What are degree days?

What are degree days?

• A. Days above the threshold temp for plant and pest growth.
• B. Days with rainfall over threshold.
• C. Days with frost events.
• D. Days with insect emergence only.

Answer: (A)

Degree days measure accumulated heat that drives plant and pest development. Each day contributes heat units equal to how much the daily average temperature exceeds a base threshold (the temperature below which growth stops for a given species). Those small daily excesses are added up over time, so you end up with a count of degree days that predicts when plants reach growth stages or pests reach emergence or other developmental milestones. For example, with a base of 10°C, a day whose mean temperature is 15°C adds 5 degree days, and these add up across days to indicate when a pest will be ready to emerge or a crop will reach a particular stage. This concept differs from rainfall days, frost events, or only noting insect emergence; degree days specifically reflect heat accumulation above the threshold that drives development.

Degree days measure accumulated heat that drives plant and pest development. Each day contributes heat units equal to how much the daily average temperature exceeds a base threshold (the temperature below which growth stops for a given species). Those small daily excesses are added up over time, so you end up with a count of degree days that predicts when plants reach growth stages or pests reach emergence or other developmental milestones. For example, with a base of 10°C, a day whose mean temperature is 15°C adds 5 degree days, and these add up across days to indicate when a pest will be ready to emerge or a crop will reach a particular stage. This concept differs from rainfall days, frost events, or only noting insect emergence; degree days specifically reflect heat accumulation above the threshold that drives development.