In every issue of The Morton Arboretum’s Plant Health Care Report, growing degree days (GDD) accumulated at Arboretum and other sites throughout Illinois are listed. Knowing what they are and how to use them enables plant professionals and home gardeners alike to plan for pests and diseases.
What are Growing Degree Days?
Growing degree days (GDD) are not really days, they are units of accumulated heat. The development of plants and insects is dependent on heat. Development speeds up as the temperature rises and slows as temperatures decrease. When 100 GDD have accumulated, certain insects are expected to begin emerging (and certain plants to be in flower). At 500 GDD there will be different insects emerging and different plants flowering.
Many plants and insects have been studied in regard to this relationship between heat and development. The flowering of a shrub or the emergence of an insect can be anticipated based on how many GDD have accumulated. At The Morton Arboretum, we 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. We are proactively looking for pests instead of waiting for them to show up. This lets us get ready to defend our gardens against these pests. GDDs are used by farmers, arborists, and landscapers as well. Even home gardeners can use them.
How Do We Calculate GDD?
There are different methods for calculating growing degree days (GDD).
At The Morton Arboretum, we use a very simple mathematical formula and base 50, because 50 degrees F is the temperature at which many plants and pests begin to grow. There are other bases that can be used, so when using data from any website, check to see what base number is being used.
To calculate GDD using this basic method, first add the maximum temperature for the day to the minimum temperature for the day. Then divide by two to get the average temperature of that day. Finally, subtract 50 (the base number). If the number resulting from this calculation is above zero, then that is the number of degree days for that day (again, think of them as units rather than days, if that is easier). If the result is zero or below, then the number of GDD is zero for that day. Growing degree days are cumulative. This calculation is done every day, and then the GDD for today is added to the sum from previous days. The GDD number never goes down. It either stays steady or goes up. 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.
How Do We Use GDD?
In the past, the emergence of insects was tied to calendar dates. That method did not always work. Accumulation of heat (and thus GDD) can vary widely from year to year due to fluctuations in weather. By tracking that information it is possible to be more accurate than just looking 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 expect to see this pest in mid-May, so 2014 was fairly average. Two years earlier, in 2012, we had accumulated 100 GDD by March 19 (nearly two months earlier than usual). If we had gone with the calendar method and waited to deal with this pest in May, we would have missed it completely.
Using GDD to know more precisely when an insect pest might arrive can help us in a number of ways. 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. Organic gardeners can be prepared to employ various non-chemical measures in a timely fashion. A floating row cover can be put in place over a row of vegetables to keep insects off the plants. Planting times can sometimes be adjusted so that the plant is not yet planted when the pest arrives.
Indicator Plants
Plants, like insects, grow and develop based on accumulated heat. Many plants can be used as indicators. A particular species being in flower tells about how many GDD have accumulated, and from that it can be surmised that certain pests will be present now or in the very near future.
When we have accumulated 100 GDD, we expect certain insects to begin emerging (and certain plants to be in flower). For example, European pine sawfly is a pest that emerges at GDD50=100 to 200. Redbud is a tree that begins to flower around that same GDD level, so it can be an indicator plant for us. When we see redbud starting to flower, we know that European pine sawfly should be starting to feed on pine needles. Even if we are not tracking GDD numbers, plants are, and they are telling us how much heat has accumulated.