Climate Matters•February 15, 2023
Winter Chill Brings Summer Fruit
KEY CONCEPTS
Winter chill is critical for fruit and nut crops—a nearly $27 billion industry in the U.S.
But warmer, shorter winters can disrupt the chill that fruit and nut crops—and related local economies—depend on.
Climate Central analyzed hourly temperature data to determine long-term trends in winter chill for 215 U.S. locations.
A majority (84%) of analyzed locations have experienced decreasing chill since 1980.
Chill has decreased in many locations in counties that grow the 10 most valuable fruit and nut crops (especially in California, Washington, Florida, Oregon, South Carolina, and Georgia).
NEW: Check the Climate Matters Editorial Calendar to see what we’ll cover in the weeks ahead.
Winter chill brings summer fruits
Many temperate fruit and nut trees need a period of winter chill in order to produce flowers and fruit each year.
Without enough sustained chilling, the timing of bud break and pollination, and the quality or yield of fruit may be compromised.
Winter chill is therefore key to non-citrus fruit and nut production—a $27 billion industry in the U.S.
But winters are the fastest-warming season in most U.S. locations. Warmer, shorter winters can disrupt the chilling requirement that fruit and nut crops—and related local economies—depend on.
Calculating chill
Winter chill generally refers to a period of cold temperatures above freezing, but there are several different ways to calculate chill accumulation.
The required chilling period also varies both among species (e.g., apples, peaches, almonds) and among cultivars of a species (e.g., Red Delicious, Gala, and thousands of other apple varieties).
Chill hours (the cumulative hours below 45°F starting November 1) is one common metric. But not every hour below 45°F contributes equally to efficient, high-quality fruiting.
Winter warm spells—such as those experienced in January 2023 across the eastern U.S. and northern Plains—can offset previous chilling.
This damaging weather whiplash is reflected in an alternate metric called chill portions—a more accurate measure of chill accumulation in fruit and nut trees.
Chill portions weigh the benefits of exposure to ideal chilling temperatures (~32–45°F) against the harms of exposure to warm or highly variable temperatures starting September 1.
Local chill trends
Climate Central used chill portions (see Methodology) to analyze trends in annual chill accumulation in 215 U.S. locations from 1980 (or the next earliest year with hourly data) to 2022.
181 (84%) of the 215 locations have experienced a long-term decrease in winter chill since 1980.
But chill trends matter most where fruits and nuts are grown, so below we take a closer look at chill trends in the places that grow the 10 most valuable non-citrus fruit and nut crops in the U.S.
The following discussion focuses on analyzed locations within top-producing counties for each crop.
Fruits and nuts: the big five
Of all non-citrus fruits and nuts grown in the U.S., the top five account for $20 billion—or 74% of all non-citrus fruits and nuts ($26.8 billion), according to USDA data. The big five are:
1. Grapes—including table and wine grapes—are a $5.5 billion dollar industry, accounting for 21% of all non-citrus fruits and nuts grown in the U.S.
California grows 94% of the nation’s domestic grapes. Washington accounts for the rest.
Key grape-growing locations in California (Bakersfield, Fresno, and Stockton) have experienced decreasing chill.
2. Almonds—a close second to grapes—are a $5 billion dollar industry in the U.S.
California grows 100% of domestic almonds and major almond-growing locations in the state (Bakersfield, Fresno, and Stockton) have experienced decreasing chill.
3. Strawberries grown in the U.S. are a $3.4 billion industry—of which California produces 88%.
Chill has decreased in Salinas and Santa Maria—California’s strawberry hotspots.
An area just east of Tampa, Fla., grows over 70% of U.S. strawberries during winter. And the Tampa Area has experienced decreasing chill.
4. Apples grown in the U.S. are a $3 billion industry—of which Washington grows 72%.
Yakima, Wa., leads the nation in apple production and has experienced consistent chill.
Apples are also a major industry in New York (near Rochester and Buffalo, where chill has decreased), Michigan, and Pennsylvania (near Harrisburg, where chill has dropped).
5. Pistachios grown in the U.S. are a $2.9 billion industry, following close behind apples.
California grows all domestic pistachios, especially near Bakersfield and Fresno.
Top 10 fruits and nuts
The local value of fruit and nuts is all relative. For smaller states, growing high-value fruit and nut crops is a major economic engine. Here, we look at chill trends in places that produce the other top 10 crops:
6. Blueberries are a $1.1 billion industry in the U.S., distributed across:
Washington (21%): Production is concentrated in the three counties to the north of Seattle, where chill has decreased.
