The growing degree day is the expression of this line. If the temperature is below 50 °F, there is no growth. If the daily temperature averages 55 °F (12 °C), some increment of growth would be evident during the 24-hour period. This growth is calculated according to a formula where the maximum temperature and the minimum temperature are averaged less some base temperature.


Equation 2.1

If the daytime high temperature is 70° F and the overnight temperature, 50° F (typical for late May), and you average those and subtract 50, the answer is 10 GDD during the 24-hour period.

If the daytime temperature were 70° F, there were probably 10 growing degree days during the 24-hour period. During a 24-hour period the value can range from 0 to as much as 36, which is the maximum value for growing degree days. Crops cannot develop any faster than that, so that is arbitrarily set as the top value. Seldom would a value of 36 be reached.

There are rules for computing growing degree days:

Rule 1: If the maximum temperature >86°F, Max=86.

86° F is the highest temperature considered. Why 86° F? The crop has its ideal growth at 93° F. Why not 93° F? Seldom do we have ideal conditions for aeration, for fertility, and for water availability to the crop. As the temperatures get higher, the crop's demand for water may become higher. At a higher demand for water, the crop may come under water stress. In the Midwest, on the average, moisture stress begins at about 86° F.

If the temperature in the day were observed at 90° F, for the purpose of calculating growing degree days, you would say the high temperature was 86° F.

Rule 2: If the minimum temperature <50°F, Min=50.

If the minimum temperature is less than 50° F, we call it 50° F. That is, if the nighttime temperatures drop down to 40° F, the crop would not be growing. ` Hence, if the temperature drops below the base, the lowest temperature at which the crop can develop, we set the minimum to 50° F. This mathematically prevents the values in the growing degree day from becoming negative. Growing degree days can vary only from 0 to 36. You cannot have negative growing degree days. We do not allow a crop to "un-grow". We have only crops that grow and develop.

Study Question 2.6
If a day's maximum temperature was 83° F and the minimum was 56° F, how many GDDs were accumulated?
Study Question 2.7
If the maximum temperature was 92° F and the minimum was 72° F, how many GDDs accumulated

By following these examples for calculating minimum temperature and maximum temperature and the growing degree days, it is rather simple to calculate and then add up the daily growing degree days to know exactly what is happening to the crop and crop development if the crop is being controlled by growing degree days. The growing degree day model for development is not exact; it is intended to be a simplistic improvement on the use of a calendar only.