Less winter fog challenges California's tree fruit, nut growers

05/20/2014 06:55:00 PM
Vicky Boyd

click image to zoompeach bloomsVicky BoydWithout a long period of cold-induced dormancy, most peach varieties will not produce a strong bloom or have a good fruit set.During the past three decades, the number of foggy winter days has declined in California's Central Valley. Although that may be good news for drivers, it's bad news for fruit and nut trees that break winter dormancy without the shroud of fog.

A study by University of California, Berkeley, researchers published this week in the journal "Geophysical Research Letters," examined the relationship between reduced fog and chill hours.

Depending on the species, fruit and nut trees require several hundred hours of chilling to maintain winter dormancy. Without adequate chill, they have erratic blooms, produce less fruit and the fruit can be of lesser quality.

Chilling is the accumulation of temperatures between roughly 35-50 degrees Fahrenheit. Temperatures less than or greater than those levels don't count toward total chilling hours.

For almonds, it is typically about 600 hours, according to University of California data.

Cherries, on the other hand, require 700-800 hours. How and when those hours occur also will affect bloom.

“The trees need this dormant time to rest so that they can later develop buds, flowers and fruit during the growing season,” biometeorologist and study lead author Dennis Baldocchi said in a news release. “An insufficient rest period impairs the ability of farmers to achieve high quality fruit yields.”

Eric Waller, a UC Berkeley doctoral student in the Department of Environmental Sciences, Policy and Management, was co-author.

The researchers examined NASA and National Oceanic and Atmospheric Administrative data from 32 consecutive winters.

When they factored in year-to-year variability, they found a 46 percent drop in the number of foggy days from Nov. 1 through Feb. 28.

Not only does the fog help keep ambient temperatures cooler, but it also acts as a blanket to protect darker-colored tree bark from the warming effects of the sun.

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