The growing degree days formula, step by step

What a growing degree day measures

Crops develop on heat, not on the calendar. A cool week and a hot week of seven days each move a corn plant forward by very different amounts, and growing degree days (GDD, called growing degree units or GDUs in corn) are the way to count that heat. Seed companies rate hybrids by the GDD they need to reach silking and black layer, so the running total tells you where a field is.

The basic formula

For one day: add the high and the low, divide by two, and subtract the base temperature. For corn the base is 50 °F, the temperature below which the plant barely grows. A day that averages below the base counts as zero, never as a negative number.

The corn method: cap the high, floor the low

The method used for corn adjusts both temperatures before averaging:

• a high above 86 °F is counted as 86, because corn does not develop faster in greater heat;
• a low below 50 °F is counted as 50, because the plant does not lose ground on a cold night.

In the week worked below, May 13 reached 90 °F and is counted at 86, and the 48 °F night on May 10 is raised to 50. Skip those two adjustments and your total runs ahead of the field on hot days and behind it after cold nights.

A week, added up

Seven days of highs and lows from mid-May come to 118.5 GDD, an average of 16.9 a day. The same heat expressed on the Celsius scale is 65.8 GDD — the number is smaller only because a Celsius degree is larger; the crop saw the same week.

Reading the total

Add each day to the running total from planting. Compare it with the GDD rating of the hybrid on the seed bag or the company’s product sheet, and you know how far the field is from the next stage — useful for timing scouting, side-dress passes and harvest logistics.