Historical and Projected Changes in Chill Accumulation and Spring Freeze Risk in the Midwest United States

In the Midwest region of the United States, the dormant or cold season has experienced significant changes over the past several decades due to human-caused global warming, and changes are projected to continue or intensify through the end of the century. Changes in chill accumulation and spring frost injury risk are particularly concerning for specialty crop growers in the Midwest. Despite their importance for the industry, relatively little work has been done to assess these changes and help guide crop management strategies accordingly. We use a combination of historical observations and CMIP6 multi-model ensemble projections to assess recent and potential future changes in chill accumulation and phenology for apple in the region. Observations show increased chill accumulation in much of the Midwest over the past 70 years, and CMIP6 projections indicate continued increases through the next 70+ years. The southern Midwest is expected to lose chill, but not at a rate that would require a substantial shift to fruit cultivars with very low chill requirements. Additionally, apple full bloom estimates using the PhenoFlex model combined with CMIP6 model projections show shifts earlier in the spring for both apple phenology and spring freeze dates. We do not find any appreciable change in spring freeze injury risk by mid-century under any scenario. This study provides an important assessment of climate change impacts on specialty crops in an understudied region of the United States for non-commodity agriculture. More collaborative work is needed between scientists, practitioners, and growers to (1) assess the current and future risks to specialty crop agriculture in the Midwest that result from climate change and (2) explore viable solutions to ensure a resilient specialty crop industry in the face of changing climatic, economic, and social systems.