The growth and development of plants, insects and other invertebrates is largely dependent on temperature. Many of these organisms slow down or block their growth and development when temperatures rise or fall above or below certain thresholds. All plants need a certain individual temperature sum to be ready for flowering. The accumulation of thermal energy over a period of time is called growth degree days or temperature sums (heat sums). It is a heuristic (as opposed to logistic) calculation in phenology.
As a rule, the growth degree days (WGT) are calculated by taking the average of the daily minimum and maximum temperatures and comparing it with a threshold value (10 °Celsius in Central Europe). The measurement of growth degree days and temperature sums is used to establish planting dates, to make predictions for flowering and the optimal harvest time, and to select the most suitable grape varieties according to their phenological ripening time (early to late).