 18 inches or less.

“Thirty pounds to the acre of a mix like this should be fine,” Handy said. “If that cost is still too much, try backing off the aesthetic farming and save some money and increase soil cover by narrowing your pre-emergent strips and minimizing mowing passes. We’ve tried mowing only two to three times per season in many orchards and seen great results: money savings, decrease in soil temperature, increase in water infiltration, increase in soil organic matter and a general increase in overall soil health.

“We have to look at trees as living organisms that exist both above and belowground. So, even though heat stress gets a lot of focus aboveground, roots and microbial activity belowground are just as important physiologically. Ideal soil temperatures will help facilitate the uptake of both nutrients and water, and that is essential during strong heat waves.”

Sunscreens
Sunscreens are another component of protecting trees during high heat.
Sunburned nuts often mold, shrivel and turn off-colors that range anywhere from a light tan to black.

As a precautionary measure, Felciano treats his trees with Surround sunscreen in preparation for the summer’s heat. “This is to help prevent sunburn, especially where trees are up against concrete and roadways,” he added.

Surround® is a white film of kaolin particles that is sprayed on the foliage and nuts as a wettable powder. When the product dries, it reflects infrared and ultraviolet radiation, yet has no negative impact on natural photosynthetic processes.

While there have been other sun-reflecting products on the market in the past, none can claim the same level of sunburn protection without interfering with photosynthesis.
For best results, Surround should be applied multiple times as a planned approach throughout the season beginning prior to a heat event.

Heat stress in walnut trees can be seen throughout the tree’s system, from leaves to roots and nuts.

Shut Down and Recovery
Research has shown that when plants hit their heat thresholds, they close their stomata to conserve water and all transpiration stops.

For every hour a plant is shut down, it can take two hours to recover. If a plant is shutdown during the day for eight hours, that’s 16 hours just for recovery, let alone resuming normal plant functions.

“The other thing to remember is potassium plays a significant role in the uptake of water because it is so heavily involved with the opening and closing of the stomata in the leaves. The ability of a plant to take water up through the roots is determined by the opening and closing of the stomata and the pressure differential between soil and air,” Handy said.

“Growers need to check potassium levels, and the best way to do that is in real time using sap analysis.”

He explained that there has to be enough potassium in the tree for the beneficial opening and closing of the stomata to efficiently utilize water and keep the tree at a healthy temperature during a heat wave.

New Varieties
“So, my take-home message is this: We can’t stick with a late-leafing, blight-avoidance scenario forever, and that includes most of the varieties we rely on now, like Chandler and Howard,” said Brown in his UCCE presentation. “You may not like to hear the message, but it’s clear we can’t rely on them as conditions change. So, can we surpass those varieties? We’re going to have to ultimately replace them because projected climate change will make them unreliable.”

The now available Wolfskill variety, bred from a cross between Chandler and Solano, has yield, quality and light color like the late-harvesting Chandler, but will mean about a 10-day- to two-week-earlier harvest for growers who can now spread out their efforts.

“That’s enough to make a discernible difference depending on how many shakes you’re doing,” Brown said. “You could be doubling your harvest window if you’re moving from Chandler to a mix with Wolfskill or the even-earlier Ivanhoe that will double your harvest window. Theoretically, you could farm twice as many acres with the same equipment.”

To diminish the doom and gloom factor, he added, “We’re planning far into the future here, which is something we need to do because walnuts take a long time. We’ve got to face these climate change-induced problems head-on and be leaders in worldwide walnut production.”

Heat and Pest Pressure
In a study published in Science of the Total Environment, researchers from three UC campuses reported that warmer growing seasons will give the navel orangeworm an extra generation to eat into growers’ profits.

The report states, “Warming temperatures may help the dreaded pest wreak even more havoc in at least two ways, including expanding their range into previously inhospitable areas and accelerating their reproductive rates, boosting their numbers.”