Grapevine growth and development is determined by several factors such as variety, temperature, day length, irrigation, winter chill accumulation and more. Growing degree day models are one simple, but effective, tool for growers that can explain most of differences in crop development year to year, and can be used to predict timing of different crop events such as bud break, bloom and verasion. While growing degree day units do not account for all the factors in determining crop phenology, they are able to explain most of the difference between growing seasons, between different ranches/ microclimates, or between regions.

Grape Growing Degree Day Models

Growing degree day models track crop growth by measuring the amount of heat “units” or time above a certain temperature. They can be estimated for each day by using max and minimum temperatures with a sine function, or by using the Pest Prophet app to automatically track growing degree day units for a given field, using hourly temperature data. For grapes, 50F is commonly used as the lower threshold for calculating growing degree units, however researchers have developed growing degree day models for 17 different grape varieties, with thresholds that also adjust to the crop’s growth stage. (Zapata et al. 2016)

Each grape varietal has it owns unique crop phenology, so using different models allows growers with multiple varietals to plan for  differences in timing between different crop stages. The following table can be used for growing degree day thresholds  and accumulation:

Re-adjusting Bio-fix Date

Growing Degree Day models require a “biofix” or starting point for when to start accumulation of growing degree day units. For predicting bud break timing, this is usually either the start of the growing season or the beginning of the year. However, after bud-break is observed for at least 50% of the vines, the biofix should be readjusted for this date in order to predict the bloom timing. Doing this after bloom will also improve accuracy of the verasion timing prediction. After a biofix is “reset” you should only count the growing degree days accumulated after this point in order to predict the next crop event.The Pest Prophet app allows growers to do this easily by changing the growth stage manually after it is observed.