What Are Growing Degree Days?
Growing degree days, often called GDD, measure accumulated warmth rather than calendar dates. Gardeners use them to estimate when plants will germinate, grow, flower, or reach harvest because development speeds up and slows down with temperature.
A daily GDD value is usually calculated from the day's high and low temperature: add them, divide by two, then subtract a base temperature. If the result is below zero, record zero GDD for that day. Many vegetable and field-crop references use a 50°F base, while cool-season plants may use a lower base, so always use the base listed by the crop-specific source.
For a 50°F base, a day with a 72°F high and 52°F low has an average temperature of 62°F and contributes 12 GDD. Add each day's value from a chosen starting date, commonly January 1 for regional insect and landscape calendars or planting day for a crop-development calculation. Weather stations, state climate offices, and Cooperative Extension services often publish local GDD data.
GDD is especially useful in variable springs. Instead of assuming tomatoes will perform the same on a particular date every year, a grower can compare heat accumulation after transplanting. It can also help small farms coordinate succession plantings, anticipate harvest windows, and schedule scouting for temperature-driven pests.
Use a nearby weather station whenever possible. Temperatures at a rural field, an urban yard, or a raised bed beside a south-facing wall can differ enough to change crop progress. Keeping a simple record of planting date, local GDD, flowering date, and first harvest for several seasons produces a more useful farm-specific guide than a generic chart.
GDD does not replace observation. Frost, drought, excess rain, day length, soil temperature, cultivar differences, and transplant size all affect growth. A warm air temperature also does not mean cold, wet soil is ready for direct seeding, so check soil conditions before planting beans, corn, cucurbits, and other warm-season crops.
For home gardens, start simply: choose a local GDD tracker with the appropriate base temperature, write down the total when you plant, and note key crop stages. Over time, compare those notes to seed-packet days to maturity. GDD is most reliable as a planning range, not as a promise of an exact harvest date.
Frequently asked questions
What does 50 growing degree days mean?
It means the day or accumulated period provided 50 heat units above the selected base temperature. It does not mean the air temperature was 50°F.
When should I start counting growing degree days?
Use the starting date specified by the crop or local Extension model. For your own crop records, planting or transplanting day is a practical starting point.
Are growing degree days the same everywhere?
No. They depend on local temperatures, the base temperature used, and sometimes the model's rules for capping daily high or low temperatures.
General guidance for US home gardens. Conditions vary by region and season; check with your local Cooperative Extension office for site-specific advice. Page updated Aug 4, 2026.