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Growing Degree Days

 County Agent News

Dan Folske

February 26, 2018

 

Growing Degree Days

            Growing Degree Days (GDD), sometimes called Growing Degree Units (GDU), and Days to Maturity are references often used to predict the suitability of annual crops for an area or to predict when an annual crop may be ready for harvest. GDDs can also be used to estimate when you need to scout for potential insect problems.

            Growing Degree Days (GDD) are used to estimate the growth and development of plants and insects during the growing season. The basic concept is that development will only occur if the temperature exceeds some minimum development threshold, or base temperature (TBASE). The base temperatures are determined experimentally and are different for each organism.

  To calculate GDDs, you must first find the average temperature for the day. The average temperature is found by adding together the high and low temperature for the day and dividing by two. If the average temperature is at or below TBASE, then the Growing Degree Day value is zero. If the average temperature is above TBASE, then the Growing Degree Day amount equals the average temperature minus TBASE. For example, if the average temperature was 75° F, then the GDD amount equals 10 for a TBASE of 65° F.

            To use this information, the GDD for a single day is really not much use. It is the accumulation of GDDs that is useful. By tracking the accumulation of GDDs you can predict when corn rootworms may emerge or when an insect like the Orange Wheat Blossom Midge may be found in a wheat field. For vegetables, you may be able to use historical average GDD accumulations to predict when you will have radishes ready for sale to a local restaurant or at a farmer’s market. Or you may be able to make an informed decision about whether or not a particular vegetable may be able to reach maturity before it is killed by frost.

You don’t have to record and calculate GDDs every day unless you want to. The NDAWN weather stations like the one just north of Bowbells records daily temperatures and can provide you with accumulated GDDs. Google “NDAWN” or go to https://ndawn.ndsu.nodak.edu/ . Under “applications” you will find links to GDD information for many of the crops raised in North Dakota.

            GDDs are much more accurate than estimated days to maturity. Unfortunately, it is more difficult to find GDD requirements than to find estimated days to maturity for many varieties of vegetables. Many seed catalogs will provide “Days to Maturity” estimates for many of the varieties they offer. These are helpful when comparing the relative maturity of different varieties.

            When working with Days to Maturity remember that it usually means days after emergence or days after transplanting. And remember that it will vary considerably if you get a colder than normal period and drought stress can also affect Days to Maturity.

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