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Growing Degree Days

by Zahid Ahmed
Published: Last Updated on
Growing Degree Days

Growing degree days (GDD) is a measure of heat accumulation and predict when a crop will reach maturity or when certain pests will appear. It is also used in horticulture to determine the appropriate time to plant and harvest crops.

It is calculated by subtracting a base temperature from the average daily temperature and summing the results over a period of time. The base temperature is the minimum temperature at which a particular plant or insect can grow or develop, and the average daily temperature is the mean of the daily maximum and minimum temperatures. Here is the formula:

GDD = (Tmax + Tmin)/2 – Tbase

Where:

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  • GDD is the growing degree days
  • Tmax is the daily maximum temperature
  • Tmin is the daily minimum temperature
  • Tbase is the base temperature

History of Growing degree days

The concept of growing degree days (GDD) has been used for over a century to predict the development of plants and insects. The first recorded use of GDD dates back to the late 1800s, when it was used to study the development of wheat rust in France.

In the early 1900s, GDD was used to predict the development of various crop pests, including the European corn borer and the Colorado potato beetle. In the 1920s, GDD was used to study the impacts of temperature on the development of fruit trees in California.

Over the years, the use of GDD has expanded to a wide range of crops and insects, and it has become a widely used tool in agriculture and pest management.

Uses of Growing Degree Days

Growing degree days (GDDs) are units that are used to measure the accumulation of heat over a period of time, particularly in relation to the growth and development of plants. Here are some examples of how growing degree days are used:

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Agriculture

GDDs are often used in agriculture to predict the optimal time to plant or harvest crops. For example, a farmer may use GDDs to determine when to plant corn, which has a base temperature of 50°F. If the mean temperature for a particular week is 60°F, then the GDD accumulation for that week would be 10 (60 – 50 = 10). If the farmer knows that corn typically requires 1000 GDDs to reach maturity, they can use this information to determine when to plant the corn in order to maximize yield.

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Pest management

GDDs can also be used in pest management to predict the lifecycle and population size of certain insects. For example, the Colorado potato beetle has a base temperature of 50°F and requires approximately 1000 GDDs to complete its lifecycle. By tracking the GDD accumulation in a particular location, a farmer can anticipate when the potato beetles will emerge and take steps to control their population.

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Horticulture

GDDs are also useful in horticulture for predicting the optimal time to prune or fertilize certain plants. For example, a gardener may use GDDs to determine when to prune their roses, which have a base temperature of 40°F. If the mean temperature for a particular month is 50°F, then the GDD accumulation for that month would be 100 (50 – 40 = 10, and there are approximately 10 days in a month). If the gardener knows that roses typically require 1000 GDDs before they are ready to be pruned, they can use this information to determine the best time to prune their roses.

Additional uses

In addition to being used in agriculture and pest management, growing degree days can also be used in a variety of other contexts. Some additional uses of GDD include:

  • Plant breeding: GDDs can be used in plant breeding to identify the optimal conditions for growing and developing new plant varieties. By tracking the GDD accumulation in different locations and under different conditions, plant breeders can identify the conditions that lead to the fastest and most successful plant growth.
  • Agricultural modeling: GDDs are also used in agricultural modeling to predict crop yields and optimize resource management. For example, a farmer might use GDDs to predict the yield of a particular crop under different irrigation or fertilization regimes.
  • Energy management: GDDs can be used in energy management to optimize the use of heating and cooling systems in buildings. For example, a building manager might use GDDs to determine when to turn on the heating or cooling systems in order to maintain a comfortable temperature inside the building.
  • Spread of diseases: Some plant diseases are more prevalent at certain GDD ranges, and tracking GDD can help to predict the likelihood of disease outbreaks.
  • Climate change: By comparing GDD data from different time periods, researchers can study the impacts of climate change on plant and insect development.
  • Suitability of a location: By comparing the GDD of a particular location with the GDD requirements of a particular crop, farmers can determine whether the location is suitable for growing that crop.
  • Forecasting the onset of spring: In some regions, the arrival of spring is marked by the accumulation of a certain number of GDDs. By tracking GDD, people can predict when spring is likely to arrive.
  • Bird migration: Some bird species migrate in response to changes in GDD, and tracking GDD can help to predict when these species are likely to migrate.

Calculation of Growing Degree Days

To calculate growing degree days (GDD), you will need to know the following:

  1. The base temperature: This is the temperature below which a plant or insect will not develop, and it varies depending on the species. For example, corn has a base temperature of 10°C, while soybeans have a base temperature of 6°C.
  2. The average daily temperature: This is the average temperature for each day over a given period of time. To calculate the average daily temperature, you will need to record the minimum and maximum temperatures for each day and take the average of these two values.

To calculate GDD, follow these steps:

  • Subtract the base temperature from the average daily temperature for each day.
  • If the result is negative, set it to zero (since GDD cannot be negative).
  • Sum the results for each day to get the total GDD.

