WeatherArchive / Growing degree days
Growing degree days calculator
Add up growing degree days (GDD) for any place and period to time planting, spraying and harvest, or switch to heating and cooling degree days to size energy use. This year’s total is plotted against the 30-year normal.
How a degree day is counted
Set one day’s high and low and a base temperature. The shaded area above the base is the heat a plant or insect “banks” that day.
Mean (24.0°C + 12.0°C) ÷ 2 = 18.0°C → 8.0 growing degree days (°C-days: 8.0 in Celsius).
Below the base, development stalls, so a cold day adds nothing rather than subtracting. Summed over weeks, these daily scraps predict when maize tassels or codling moths hatch far better than the calendar does.
Which base temperature?
Each crop or pest has a threshold below which it barely develops. Common choices: 10 °C / 50 °F for maize (corn), soybeans and many warm-season crops, usually with an upper cap of 30 °C / 86 °F (the “86/50” method used by US extension services); 5 °C / 41 °F for wheat, barley and grass; 4.4 °C / 40 °F for peas and potatoes. For insects, use the threshold from your pest model.
Heating degree days measure how far the daily mean fell below a comfort base, commonly 18 °C in Europe or 65 °F (18.3 °C) in the US: energy analysts use them to normalise heating bills. Cooling degree days are the mirror image for air conditioning.
Questions people ask
How do you calculate growing degree days?
Take the day’s high plus low, divide by two, and subtract the base temperature. If the result is negative, the day counts as zero. Add the days up over the season.
What is the 86/50 method?
A US convention for corn: highs above 86 °F are treated as 86 and lows below 50 °F as 50 before averaging, because growth stops increasing above that heat.
How do heating degree days relate to energy use?
Heating demand is roughly proportional to heating degree days, so dividing a heating bill by HDD lets you compare winters of different severity or check whether insulation helped.
Why compare with the normal?
A season running ahead of its normal GDD means crops and pests will reach their stages earlier than the calendar suggests.