Tracking plant growth and pest development is essential for timely decisions throughout the growing season. Agronomists and growers utilize Growing Degree Days (GDD) – a measure of daily heat accumulation – to predict the developmental stages of crops and pests, guide field scouting, and assess intervention techniques. GDDs are accumulated daily, starting from a user-selected beginning date. Common starting dates are April 1st, April 15th, or May 1st.
Depending on the specific crop or pest, different temperature thresholds are applied for GDD accumulation. A standard temperature threshold (base temperature, Tbase) used in agriculture is 50°F, assuming plant growth occurs above this temperature. The daily GDD is calculated using the maximum (Tmax) and minimum (Tmin) temperatures and Tbase. For example, if the maximum temperature was 90°F and the minimum temperature was 47°F, the daily GDD would be 18.5.
When maximum temperatures exceed a certain threshold, typically 86°F, crops may show signs of stress and limited growth. Modified Growing Degree Days (MGDDs) incorporate a ‘ceiling’, which caps the Tmax at the specified ceiling temperature. Additionally, if the low temperature falls below the Tbase, it is adjusted to the base temperature (since no meaningful growth occurs at lower temperatures).
Most commonly, MGDDs implement a base of 50°F and a ceiling of 86°F. In other words, if Tmax exceeds 86°F, Tmax is set to 86°F. If Tmin is below 50°F, then Tmin is set to 50°F. Using the Tmax and Tmin from the previous example, the daily MGDD calculated would be 18. There is not a big difference in this scenario. However, larger differences could be seen earlier in the season, leading to an underrepresentation of accumulated heat units.
It is essential to pay attention to how different tools calculate the GDDs, as this can explain why the products may show different accumulations. The Midwestern Regional Climate Center (MRCC) tracks accumulated MGDD (ceiling: 86°F, base: 50°F) throughout the growing season with two start date options: April 1 and May 1. Both include departures from the 1991-2020 climatological average. For example, the April 1 to May 14, 2025 map indicates that Indiana has seen between 300 and 550 MGDDs, which is within 20 units of normal.
The MRCC’s Corn GDD tool uses MGDD to track corn growth from a specified planting date and estimate key growth milestones. Users select their location and customize the GDD start date and maturity rating. The tool monitors GDD accumulation from planting to the current date and then uses typical GDD accumulations based on 30-year average (1991-2020) data to estimate future GDDs through the season. The tool also indicates when silking is expected and provides a range for achieving full maturity (black layer). In the provided example, Farmer P. Pete from Tippecanoe County, Indiana, chose May 5, 2025, for planting with a 108-day maturity hybrid. As of May 15, 2025, P. Pete’s corn silking is expected around July 15, with full maturity by September 13, assuming temperatures accumulate normally. Corn growth dates may shift based on weather patterns across the growing season.
The MRCC recently released the experimental Weed Emergence Scouting Tool, which helps producers track the estimated and peak emergence dates for giant ragweed and waterhemp based on GDD accumulation. Be sure to check it out or read our recent news release.
And beyond plants, GDDs are commonly used to track insect growth stages. The Iowa Environmental Mesonet offers a GDD tracking tool for Alfalfa Weevil and other pests, using a base temperature of 48°F and a ceiling of 90°F from entomology research. Users can set the start date and pest, receiving a map and GDD data for egg hatch (300 for Alfalfa Weevil) and peak larval feeding (575 for Alfalfa Weevil). It also predicts GDD accumulation for early pest detection over the next 7 and 14 days.
There are dozens of GDD calculators, maps, and tools; this article notes just a few. For more information, contact Austin Pearson at pearsona@purdue.edu.
