At the height of summer, tree nut growers typically aren’t thinking about winter frost events, unless they’re dealing with the fallout from damage in their own orchards. But it’s never too soon to start thinking about the next winter, researchers say.

University of California Cooperative Extension researcher Allen Vizcarra urges growers to “keep in mind the lessons that we learned” after a mid-February freeze that came at the heart of almond bloom and caused varied damage to tree nut crops. In preparation for bloom 2019, growers should make sure pump and irrigation systems are ready to go, he advises in a post on the UC “The Almond Doctor” blog.

When designing a system in frost-prone areas, make sure there are high enough flows to provide warming. And always pay attention to the weather. “Frost events can be complex and difficult to predict,” Vizcarra says, “as they are heavily influenced by humidity and pressure at specific temperatures. Weather services can help by giving you a short notice advisory for oncoming events.”

From Arvin to Red Bluff, almonds were exposed in February to freezing temperatures that, in some areas, were reported in the upper teens. Two reports of temperatures as low as 19 degrees — one in eastern Stanislaus County, the other east of Madera — suggested the possibility of complete damage in some orchards.

EARLIER VARIETIES HURT

“There’s really a lot of damage,” UCCE Orchard Systems Advisor Katherine Jarvis-Shean told Western Farm Press in April. “The earlier varieties really took a hit. Some trees even dropped their nuts due to frost damage. It’s pretty bad in some orchards.”

At full bloom, temperatures below 28 degrees F. can cause almond crop loss. Freezing temperatures occurred throughout the Sacramento and San Joaquin valleys, but damage was not as severe in some areas. Nonpareils, the most valuable variety, seemed to escape most of the damage.

Common practices that growers use to mitigate frost damage include running irrigation water, mowing vegetation, and increasing air flow to the orchard by using fans to mix cold and warmer air, Vizcarra says. Doing any or all of these could raise the temperature by one or two degrees, which can be enough to avert damage.

Almond sensitivity to cold temperatures increases progressively from buds to small nuts, emphasizing the concern over cold temperatures during and after bloom, he says. This year, researchers noted the survivability of almonds in areas that were colder than what is typically considered critical temperature, Vizcarra says — probably because the varieties planted may be more tolerant of cold than researchers thought.

DAMAGE FROM ICE

One thing researchers noticed in an orchard near Chowchilla, Calif., using micro-sprinklers, was the formation of icicles hanging from the lower canopy, which formed when irrigation water hit parts of the canopy by either unleveled sprinklers or from a high-angled spray pattern.

In general, flowers in the upper canopy are better off than the ones that are covered in ice, Vizcarra says. For one thing, damage may occur to the iced-over flowers when the ice thaws and causes an endothermic reaction, in which heat is being conducted out of the frozen buds, flowers, and branches.

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