Example 1

For example, let’s say that the base temperature for a particular plant is 10°C and the average daily temperature for a given week is as follows:

Day 1: 15°C
Day 2: 12°C
Day 3: 18°C
Day 4: 16°C
Day 5: 13°C
Day 6: 20°C
Day 7: 14°C

To calculate the GDD for this week, we would subtract the base temperature of 10°C from the average daily temperature for each day and sum the results:

Day 1: 15°C – 10°C = 5°C
Day 2: 12°C – 10°C = 2°C
Day 3: 18°C – 10°C = 8°C
Day 4: 16°C – 10°C = 6°C
Day 5: 13°C – 10°C = 3°C
Day 6: 20°C – 10°C = 10°C
Day 7: 14°C – 10°C = 4°C

The total GDD for the week would be 38°C (5°C + 2°C + 8°C + 6°C + 3°C + 10°C + 4°C).

Example 2

As a second example, a day with a high of 15°C and a low of 7°C (and a base of 12 °C) would contribute:

Version A: (15 + 7) / 2 – 12) = -1 = 0 (If the result is negative, set it to zero (since GDD cannot be negative).
Version B: (15 + 12) / 2 = 13.5 – 12 = 1.5

Optimal base temperature of crops

The base temperatures provided are general guidelines and may vary depending on the specific variety of the crop and the conditions in which it is grown. It is important to consult reliable resources, such as a trusted horticulture expert or a reputable reference manual, to obtain accurate base temperatures for GDD calculations.

Here is a list of some common crops and their optimal base temperatures for growing degree days (GDD) calculations, according to the United States Department of Agriculture (USDA):

  • Corn: 50°F (10°C)
  • Soybeans: 50°F (10°C)
  • Wheat: 32°F (0°C)
  • Oats: 32°F (0°C)
  • Barley: 32°F (0°C)
  • Potato: 40°F (4.4°C)
  • Tomato: 50°F (10°C)
  • Pepper: 50°F (10°C)
  • Eggplant: 50°F (10°C)
  • Squash: 50°F (10°C)
  • Watermelon: 50°F (10°C)
  • Cantaloupe: 50°F (10°C)
  • Cucumber: 50°F (10°C)
  • Carrot: 40°F (4.4°C)
  • Beet: 40°F (4.4°C)
  • Radish: 40°F (4.4°C)
  • Onion: 40°F (4.4°C)
  • Garlic: 40°F (4.4°C)
  • Lettuce: 40°F (4.4°C)
  • Spinach: 40°F (4.4°C)

Optimal base temperature of insects

The optimal base temperature for calculating growing degree days (GDD) for insects varies depending on the species of insect. Here is a list of some common insect pests and their optimal base temperatures for GDD calculations, according to the United States Department of Agriculture (USDA):

  • Colorado potato beetle: 50°F (10°C)
  • Corn earworm: 50°F (10°C)
  • Cutworms: 50°F (10°C)
  • Armyworms: 50°F (10°C)
  • Cabbage worm: 50°F (10°C)
  • European corn borer: 50°F (10°C)
  • Imported cabbageworm: 50°F (10°C)
  • Corn rootworm: 50°F (10°C)
  • Soybean aphid: 50°F (10°C)

Growing degree days limitations

Growing degree days (GDD) is a useful tool for predicting the development of plants and insects, but it has some limitations that should be considered when using it.

One limitation of GDD is that it is based on temperature alone and does not take into account other factors that can affect plant and insect development, such as moisture, nutrients, and sunlight. As a result, GDD is not always a reliable predictor of plant and insect development in all situations.

Another limitation of GDD is that it assumes a linear relationship between temperature and development, but this relationship is often not linear in reality. In other words, the rate of development may not increase evenly as temperature increases. This can lead to inaccuracies in GDD predictions.

Additionally, GDD calculations are based on the assumption that there is a single base temperature below which development does not occur. However, many plants and insects have a range of base temperatures within which development is optimal, and GDD calculations may not accurately reflect this.

Finally, GDD calculations are based on daily average temperatures, which may not accurately represent the temperatures experienced by plants and insects over the course of a day. For example, plants and insects may be exposed to higher temperatures during the day and lower temperatures at night, and GDD calculations do not take this diurnal variation into account.

Despite these limitations, GDD can still be a useful tool for predicting plant and insect development in many situations. Therefore, it is important to consider the limitations of GDD when using it and to supplement it with other information as needed.

Conclusion

Growing degree days (GDD) is a measure of heat accumulation that helps predict the development of plants and insects. It is commonly used in agriculture and pest management to forecast crop development and insect emergence. While GDD is a useful tool, it has limitations, including that it only considers temperature and does not account for other factors that can affect development, such as moisture, nutrients, and sunlight. Despite these limitations, GDD remains a valuable tool for predicting plant and insect development in many situations.

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