Growing Degree Days (GDD) aka "Heat Units"

Temperature governs so many biological reactions, including the growth of plants, the development of plant diseases and insect pests, and emergence of planted seed. Growing degree days (sometimes referred to as "heat units") are a measure of the accumulated heat over time. Many "base temperatures" are used depending on how the species in question respond to temperature. Cool-season crops or early-emerging weeds are best predicted by temperatures accumulating above freezing.

📸: @mvburling via Canva/Featuring corn emergence and development are well predicted by base 50 °F GDD.

Calculating GDD

GDD is calculated using a base temperature and a high cutoff temperature (for most crops). A base of 50 °F is used for corn and soybeans, meaning that temps below 50 °F are not conducive to any advance in growth for these crops. There is also a "high cutoff" assumed to be 86 °F, which means that when calculating GDD, there is no further benefit for temperatures above 86 °F. The GDD is the "average" between the high and low that is then compared with the base temperature to denote " how much higher was the average daily temp compared to the baseline."

The general formula is (High Temp + Low Temp) /2 -50, with the caveat that if the low temp is below 50 °F, use 50, and if the high temp is above 86 °F, use 86.

Here are a few examples for GDD50:

The temperature never gets above 50 °F all day. Use a baseline low of 50 in the calculation

GDD50 = 0 (50 + 50) / 2 - 50 = 0.

Low temp 45 °F, High temp 58 °F. Use 50 for low temp

• GDD50 = 4 (58 + 50) / 2 - 50 = 4 Low temp 65; High temp 90. Use 86 for high temp: GDD50 = 25.5 (86 + 65) / 2 - 50 = 25.5
• The highest theoretical GDD 50 would be a day when it never got cooler than 86 °F: GDD50 = 36 (86 + 86) / 2 -50 = 36

It is, however, unlikely that most crops would prosper under this heat unless the soil was very moist.

Growing Degree Day data can be found for your location on an excellent website that Michigan State University provides: https://gddtracker.msu.edu/?model=7&offset=0&zip=61822. This is a simple way to get a heads-up on local conditions and to see the advance of GDD (or heat units) from south to north over time.

📸: Michigan State University GDD tracker/Featuring an example of GDD32, where the green color denotes the optimum time for a PRE herbicide to control crabgrass or annual bluegrass (250 GDD32).

Corn

Corn is a daylight-neutral crop whose growth is predictable by accumulated heat units. The advancing phenology from planting to emergence through the V stages and ultimately maturity is fairly well predicted by GDD50.

Winter wheat

Winter wheat development is predicted by a GDD32 that straddles the winter from fall planting until maturity the next year. Knowing the crop growth stage if a late freeze occurs is helpful in understanding potential damage to the crop. Research done in Kentucky https://wheatscience.ca.uky.edu/sites/wheatscience.ca.uky.edu/files/rr_2016_carrie_knott_-_estimating_the_number_of_growing_degree_days.pdf observed wheat green-up between 800 and 1100 GDD32 depending on planting date. When corrected for fall development by the earlier-planted wheat, a post-winter accumulation of 800 GDD32 predicted green-up.

Alfalfa

Alfalfa growth is predicted by GDD41 and is important in determining the last cutting before a hard freeze occurs in the fall. This calculation can be made by looking at a 10-day forecast and utilizing long-term average temperatures.

📸: @ImagineGolf via Canva/Featuring the final annual alfalfa harvest should be completed with 200–300 GDD41 remaining before a killing frost.

Weed species

Emergence of important weed species can also be approximated using GDD, either base 32 or base 50, depending on species. Early emerging weeds such as annual bluegrass, knotweed, and others use the 32 base for predictive purposes. Emergence of most of the summer annuals is better predicted with GDD50.

Pests

Emergence and feeding by adult Japanese beetles are one of many predictable events using GDD. Many other insects run through their life cycles based on the accumulation of heat units.

GDD calculations are a valuable tool to predict crop development and progression of insect and disease pressure.

Coming next: Soil fitness for Spring Tillage

This content is being brought to you in partnership with Grow Smart^® Live and contributing guest authors. BASF provides the information in this article as a service to its customers; however, the views expressed by guest writers are their own and do not necessarily reflect the views of BASF.