Growing Degree Days

KEY CONCEPTS

• Growing degree days (GDD) measure heat accumulation over the growing season, and are an important indicator of plant and pest development. 

• Between 1970 and 2021, the number of GDD increased at 97% of 246 stations analyzed across the contiguous U.S.

• The largest increases in GDD since 1970 were recorded throughout the West, Southwest, and South.

• Some plants and animals may benefit from more GDD and longer growing seasons, but more GDD can also mean more water and heat stress, and longer pollen and pest seasons.

Meteorological spring is underway and many parts of the country are looking forward to a productive growing season. As soils thaw and the first blooms appear, we take a close look at nationwide trends in a critical growing season indicator: growing degree days.

Growing degree days matter—for agriculture, ecology, and even health.

The rate of plant and insect development is largely determined by how much heat they are exposed to over the growing season—from planting to harvest. Growing degree days (GDD) are a measure of this heat accumulation. 

Each plant species has a different base air temperature—a threshold below which growth is limited. GDD is the difference between average daily temperature and the base temperature. For example, corn, soybeans, sorghum, and tomatoes all have a base temperature of 50 °F. For these crops, a day with an average temperature of 60 °F would equal 10 growing degree days, or GDD.

In agriculture, the running sum of daily GDD over the growing season, or accumulated GDD, can be used to track crop (and pest) growth stages and predict when they will be ready for harvest. For example, corn typically reaches maturity around 3,000 accumulated GDD.

But GDD isn’t all about agriculture. It also impacts the timing and length of pollen seasons that cause allergies and other health effects. And the development of many other plants and insects in the wild and in home gardens—both beneficial (e.g., pollinators) and harmful (e.g., pests)—is also linked to GDD.

Trends in growing degree days since 1970

Understanding long-term trends in GDD can help inform decisions in agriculture, natural resources management, and public health as each of these sectors adapt to the impacts of climate change.

Climate Central analyzed trends in GDD with a base temperature of 50 °F at 246 locations across the U.S. from 1970 to 2021 and found that:

• Between 1970 and 2021, the number of GDD increased at 97% of the 246 analyzed stations across the contiguous U.S.

• Over this period, GDD increased by 525 on average, ranging from -140 in Great Falls, Mont. to +1818 in Reno, Nev. The average rate of change was 10 additional GDD per year.

• The 25% of stations with the largest increases in GDD (+687 or more) were mainly located in the West, Southwest, and South.

• The five largest increases in GDD were in: Reno, Nev. (+1818), Las Vegas, Nev. (+1816), Tucson, Ariz. (+1585), El Paso, Texas (+1585), and Phoenix, Ariz. (+1537).

Impacts of more growing degree days 

Temperature is a key factor for plant development and some plants may benefit from more GDD. But the GDD metric doesn’t consider other important factors such as precipitation. Warmer growth conditions cause more evaporation from the land surface and greater potential for water stress in plants, especially in the drought-prone regions of the West and Southwest that have seen some of the largest increases in GDD since 1970.

Regardless of rainfall, changes in GDD also mean changes in farmers’ planting, irrigation, pest management, and harvest schedules and strategies. 

It's also important to note that Climate Central’s GDD analysis does not include a maximum temperature threshold, and therefore doesn’t account for the occurrence of extreme heat and its impacts on plant growth, which can include heat stress and reduced productivity and yield.

There is also a correlation between GDD and longer and/or earlier pollen seasons for grasses, oaks and birch. Allergic responses to pollen such as hay fever and asthma already affect millions of Americans and account for tens of billions of dollars in economic losses each year. And these impacts could worsen with more GDD and longer pollen seasons.

POTENTIAL LOCAL STORY ANGLES

What’s the national outlook for GDD in 2022? The USA National Phenology Network has a visualization tool that can be used to map and explore the current and past nationwide accumulated GDD and other phenological indicators. 

And the regional outlook for GDD? In the Northeast, Cornell University’s Climate Smart Farming program has a GDD outlook for the 2022 growing season, searchable by zip code for a range of base temperatures. In the Upper Midwest region, the North Dakota Agricultural Weather Network has GDD primers and data for regionally-important crops. 

How could GDD change in the future under climate change? The U.S. Department of Agriculture (USDA)’s interactive map, Climate Change Pressures in the 21st Century, compares projected future changes in GDD under different emissions scenarios.

How is the allergy season progressing this year? The National Allergy Bureau’s Aeroallergen Network provides station-level allergen reports across the U.S.

LOCAL EXPERTS 

Looking for local agriculture and climate expertise? The USDA’s Land-Grant University Partner Map provides links to extension offices in all 50 states to get connected with scientists, educators, and extension staff in your local area. 

The SciLine service, 500 Women Scientists or the press offices of local universities may be able to connect you with local scientists who have expertise on growing seasons, phenology, and climate change. The American Association of State Climatologists is a professional scientific organization composed of all state climatologists. 

NATIONAL EXPERTS 

• William Anderegg, PhD
  Associate Professor, School of Biological Sciences, University of Utah
  Contact: anderegg@utah.edu
  Related expertise: climate change impacts on forest ecosystems and pollen seasons

• Suat Irmak, PhD
  Professor and Head of Agricultural and Biological Engineering
  The Pennsylvania State University
  Contact: sfi5068@psu.edu
  Related expertise: climate change impacts on agricultural productivity

• Fiona Lo, PhD
  Postdoctoral Researcher, School of Public Health, University of Washington
  Contact: fionalo@uw.edu
  Related expertise: climate change impacts on allergenic pollen seasons and health

• Meetpal S. Kukal, Ph.D.
  Assistant Research Professor
  Department of Agricultural and Biological Engineering
  The Pennsylvania State University
  Email: msk5779@psu.edu
  Related expertise: Climate sensitivity & resiliency of ag systems

METHODOLOGY

Analyses used 1970-2021 data from the Applied Climate Information System. A base temperature of 50 degrees was used to calculate growing degree days. Growing degree days include the entire year. Displayed trend lines are based on a mathematical linear regression. Climate Central's local analyses include 247 stations. However, for data summaries based on linear trends, only 246 stations are included due to large data gaps in Wheeling, W. Va.