What Are Growing Degree Days, and Why They Matter Now More Than Ever

Andy Paterson • July 21st, 2025.

Climate change will impact crops in numerous ways. With extreme heat, drought, and extreme weather events all having an impact. These changes are measured in litres, degrees Fahrenheit, and tons of yield lost, but one other lesser-known, yet equally important measurement is growing degree days (GDD). 

GDD is a unit of measurement that helps growers and agribusinesses determine when to plant, when to harvest, and whether to worry about pests for the crop they are growing and where they are growing it. 

Measuring GDD requires a simple equation. However, the temperatures used in the equation need to be highly localized and specific to each crop. Climate change will further complicate this, making historical averages unreliable and real-time decision-making essential. 

This article will dive deeper into what GDD is, its importance for growers, how it’s measured, and why accurate forecasting using weather intelligence tools is essential, especially in a quickly changing climate.

What Are Growing Degree Days?

A Growing Degree Day (GDD) is a measure of heat accumulation used to estimate plant phenology—the timing of plant life cycle events, such as flowering and harvest.

GDDs are also used to predict pest development cycles, helping determine when specific pests are likely to emerge based on accumulated heat over time. This makes them a critical tool in planning pest control strategies and aligning them with actual conditions in the field. 

GDDs work by tracking the number of days where temperatures exceed a certain baseline (or threshold), typically the minimum temperature needed for plant development (50°F for corn and soybeans).

More GDDs usually mean faster plant growth. However, rapid accumulation of GDDs can lead to early maturation, crop stress, or increased pest activity. Think of GDDs as a way of finding the Goldilocks zone for both plant growth and pest management, not too cold, not too hot, but just right.

GDDs can be used retrospectively to estimate the optimal time to harvest or can be forecasted for the upcoming season to predict when key growth stages will occur, thereby determining the best time to plant and to plan out operations for the growing season. 

Why Are GDDs Important?

• More accurate than calendar days: Farmers have traditionally relied on planting calendars, which provide a generic number of days for each crop to reach maturity, to estimate crop development. However, GDD offers a more precise way to track plant progression throughout the growing season.
• Predicts key crop milestones: GDD helps determine when a crop will reach stages such as flowering, fruiting, or maturity, enabling better planning for harvest, irrigation, and the use of fertilizers and pesticides.
• Improves pest management: GDD is a reliable proxy for predicting pest development. It supports Integrated Pest Management (IPM) by identifying the optimal timing for insecticide or pest control applications.
• Essential under climate change: As climate patterns shift, traditional benchmarks like average frost dates and fixed maturity timelines from planting calendars become less reliable. GDD adapts to these changes, offering a dynamic tool for adaptation and resilience.
• Supports supply chain & processing coordination: For crops like potatoes, GDD tracking can help forecast daily harvest volumes, providing downstream teams and processors with advance notice of what’s coming. This enables better capacity planning, minimizes bottlenecks, and reduces waste at facilities.

How Do You Measure Growing Degree Days

To determine the number of GDDs for each day, a simple equation is used, which takes the maximum and minimum temperatures for a day and measures them against the threshold temperature for each crop.

What is the Growing Degree Day Equation

The equation to find that number looks like this: 

(TMax + TMin) /2 – Tbase= GDD

• Tmax = daily maximum temperature capped at a crop-specific upper threshold
• Tmin = daily minimum temperature with a crop-specific capped minimum
• Tbase = base temperature for plant growth

To use winter wheat as an example. Winter wheat is a cold-weather crop, so its baseline (Tbase) would be 32°F (0°C). If a day had a Tmax of 80°F and a Tmin of 64°F, the equation would be:

(80 + 64) /2 – 32= 25 GDD

This would mean this day has a 25 GDD to add to the season total. Each crop needs a specific amount of GDDs to reach certain points of maturity for winter wheat. It varies by region and species, but on average, the total growing season requires 2,000–2,400 GDDs, 1,200–1,400 GDDs from seeding to flowering, and 800–1,000 GDDs from flowering to maturity.

