Climate change and winter chill hours

Dormant peach orchard

A recent study, Climate Change Affects Winter Chill for Temperate Fruit and Nut Trees, describes the impacts that climate change will have on the temperate fruit and nut industry. This report is of particular importance to the long-term viability of California agriculture which produces almost half the country’s fruits and nuts, valued at $11.8 billion in 2009.

Outside of farming circles, the role of “winter chill hours” in fruit and nut production is little known. Chill hours are defined as the amount of time the temperature is below 45 degrees F. In order to produce, many species of fruit and nut trees require a certain number of winter chill hours of dormancy in order to achieve high yields and quality in the spring; the required number varies by species and cultivar.

In many regions including California, Chile, and Australia, climate change is already causing a reduction in the number of chill hours, threatening reduced yields or complete crop failures of these industries. In an interview one of the authors remarked “in California… I’d be worried about pistachios, walnuts, plums and peaches.”

With these four crops being a substantial part of California’s agricultural economy, a decrease in production of these crops would have an large impact. Approximate production values in 2009 were $583 million for pistachios, $740 million for walnuts, $257 million for dried and fresh plums , and $326 million for peaches. California accounted for nearly all of the national production of Clingstone peaches, pistachios, dried plums, and walnuts. These production values not only reflect the demand of Americans, but international demand as well — the number one export of California  was tree nuts, followed closely by fruit.

What can be done to ensure that these products and industries will continue to be available? The authors wrote of the possibility of moving the industries northward, towards the Pacific Northwest. From the perspective of farmers who have homes, community ties, capital investments, and in many cases long family histories where they now farm, the prospect of “moving north” is probably a non-starter. Remember also that these are trees that take years to mature and require tremendous investments in infrastructure and plant material.

There are certain agricultural management practices that can help with the adaptation to changes in winter chill hours. Although irrigation and shading can reduce the temperature in an orchard, these solutions are resource and energy intensive and therefore not very sustainable. Newly developed cultivars can lower chill hour requirements, but require costly and long-term research endeavors. It is of extreme importance to increase the efforts to develop adaptation tactics with a focus on breeding new cultivars that can better manage with climate change.

This study provides yet another compelling set of data to add to our understanding of the vulnerability of California agriculture to climate change. In order to address these issues, policies must be put in place to support research, technical assistance and direct incentives for growers to adapt to climate change and contribute to mitigating it. CalCAN’s report entitled “Ready…or Not? An Assessment of California Agriculture’s Readiness for Climate Change” provides more background on the expected impacts of climate change and assesses the readiness of the state’s agriculture system to address them. The indications are clear about the high stakes, but the policy tools continue to lag.

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