What Does “Growing Degree Days” Mean?

Growing Degree Days (GDD) are used to estimate the growth and development of plants and insects during the growing season, and to determine the best time to apply treatments for specific pests.

So what exactly are GDDs?

GDD is based on the number of days (since January 1 of the current year) during which the average temperature was higher than the minimum temperature needed for a plant, pest or disease organism to develop.

Each plant, insect and pathogen has a minimum temperature below which it cannot grow. That temperature is called the base temperature (TBASE). Once the average daily temperature goes above TBASE for that organism, it starts to grow.

Here in New Jersey / Pennsylvania, we use 50F as the base temperature in most calculations because that’s when woody plants in our area start to grow. So each day we look at the average temperature (the maximum temperature plus the minimum temperature, divided by 2) and subtract 50 to get the GDD for that day. If the average temperature is below 50F then we have 0 GDD. If, for example, the average temperature is 70F, we would have 20 GDD (i.e., 70 – 50).

There’s Method to Our Madness: Why We Schedule Visits To Your Property When We Do

Growing Degree Days (GDD) formula

Growing Degree Days (GDD) formula

Believe it or not, there’s a lot of planning and science that goes into when we schedule your property visits. And one important factor is the number of Growing Degree Days.

Tracking GDDs can be a bit time-consuming – unless you download a cool app to do it for you.

Or, since plant development is temperature dependent, the development of specific plants can be used to track growing degree days and predict pest development.

For example, we know that just after Star Magnolia and Forsythia first bloom (at about 83-86 GGD), Eastern Tent Caterpillar eggs will begin to hatch (around 90 GDDs).

So we often use the flowering sequence of plants as a biological calendar to predict pest activity and schedule pest management appointments to your property.

Since organic pesticides are usually specific to just one target pest, and don’t stay active long in the environment, it’s important to time applications when they will be their most effective.

Grubs, for example, are at their most vulnerable during their early development stages.  Treatment after that, even with more toxic, synthetic pesticides, is fairly ineffective and does more harm to the environment than good (and wastes a lot of your money).

Bill Grundmann

Bill Grundmann

Bill is the owner of Organic Plant Care, LLC. He takes an integrated approach to tree care, focusing not only on the trees themselves but also on the surrounding environment, managing tree health from the "ground up" - healthy soil equals healthy plants. Bill is a New Jersey Licensed Tree Expert and NJ Approved Forester with over 35 years of experience in the tree care industry.