James A. Schrader, Ph.D
“Climate change,” you hear the term almost every day. For most, the term is nearly synonymous with another term, “global warming.” In recent years these two terms have been used extensively in discussions and in the media to explain a predicted increase in global temperatures that will, among other things, cause oceans to rise, cause warm areas of the globe to become hotter, and cause temperate regions to warm in ways that will alter the normal environment for plants and animals that live in those regions. In the areas of agriculture and horticulture, some of the greatest impacts are predicted to be in the form of increasing extreme-weather events and increasing pressures from pests and pathogens. Some changes could be beneficial. It is believed that global warming may benefit some crops that are typically grown in certain regions, and/or it may allow growers to switch to crops or cultivars that are currently grown only in warmer areas.
So how will climate change impact the practice of viticulture in the midwestern United States? With the amount of publicity that climate change receives, one might feel compelled to make radical changes in the way they manage their vineyard or in the cultivars that they choose for their next planting cycle. In this article we will examine one important aspect of the climate change discussion as it relates to midwestern viticulture, and we’ll consider how best to respond. Because cold hardiness is of utmost importance when choosing grape cultivars for use in the upper midwest, we will examine Iowa air temperature data from the last 32 years to answer one specific question: “has midwestern climate warmed enough for vineyards to begin using cultivars that are less cold hardy than those currently grown here?” With the higher acid content commonly found in cold-climate grape cultivars, it could be beneficial for midwestern viticulture if cultivars grown in warmer climates (many of which have lower organic acid content at harvest) could be grown successfully in the upper midwest.
In the upper midwest, winter extreme low temperatures are among the greatest stressors that determine if a grape cultivar will survive and/or provide a consistent supply of fruit from year to year. Both vines and buds must be hardy enough to survive winters in order to provide a consistent crop. If midwinter extreme lows have risen to a consistently warmer level, then warmer-climate cultivars might be worth considering for the midwest. Examining data collected from three long-term climate stations managed by Iowa State University (one each in northern, central, and southern Iowa), we see that annual extreme low temperature (the lowest temperature of each year) has varied greatly from year to year, but there has been no sustained rise in mid-winter extreme lows over the last 32 years (see Figure 1). Data beginning in 1988 were selected because these data are considered to be of “high quality” in terms of the accuracy of the sensing equipment and data collection systems used. Over the last 32 years, annual extreme lows ranged from -11.1 to -30.3 °F in northern Iowa, -5.4 to -28.5 °F in central Iowa, and -2.8 to -27.5 °F in southern Iowa (Figure 1). The variation across years is so great that no reliable trend can be assigned to the data. There are years with extreme lows that were warmer than average, other years that were colder than average, and a few obscure multi-year cycles that varied from colder to warmer, then back again, but there has been no significant increase in annual extreme low temperatures over the last 32 years.
In northern Iowa (Sutherland weather station) the coldest temperature of the last 32 years was recorded in 2010 (-30.3 °F), and the annual extreme low in 2018 was the 5th lowest over the last 32 years (-26.8 °F). In central Iowa (Ames weather station) the coldest temperature of the last 32 years was recorded in 1996 (-28.5 °F), and the extreme low in 2019 was the 4th lowest over the last 32 years (-22.2 °F). In southern Iowa (Chariton weather station) the coldest temperature of the last 32 years was recorded in 1989 (-27.5 °F), and the extreme low in 2019 was the 5th lowest (-23.2 °F) and nearly identical to the 2nd, 3rd,and 4th lowest extreme lows that were recorded in 1988, 2009, and 2013 (-23.4, -23.7, and -23.6 °F, respectively). Regardless of what a person believes about the long-term predictions of global warming and climate change, now is not the time to plant cultivars that are less cold hardy than those currently in use in the midwest. A decision to do so would carry a high risk of failure from cold-temperature damage to vines and buds. Perhaps a sustained change in winter extreme low temperatures will develop in the future, but based on the actual data, such a scenario does not yet exist in Iowa.
Since the 1980s, there has been consistent progress in breeding and selection of cold-hardy, disease-resistant grape cultivars for use in the midwestern United States and other cold regions. A review publication describing many of these cultivars can be found at: https://store.extension.iastate.edu/product/A-Review-of-Cold-Climate-Grape-Cultivars. Based on the current climate reality of the upper midwest, these cultivars and others with similar or greater cold hardiness should top the list of those considered for commercial use in Iowa and other areas of the upper midwest. Using cultivars that are less hardy than these would be a costly gamble that one would most likely lose.
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