Emmalea Ernest, Scientist – Vegetable & Fruit Crops; emmalea@udel.edu
Growing Degree Days (GDDs) are a useful tool for timing plantings of certain crops like sweet corn and peas to achieve a continuous harvest. Closer to harvest, degree days can also be used to predict when a crop will mature. Degree days are calculated based on the daily high and low temperatures. The simplest method for calculating daily degree days uses this formula:
The base temperature is crop dependent and is the approximate threshold temperature for growth. For peas the base temperature typically used is 40°F, for corn the base temperature is 50°F. I use GDDs to predict harvest for the pea variety trials and, in the past, have used a spreadsheet to calculate accumulated GDDs. I would copy and paste in the temperature data from the DEOS weather station at the Carvel Research & Education Center in Georgetown, DE into my spreadsheet every few days. This year it is even easier to track GDDs because the DEOS weather station data is being fed into the NEWA system and NEWA has a GDD calculation tool. DEOS has more than 30 weather stations in Delaware (plus a few more in Chester Co., PA), so if you are in Delaware there is probably a station that is close to your farm.
NEWA (https://newa.cornell.edu/) has many weather-based tools for making crop management decisions and anyone can create an account and use the system. The GDD Calculator is accessible through the home page under “Weather Tools” (Figure 1).
To use the calculator, select the closest weather station to your field, choose start and end dates and indicate the appropriate base temperature for your crop (Figure 2). There is a table-based output, which includes the daily and accumulated GDDs, and a graph of accumulated degree days. The table-based output can be exported to a spreadsheet. One potentially useful feature is the inclusion of projected GDDs based on the six-day weather forecast. I look forward to using the forecast GDDs to predict which days to expect pea plots to harvest.
Figure 1. The degree day calculator tool (circled) is located under “Weather Tools”
Figure 2. The NEWA Degree Day Calculator menu.