Growing degree day

A measure of accumulated warmth calculated from temperatures above a chosen lower threshold. It estimates the development rate of plants, insects, and other organisms.

Base temperature: A lower temperature threshold below which an organism’s development is treated as negligible. Growing degree days count warmth above this threshold.
Phenology: The study of recurring life-cycle events and their timing in organisms. Degree-day totals help predict when seasonal stages occur.
Temperature: A physical quantity describing how hot or cold a system is. Temperature observations supply the input for thermal accumulation.
Calendar days: Units of elapsed time based on successive periods of 24 hours. Unlike calendar time, degree days give warmer days greater developmental weight.
Degree-day model: A model that estimates biological development by accumulating thermal time above a temperature threshold. Growing degree days are the accumulated values at the heart of these models.
Plant phenology: The timing of recurring plant events such as flowering, leafing, and fruit development. Crop stages can be forecast from accumulated warmth.
Biological development: The changes through which an organism grows, differentiates, and progresses through its life cycle. Degree days approximate the temperature-dependent pace of these changes.
Chilling hours: An accumulation of hours within a specified cool-temperature range, often used to assess plant dormancy release. Many fruit crops require winter cold, not just accumulated growing-season warmth.
Daily mean temperature: The average air temperature over a 24-hour period. A common calculation estimates each day’s thermal contribution from its mean temperature.
Insect phenology: The timing of insect life-cycle events in relation to seasons and environmental conditions. Thermal accumulation predicts emergence and other developmental stages.

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