How to Understand Growing Degree Days

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In every issue of the Plant Health Care Report, we list growing degree days (GDD) accumulated at The Morton Arboretum and other sites throughout Illinois. Knowing what they are and how to use them enables plant professionals and home gardeners alike to plan for pests and diseases.

The development of plants, insects, and fungi is dependent on heat. Development speeds up as the temperature rises and slows as temperatures decrease. Many plants and insects have been studied in regard to this relationship between heat and development. We can anticipate the flowering of a shrub, or the emergence of an insect, based on how many growing degree days have accumulated. We can give this information to our scouts and ask them to look for specific problems based on GDD. This helps to refine the process of scouting. 

Accumulation of GDD can vary widely from year to year, and by tracking that information we can be more accurate than if we just looked at the calendar. For example, eastern tent caterpillars hatch out of their eggs when GDD base 50 is between 100 and 200. In 2014, we had accumulated 100 GDD by May 9. We often do expect to see this pest in mid-May, so 2014 was fairly ‘average.’ In 2012, we had accumulated 100 GDD by March 19 (nearly two months earlier than ‘normal’). If we had gone with the calendar method and waited to deal with this pest in May, we would have missed it completely. If we plan to use any type of insecticide for control, having a better idea of when a pest might actually be present allows us to use that insecticide more responsibly and accurately.

GDDs are fairly easy to calculate: We use GDD base 50. Add the maximum temperature to the minimum temperature for a day, divide by two, and subtract 50 (the base number). If the number resulting from this calculation is above zero, then that is the number of degree days (think of them as ‘units’ rather than days, if that is easier) for that day. If the result is zero or below, then the number of GDD is zero for that day. These growing degree days are cumulative. When we have accumulated 100 GDD, we expect certain insects to begin emerging (and certain plants to be in flower). When we get to 500 GDD there will be different insects emerging and different plants flowering. We use base 50 because 50 degrees F is the temperature at which most plants and pests begin to grow.