Some Limitations of the Degree Day System as Used in Viticulture in California

Abstract

The average and range in dates of budbreak, bloom, and fruit maturity of five grape cultivars grown in five different climatic regions of California were determined over a period of two to six years. For each cultivar and climatic region, a comparison of the variation in the average number of degree days and duration (in days) for three developmental periods (budbreak to bloom, bloom to fruit maturity (20°Brix), and budbreak to fruit maturity) was made. No statistically significant difference was observed between the two procedures. Determining the number of degree days by digitizing the area under the thermograph curves for five different locations gave degree day values for the period 1 April to 31 October that ranged 6% to 22% lower than the traditional method of mathematically averaging the maximum and minimum temperature per day. For a given cultivar, there were wide differences in the number of degree days required for the developmental periods between the five locations. Possible reasons for these differences are discussed.