Understanding growing degree days
Growing degree days sum daily heat above a crop's base temperature and predict development more reliably than calendar dates.
13 min · advanced
You will learn
- Define growing degree days and base temperature
- Calculate daily and cumulative GDD for a crop
- Use GDD to predict development and improve harvest timing
What GDD measures
Growing degree days accumulate daily heat units above a crop-specific base temperature below which development stops. University of Minnesota Extension uses a base temperature of 50 degrees Fahrenheit for corn development tracking. Each day contributes the average of daily high and low minus the base, down to zero if below base. Sum from planting or a biofix date through the season.
Calculating GDD
A simple formula: GDD equals the maximum of zero and the quantity daily high plus daily low divided by two minus base temperature. If the average is 65 and base is 50, that day adds 15 GDD. Local weather stations and extension services often publish accumulated GDD for standard bases. Different crops use different base temperatures.
Why GDD beats calendar dates
A cool spring delays phenology; a warm one advances it. Calendar-based harvest predictions miss this variation. Extension research links GDD totals to developmental stages like tassel emergence in corn. Comparing your accumulated GDD to published thresholds improves harvest forecasting and pest timing, since many insects also develop by heat units.
Applying GDD in the home garden
Record planting dates and track local daily temperatures or use online GDD calculators keyed to your nearest station. Compare totals to variety descriptions and extension guides for your crop. GDD helps decide when to scout for specific pests and when to expect first harvest. It is most valuable for comparing seasons and refining plans over multiple years of records.