Growing degree days (GDD) are a measure of accumulated heat used in agriculture and viticulture to describe seasonal temperature patterns and estimate crop development. In grape growing, a common method calculates each day’s heat accumulation above a base temperature, often 10°C (50°F), and adds the values over a defined growing season. The exact calculation period and base temperature depend on the model being used.
In a simple model, the daily mean temperature is calculated from the day’s maximum and minimum temperatures. The base temperature is subtracted, and only positive values are counted. These daily values are then added over the chosen period. More advanced models may cap high temperatures or use different calculations.
Example: if a day’s mean temperature is 18°C and the base temperature is 10°C, that day contributes 8 growing degree days. A season’s GDD is the sum of the daily values over the selected dates.
GDD can help compare growing seasons, estimate phenological timing and assess the suitability of a site for different grape varieties. It is often used alongside rainfall, frost dates, sunlight, soil moisture and other climate data. GDD does not directly measure grape quality or guarantee ripeness.
Growing degree days are one heat-accumulation measure. Wine-region climate classifications may use GDD, average temperatures, growing-season length and other variables. Different systems may use different base temperatures, dates and thresholds, so figures should be compared only when the calculation method is consistent.
Chef’s note: GDD helps explain why some varieties suit certain regions, but the final wine also depends on site, vintage, vineyard management and winemaking.
This term is one of hundreds covered in depth across OCS courses, learning paths, and 1-on-1 coaching with Chef Eric.
