The Indiana State Climatologist's Office recently calculated when growing degree days reached certain levels as part of a project for Indiana Prairie Farmer. Growing Degree Days basically measure how much energy is available ot the plant to grow, based on temperatures.
The system used has a base temperature of 50, which means the low lowest temperature used in the formula. The ceiling is 86 degrees. Agronomists suggest corn does not grow and develop further at temperatures above this level.
This year central Indiana reached 2,900 growing degree days on August 28, considerably ahead of normal. Figuring out why is probably not hard. During the 23-degree streak of 90 degree or higher days, the ceiling was reached every single day. All in all, the year featured more than 40 degrees of 90 or higher in central Indiana, more than twice the normal number of days. Those are all days when the ceiling temperature of 86 degrees goes into the formula.
Meanwhile, for many of those 40 plus days, nighttime temperatures remained well above the minimum of 50 degrees. The higher the nighttime temperature, the more growing degree days that can accumulate. However, that's not always positive for the plant, especially not for corn. Corn needs nighttime lows in the mid-to-low 60 degree level to 'cool off' properly and make maximum use of sugars formed during photosynthesis during the day,. Otherwise, if nighttime temperatures remain in the 70's or even low '80s, the plants continue to use up sugars that should otherwise have been converted into starch and placed in developing kernels.
So why are all crops, especially some soybeans, not 100% safe from frost? It's because growing degree days that accumulate before the crop is planted don't do the crop any good. So if corn was planted June 1 and 1,000 GDDs had already accumulated, the GDD clock for that corn hybrid reset to zero, and started over. If the hybrid needed 2,500 days to mature, it would have still been 600 days short on Aug 28 in central Indiana of reaching black layer maturity, based on this example.
One factor plays in favor of the corn crop. When planted late corn doesn't need as many GDDs to reach maturity. In fact, it may need 200 less GDDs than the same hybrid planted during the normal planting time, say before May 10. So the crop can catch up toward maturity. The sacrifice, of course, is that it probably made decisions along the way, like rows of kernels per ear and size of kernels, which will work against yield at harvest.