Climate change is impacting California tree crop farms

Rising temperatures appear to be reducing the number of hours tree crops in the San Joaquin Valley are subjected to chill during the winter, a critical factor in producing a profitable yield, reported Ezra David Romero on Valley Public Radio, KVPR-FM.

Pistachios, for example, require temperatures between 32 and 45 degrees for about 700 hours each winter, but for the past four years have had less than 500 chill hours.

UC Davis researcher Hyunok Lee recently published a study about climate change impacts on agriculture in UC Agriculture and Natural Resources' peer-reviewed journal California Agriculture. The study found that winter temperatures are increasing more than any other time of year. Her modeling looks at the year 2050 in Yolo County.

“Our agriculture will continue,” Lee said. “But if you look at . . . like 20 years or 30 years. The pattern may change a little bit, crops may move a little bit north.”

Insufficient chill hours can delay the opening of leaf and flower buds in crops such as walnuts, which may result in a smaller yield. (Photo: Will Suckow))

Romero spoke to UC Cooperative Extension farm advisor Craig Kallsen, who holds the UC Cooperative Extension Presidential Chair for Tree Nut Genetics. Kallsen is conducting trials aimed at finding pistachio varieties with novel nut, tree growth and yield characteristics, and varieties that produce a high yield even under low-chill conditions.

"We're trying to use the other species of pistachios actually to see if we can come up with something that has a low chill requirement. It's pretty hypothetical at this stage,” Kallsen told Romero. “We made quite a few crosses this spring and we actually hope to put a trial in a low chill area.”

David Doll, UC Cooperative Extension farm advisor in Merced County, is studying other tactics to improve winter chill, such as using overhead sprinklers to cool the trees and painting them white with liquid clay to reflect sunlight.

"So this is something that could impact a lot of farmers over the next 10, 20, 30 to 40 years,” Doll said. “And in fact it's already impacting farmers on random given years across the state."

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