Current and Future Trends in Chilling, Spring Temperatures, and Spring Freezes in Tennessee

Climate change is likely to affect future winter and early spring temperatures and could have a major impact on the U.S. fruit and nut industry, mostly by impacting the chilling that must occur during winter. The traditional definition of a chill hour is any hour under 7.2C (45F). Some say that chilling only counts if the temperature is between 7.2C (45 F) and 1C (34F), or that hours must be subtracted if the hourly temperature goes above a threshold, such as 15.6 C (60F). Therefore, these accumulations are no longer strictly a count of hours below a threshold, and the term used is “chill unit”. Many of the crops grown in Tennessee require an accumulation of chill hours or chill units to end their winter dormancy. For example, peach trees require accumulations of 600-1000 chill units, depending on the variety. If the buds do not receive sufficient chilling temperatures during winter to completely end dormancy, plants may have problems with delayed leafing out, reduced fruit set and increased buttoning, and reduced fruit quality. Winter wheat varieties require up to 1080 chill units at the growing point to vernalize, which is necessary for optimal grain production. Once these plants break dormancy, their rate of development depends on heat accumulations such as growing degree (hours or days) to reach critical stages such as bud burst and flowering. The seasonal average chill hours in Tennessee range from 900 to 1600, the seasonal average chill units range from 700 to 1200, and the last spring freeze 0C (32F) date ranges from the early March through the middle of April, depending on latitude and elevation. The objectives of this study were to calculate and assess the trends in the seasonal accumulation of chill hours, chill units, spring growing degrees, and last spring freeze dates for a cross section of locations in Tennessee, using daily maximum and minimum temperature data from 1981 to 2016. Preliminary results indicate that there has been a significant decrease in both chill hours and chill units throughout the state. As a result, crops requiring chilling are likely to suffer setbacks in production. In addition, there has been an upward trend in spring growing degrees, thus hastening spring development of these crops which does not favor optimal production. However, there has not been a significant trend in dates of last spring freezes, meaning that many crops requiring chilling are likely to be in a more sensitive stage of development when a freeze occurs. The interplay of these 3 factors – winter chilling, spring warming, and spring freeze dates – will essentially determine the future viability of these crops in Tennessee.