Cold stress and frost damage have the potential to impact perennial fruit crops during different times of the year, and springtime in the Upper Midwest is no exception. While cold stress can happen at warmer temperatures, frost occurs when ambient temperatures fall below freezing (32°F). When a spring frost happens, it can harm vegetation, and negatively impact bloom and fruit set. This is problematic recognizing that many perennial fruits exit dormancy and begin growing in Minnesota before the threat of spring frost has passed. Crop loss due to frost damage can be devastating and many strategies that help annual crop growers (e.g., delayed planting) are not usually possible for perennial crops.
What happens when dormant chilling requirements are ahead of schedule?
During winter seasons when more chilling hours (total hours during dormancy where ambient temperatures are between 32°F and 45°F) have accumulated than normal, many fruit crops may break dormancy and “wake up” earlier. This means there is a larger window of time between the initial bud break stage and the last day when freezing temperatures could occur. Minnesota has generally accumulated between 100 and 600 more chilling hours this year than average, with the highest deviation of chilling hours documented near the Twin Cities metro, the Arrowhead region, and Eastern Minnesota.
Critical temperatures (Tc) impact how severely a frost impacts a fruit crop at different growth stages
There is variation in both winter chilling requirements as well as the temperature in which frost damage can occur (i.e., the critical temperature, Tc) between fruit species (e.g., apples vs strawberries, or elderberries) and across different varieties of the same fruit species. More importantly, each specific plant can be more or less vulnerable to frost damage based on its growth stage (see chart below).
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Categories: Minnesota, Crops, Fruits and Vegetables