Growing degree days (GDD) are used for what purpose in crop management?

• A. A unit of rainfall measurement.
• B. A unit of soil moisture.
• C. A pesticide degradation rate.
• D. A measure of heat accumulation used to predict plant development stages and timing of events like flowering and harvest.

Answer: (D)

Growing degree days are a way to quantify how much heat a crop experiences over time. Plants need heat to develop, so by summing the amount of heat above a base temperature day by day, we can estimate when growth stages will occur—like leafing, flowering, fruit set, and harvest. This helps with scheduling planting dates, irrigation, fertilization, pest scouting, and harvest timing so actions align with when the crop will reach those stages. The daily heat unit is calculated from the average of the daily high and low temperatures, subtracting a base temperature below which growth doesn’t proceed; if that result is negative, it’s treated as zero, and these values are accumulated over the growing season. For example, with a base of 10°C, a day whose average temperature is 20°C adds 10 GDD to the tally. This concept is about heat accumulation, not rainfall, soil moisture, or pesticide degradation, which relate to water, soil conditions, or chemical breakdown, not plant development driven by temperature. Therefore, GDD is a measure of heat accumulation used to predict plant development stages and timing of events like flowering and harvest.