The faster or slower these GDDs are accumulated will determine the speed of growth that season. The slower it happens, the longer the growing season will be. The quicker it accumulates, the greater the chance of early maturity, but that also increases the likelihood of pest infestations.

Growing Degree Days and Climate Change

GDD is a reliable method for assessing the impact of climate change on specific crops, their surrounding ecosystems, and how undeveloped regions will become viable in the future. One study that used GDDs to evaluate new growing zones in the U.S. found that viable agricultural zones could shift 1,200 KM north by 2099. 

Typically, a warmer climate due to climate change means more growing degree days, which accelerate crop development. However, the rapid accumulation of GDDs at certain stages can increase the risks of reduced yields and pest infestations. 

Another study on climate change and GDDs suggests that GDDs are accumulating at a faster rate, resulting in most crop stages occurring approximately one day earlier per decade, with large variation across different regions. This shift, combined with increased regional variability, underscores the growing need for highly localized GDD forecasting and adaptive crop management. 

The Importance of Accurate Forecasts for GDD

Climate change is making historical averages an unreliable source to measure GDD. Growers need highly localized, accurate, and real-time GDD information to make informed decisions. 

Historically, GDD measurements have been tracked on spreadsheets that lack the regional granularity and crop variety specificity necessary to be decision-useful. ClimateAi’s new GDD tool uses our proprietary climate forecasts and 30-year climatology to provide real-time, field-level visibility into crop progress, enabling proactive decisions that directly improve yield quality, reduce losses, and streamline operations.

ClimateAi’s new GDD tool will enable: 

• Visualization of GDD accumulation with clear graphs and field-level maps that can track multiple crop varieties and locations simultaneously with daily updates.
• Tracking of crop stages with GDD Targets specific to each variety.
• Accurate predictions of harvest readiness date and key growth milestones (e.g., bulking, training, flowering, ripening)
• Growers to have better calendar planning for harvest, labor, and logistics
• Iteration year-over-year, incorporating grower on-the-ground knowledge to improve accuracy
  

How Pea, Potato, and Hop Growers Benefit From ClimateAi’s GDD Tool

As climate change evolves, decisions around when to harvest and how to organize labor and logistics become more complex. Accurate GDD tools are among the most effective tools in a grower’s arsenal, enabling them to produce the highest-quality product possible and optimize their operations. 

Here is how our GDD Tracker is helping growers of the following crops:

• Peas: For peas, the timing of harvesting and the tenderness of the crop can significantly impact the price they can sell for. The window for perfect pea harvesting is 24 hours. Our tool enabled growers to measure an accurate harvest time, allowing them to pre-plan labor and other operations and make localized, field-by-field decisions to secure the best possible price. 
• Potatoes: By monitoring GDDs, soil temperatures, and heat/cold stress, we can accurately track potato development from sprouting to harvest. This allowed for better disease management, optimized harvest timing, and improved yield forecasting across multiple fields. Additionally, potato growers were able to understand what volume of potatoes is likely to be harvested for processing on any given day. 
• Hops: We were able to provide precise timing of key crop management tasks, such as trimming, pruning, and fertilization, across complex, sensitive growth stages. Integration with weather systems and growth stage-specific GDD targets enhanced disease control and harvest planning, resulting in more efficient resource utilization and better coordinated operations.

Growing Degree Days are a critical tool for growers to understand how their crop varieties will grow in their region and to make informed decisions. Historically, this has been achieved by relying on regional forecasts and historical data tracked through spreadsheets. But to get the most out of GDDs, forecasts require hyper-local, field-level intelligence with daily dynamic updates. 

With climate change making the need for advanced GDD tools even more pressing, our team is ready to enable you with the granular information you need to get the most out of the crops you grow where you grow them

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