Oregon (16%): Eugene and Portland, on the southern and northern ends of the blueberry-producing Willamette Valley, have both experienced decreasing chill.
California accounts for 20%, followed by: southeastern Georgia (near Savannah, where chill has decreased), Michigan, Maine, New Jersey (near Atlantic City, where chill has decreased), Florida, and North Carolina’s southern coastal plain (near Wilmington).
7. Walnuts are a $1 billion industry in the U.S., with 100% produced in California’s Central Valley, where places such as Bakersfield and Fresno have experienced decreasing chill.
8. Cherries are a $950 million industry in the U.S.—and Washington (Yakima) accounts for 50%.
California produces 34% of total cherry value, mainly in the San Joaquin valley. Nearby Stockton has experienced a decrease in winter chill.
Oregon produces $67 million of cherries, mainly in Hood River and Wasco counties directly east of Portland, where winter chill has decreased.
Cherries are also a major crop in Michigan. The state’s $58 million in cherry industry is based near Traverse City, where winter chill has increased.
9. Peaches—a $624 million industry in the U.S.—are grown throughout the country, from California to New Jersey and from Michigan to Georgia.
California accounts for 60% of the national peach industry, with production concentrated in Sutter County (just north of Sacramento, where chill has decreased) and near Fresno.
Chill has also decreased in peach-producing regions of South Carolina (near Upstate), which grows $106 million, and Georgia (near Macon), which grows $35 million.
10. Pecans are a $550 million industry in the U.S.
About a third of U.S. pecans are grown in New Mexico. Another third of pecans are grown in Dougherty County, Ga., where Albany has experienced a decrease in chill.
The remainder are grown in Arizona, Texas, and Oklahoma.
LOCAL STORY ANGLES
How are local growers adapting to changing chill—now and into the future?
The USDA’s annual Fruit and Tree Nuts Outlook gives a national overview of forecasted growing conditions during the current season, including potential chill-related challenges for specific crops and regions. Options for adapting to decreasing or unreliable winter chill also include breeding or planting low-chill cultivars. Researchers are also improving dormancy and chilling forecasting models to more accurately inform growers on seasonal timescales. Other research looks decades into the future, using climate models to explore chill accumulation scenarios over the coming decades in key growing states and regions including Georgia, the Pacific Northwest, and California.
What are the chilling requirements for specific crops in your local area, and how are they changing?
USDA’s interactive Farm Income Atlas quickly maps and compares the state-level economic impact of different crops, including dozens of fruits and nuts. The UC Davis Fruit & Nut Research & Information Center provides detailed tree crop chilling data for California fruit and nut growers, including crop-specific chilling models. UC Merced’s interactive Climate Mapper tool can be used to map projected future cold hardiness (at state and county levels) over the contiguous U.S. and projected future growing conditions for perennial crops across the western U.S. Contact agricultural extension agents at your nearest Land-Grant University for more detailed local information.
CONTACT EXPERTS
Lauren E. Parker, PhD (she/her)
Research Program Manager
USDA California Climate Hub
Related expertise: Climate change impacts on fruit production
Contact: leparker@ucdavis.edu
FIND EXPERTS
Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.
Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.
Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices.
Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area.
METHODOLOGY
Many chilling models have been developed; all are empirical proxies of actual chill experienced by fruit and nut trees. One of these chilling models, the Dynamic Model, calculates the annual accumulation of chill portions in a two-step process. In the first step, the reversible accumulation and destruction of chill is temporarily tallied based on hour-by-hour temperature fluctuations within or above an ideal chilling range. When a critical amount of temporary and reversible chill accumulates in the first step, it is permanently and irreversibly counted toward the annual total chill portions in a second step. Over the annual dormancy period, chill portions accumulate in this dynamic, two-step process.
The chill portions analysis was based on hourly GHCN temperature data accessed through ACIS. Data was retrieved beginning in 1981, however the full set was not available for many stations. There are also fewer stations collecting hourly data than daily data, resulting in a smaller set (215 total locations) than most of Climate Matters analyses. The calculation of chill portions was performed using the chillR package for the R programming language. chillR includes a function that calculates the chill portion based on the complex Dynamic Model of chilling. For this analysis, Climate Central used the base function and did not make any adjustments based on local factors. An analysis of particular crops would require making some of these model adjustments to properly capture the chill needs of specific crop or cultivar.