timing
Growing degree days
Last reviewed 2026-10-04
Growing degree days, or GDD, estimate how much heat has accumulated during a season above a chosen base temperature. The total can help predict development stages for crops, weeds, and insect pests. A GDD estimate is useful only when the crop, base temperature, and calculation method are stated.
What it means
A day contributes heat when its temperature is above the selected base threshold. Daily contributions are added over time. Different plants develop at different rates, so one GDD total cannot describe every crop. Extreme heat, drought, disease, and light can also change actual development.
How it affects the garden
GDD can improve timing when a crop or pest has a tested threshold. It is less useful as a universal replacement for observing the plant. Gardeners can combine local temperature records with crop observations and a maturity estimate instead of treating one number as a promise.
The harvest date calculator provides a simpler date estimate from planting and maturity inputs. A microclimate may make conditions at one bed differ from a weather station.
Example
A pest forecast may use accumulated heat to estimate when larvae are likely to appear. The gardener still checks the actual plants before taking action.
Common confusion
Growing degree days are not calendar days. They are a heat accumulation model, and the result changes when the base temperature or data source changes.
Sources
Michigan State University Extension explains growing degree days and their use in crop and pest timing.
