Farming

Understanding Growing Degree Days

Published:

October 30, 2024

Key Takeaways:

Growing degree days measures the amount of heat a crop receives during its growing season.
Measuring the accumulated GDD for a particular crop helps farmers monitor plant development and organize plant management activities.
Automated crop monitoring solutions can automate and simplify data collection and help calculate GDD more accurately.
GDD helps farmers figure out the best crops for their land and predict yields.

What Are Growing Degree Days

Growing degree days (GDD) is a measure farmers use to estimate the growing time for their crops. It's calculated based on the cumulative daily average temperatures from the day crops are planted until they are ready for harvest.

GDD measurements show the amount of heat energy crops receive, which is almost directly related to healthy growth (considering the absence of negative factors such as extreme weather, drought, or disease). Other terms commonly used for GDD include heat units and crop growing degree days.

Why Are Growing Degree Days Important to Measure?

GDD measures the cumulative average daily temperature during the growing season, also taking weather variations into account. For example, if a growing season is unusually cold, the accumulated GDD at a point in time is lower than it would be in a warm growing season.

Each plant species requires a certain number of heat units to grow, so measuring GDD allows a farmer to predict each stage of the crop’s development. For example, a crop like wheat goes through various stages, such as tillering, stem growth, head development, flowering, and fruit development. Knowing when these stages will take place allows the farmer to plan the best times for the application of fertilizers, fungicides, and other interventions as well as define when the crop will be ready for harvesting.

How to Calculate a Growing Degree Day

Key measures used in the GDD calculations include:

The daily average temperature.
The crop base temperature — the temperature above which a particular plant will grow.
The cumulative GDD value for a particular crop.

The first step in calculating GDD is to figure out the right parameters for the crop, such as its base temperature and required growing degree days. Once done, there are two main methods for calculating growing degree days: temperature averaging and the Baskerville-Emin method.

Temperature Averaging GDD Calculation. To calculate the GDD using the temperature averaging method, you measure the maximum air temperature (Tmax) in the field during the day and the minimum nighttime temperature (Tmin), usually in the early morning. You average these temperatures and subtract the crop's base temperature (Tbase) to calculate the GDD for that day. The accumulated growing degree days are the sum of the daily measurements.

This makes the growing degree days formula as follows:

Accumulated GDD = SUM ((Tmax + Tmin) / 2 – Tbase)

Baskerville-Emin Method. The Baskerville-Emin method starts with making the same daily calculation as in the temperature averaging method. Then, you have to fit the data to a sine curve and calculate the cumulative area under the curve. This method is more exact but is difficult to do by hand. However, you can automate

Use a crop monitoring solution such as Cropler’s innovative wireless agri-camera and web platform to gather detailed data on daily temperature.
Use custom formulas in Microsoft Excel, agricultural software, or online Baskerville-Emin calculators to determine GDD.

Resolving Difficulties With the Temperature Averaging Method

If you use the temperature averaging method to calculate GDD, remember the following points as they can cause inaccuracies:

Maximum day temperature: If the maximum day temperature doesn’t reach the crop’s base temperature, you should ignore the daily GDD calculation and enter a zero into your records for that day.
Average temperature: If the average temperature calculated for the day is below the base temperature, you should also ignore the result as it indicates there was no plant growth during that day or it was insignificant.
Upper threshold temperature: Most crops have an upper threshold temperature above which their metabolism shuts down and growth is limited. For example, for corn, it's around 86 °F (30 °C). If the maximum temperature exceeds this figure, use the upper threshold temperature in the calculation.

Benefits of Calculating GDD

The benefits of using GDD allow you to:

Predict crop growth and yields more accurately: You can use this technique to track plant development so you're prepared for each growth stage. It's also useful for estimating crop yields, which helps you plan harvesting time, storage and processing capacity, and the amount of crop available for sale.
Know when and how to fertilize: Using GDD calculations, you can determine the best time to apply organic or inorganic fertilizers for maximum yield and to measure their effectiveness.
Improve crop management: Over time, you'll be able to figure out growing degree days for each crop, as well as where and when to plant sensitive crops such as wheat. You can identify the average heat units each part of your farm gets and discover the best areas for the different crops you grow.
Enhance CSA practices: As you accumulate historical heat unit information, you'll be able to improve your climate-smart agriculture practices and optimize resource utilization.

When you have questions, we'll help you find the answers you need to take control of your fields and the harvest you care about. Contact Cropler to discover more about our products and the ways we help you proactively protect your operations from the effects of crop diseases. Learn More

Should You Measure the Soil or Air Temperature?

GDD measurements typically use the ambient air temperature. However, some authorities recommend using soil temperatures, especially early in the season. Their rationale is that the soil must reach a minimum temperature before seeds can germinate. Despite this, the most common way to calculate growing degree days is by measuring air temperature. Plant growth, such as for soybeans, is more closely correlated with the heat of the sun on the plant than with soil temperature.

Summing Up

The measurement of growing degree days is a well-established method used by farmers for monitoring crop growth and predicting yields. With the help of remote monitoring tools, the technique becomes simple, reliable, and predictable. It's also extremely useful for determining the most appropriate crops for different agricultural lands.