Use of Growing Degree-Days in Viticulture
Degree-days (also called heat units, heat accumulation, or heat summation index) are used as a rough measure of the cumulative amount of functional heat experienced by grapevines during the growing season of April 1 through October 31. Degree-days are determined by subtracting 50ºF from the mean daily temperature (maximum + minimum divided by 2), and calculating the cumulative sum through the defined growing season. The base temperature of 50ºF is used in calculations because grapevine shoot growth is essentially nil below this temperature. Many crops and pest management systems utilize degree-days, but be aware that calculations may differ in the base temperature, the seasonal duration, or other factors. Use only grape growing degree-days.
The rate of growth of grapevines and development of fruit are directly influenced by temperature. The term phenology describes the study of life cycles in response to climatic changes. Thus, degree-days are sometimes used to predict when important phenological stages, such as bloom or veraison, will occur. Adaptability of grape varieties to the climate of a region can be partially assessed by consideration of the average degree-days accumulated by the region. Degree-days are also used to recommend crop coefficients at varying times of the year in irrigation scheduling by the Potential Evapotranspiration method. Emergence of certain insects can also be predicted on the basis of degree-days.
See our dynamic Growing Degree Days for Texas map and database.
