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Growing Degree Units

Understanding growing degree units and how they help make crop management decisions

There are certain variables in farming that growers can control: seed type, fertilizer amounts and pest control actions, just to name a few. There are certain variables, however, that cannot be controlled, such as the weather. Ultimately, plants depend on the climate to grow.

Growers try to work with Mother Nature to make the best decisions for their crops. One way to do this is by using growing degree units. The growth and development of many crops depend heavily on temperature. Growing degree units, also known as growing degree days, are a tool used to measure the cumulative amount of heat to which the plant has been exposed.

Calculating growing degree units

To calculate growing degree units, farmers take a base threshold temperature and compared it to the average daily temperature to determine how much heat the plants have been exposed to.

The base temperature is a set figure based on the growth of the plant. For example, corn plants do not begin to develop and grow until temperatures reach 50 degrees. Therefore, when calculating the degree units for corn, growers use a base temperature of 50 degrees.

Growers find the average daily temperature by taking the low and high temperature of the day, adding them, and dividing by two. Note: generally, plants have a maximum temperature after which growing occurs no faster than before. For corn, the maximum temperature is 86 degrees, so even if the high that day is 90 degrees, growers would use 86 degrees as their high.

To calculate the growing degree units for a day, farmers take the average temperature and subtract it by the growth threshold (50 in the case of corn.)

Then, either after planting or after plant emergence, depending on what type of plant, the growing degree units for each date are added together to determine cumulative growing degree units.

Knowledge that pays

This cumulative measurement of growing degree units is very important to farmers as it can help decide future actions that need to be taken, not to mention influence preseason decisions as well.

For example, farmers deciding on seed hybrids look at the growing degree units to make sure that, on average, the seed they choose can grow well in the type of climate in their area based on past growing degree days.

During the season, knowing the cumulative growing degree units can help farmers determine the growth stage their crops have reached and approximately when they will reach the next stage. Essentially, the warmer the weather has been, the faster that plants will grow. This knowledge can help growers predict what is in store for plants and what assistance they might need. For instance, if the total growing degree units are less than what they typically are, an earlier nitrogen or herbicide application might be necessary.

Growing degree units also help farmers choose the right time to apply pesticides to avoid injury or improve effectiveness. In addition, growing degree units may indicate scouting for pests is needed at a different time based on the development of the plant, or farmers may use growing degree units to schedule irrigation.

Ultimately, employing growing degree units as tool can significantly assist you during the season and help you make the most of your crop. For more information on growing degree units, please consult your local Southern States Agronomy Expert or local Extension office.

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