What is a Growing Degree Day?

Many of our products need to be applied at a certain time in the life cycle of an insect or fungal pathogen in order to achieve maximum efficiency.  Fortunately for us we have a reliable way to predict the emergence of these problems: the Growing Degree Day (GDD)!  Many species of insects are known for arriving during a specific time of the summer, but years of research by dedicated scientists have shown that the emergence of various stages of insects can reliably be timed to a certain number of degree days.  Tree development can also be timed in this way.  GDDs are the way entomologists, botanists, and arborists (like us!) gauge our timing of treatments, and can be reliably calculated using basic weather data available from whatever your favorite weather information source may happen to be!

Growing degree days are actually am measure of the accumulation of heat in a given area, and the number of GDDs accumulated per day are calculated using a specific formula: GDD= (High Temperature+Low Temperature/2)-Base Temperature.  Most people use 50 degrees F for a Base Temperature.  You add up the GDD’s per day for a given season, and you begin to see patterns emerge over time as to when things start happening based on the GDDs.

For Example:

Hypothetical day 1 has an overnight low of 42 degrees F and a Daytime High of 70 degrees F.
42+70= 112.
112/2= 56.
56-50= 6 Growing Degree Days.
Hypothetical day 2 has an overnight low of 44 degrees F and a daytime high of 73 degrees F.
58.5-50= 8.5 Growing Degree Days.
Those two hypothetical days have a total of 14.5 Growing Degree Days accumulated.

If the spring has been long and cold with a gradual warming, you’d expect to see lower GDD- which would lead to later development of plant structures, such as flowers and leaves, and later emergence of insect pests and fungi.  Earlier, warmer springs would lead to earlier emergence- but it’s still tied to the accumulated Growing Degree Days.

For information on the Growing Degree Days in your area, check out page of regional GDD information!

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