Growing Degree Days (GDU) Calculator for Crops

Growing degree units treat temperature as the currency of development. Above a species-specific base temperature the rate of development is approximately linear in temperature, so summing each day's excess heat predicts crop stages far better than counting calendar days. The standard daily calculation averages the maximum and minimum temperature and subtracts the base. The modified form used for corn first caps the maximum at 86 degrees F and raises the minimum to the base of 50 degrees F, because corn development neither accelerates meaningfully above about 86 degrees F nor proceeds below about 50 degrees F. Base temperatures differ by organism: 50 degrees F is standard for corn, sorghum and soybean, while 32 or 40 degrees F is used for wheat, barley and many cool-season plants, and insect models carry their own values, such as the roughly 48 degrees F often used for alfalfa weevil. Corn hybrids are sold by GDU rating, roughly 2,100 to 2,300 units for early material and 2,700 or more for full-season hybrids, with emergence near 120 units and silking near 1,200. The limitations are substantial: a daily maximum and minimum is a crude proxy for the true temperature curve, the method ignores soil temperature at planting, moisture stress, and photoperiod-sensitive species, and predictions drift after unusual cold snaps or extended heat.

Accumulated heat as a predictor of plant development is one of the oldest quantitative ideas in agriculture. Rene Antoine Ferchault de Reaumur described summing daily mean temperatures in 1735 to compare the timing of the same crop across different years and places, and the resulting thermal sums appeared throughout nineteenth and early twentieth century phenology under names including heat units, thermal time and remainder indexes. The specific form used for North American field crops was worked out in the middle of the twentieth century. Agronomic research published in the 1950s, including work by E. C. Gilmore and J. S. Rogers in the Agronomy Journal in 1958 on heat units as a measure of corn maturity, tested competing heat unit systems and established that capping high temperatures improved prediction over a simple mean-minus-base sum, which is the origin of the 50 and 86 degree F pair still in use. Seed companies adopted GDU ratings in place of days-to-maturity labels during the 1970s because a heat unit rating transfers between latitudes and planting dates in a way that day counts do not. Extension services and national weather agencies now publish running accumulations by weather station and by pest model, which has turned the daily calculation into a lookup for most growers rather than a field exercise.