Growing Degree Day Meaning for Plants and Insects

As a follow-up to the previous piece on pollinators and native plants, I thought a brief discussion of Growing Degree Days (GDD) would be interesting. I seem to do this every year or two, but it really helps to reinforce our previous discussion on the relationship between plants and pollinators. Nature provides plants at the right time for pollinators simply because they are on the same schedule.

Plants and insects flower and hatch based upon the weather, specifically temperature, and not an actual date. If you think back to previous Springs, you realize, that some years when we have warm stretches, all the plants tend to flower at the same time and for shorter periods, but some years when Spring comes gradually, plants tend to flower longer and not all bunched together. Plants start and finish flowering on pretty specific GDDs. Magnolias in Spring are a great example; They can flower for weeks with a cool Spring or a few days if there is a heat wave, depending upon how quickly we accumulate GDDs.

This comes down to a very simple formula. Sorry, I am geeking out a bit like in the previous newsletter, but understanding some of the science in horticulture can be helpful. Basically, you take the high temperature for a day and add it to the low temperature and divide by 2 to get the average.  Then subtract 50º from that average.  If we get a positive number, the we add it to previous positive numbers in the season. In the Spring, we are often adding single digits and slowly accumulating a few hundred GDDs, but by the end of summer we have accumulated many thousand GDDs. 

Max Temp (ºf) + Min Temp (ºf)
—————————————-   – 50  =  GDD
                        2

Since it is cold in the Winter, plants don’t grow and insects don’t hatch, but once we start to accumulate GDDs, the world comes alive.  This relates to our earlier discussion because when our native insects and pollinators emerge, they have specific plants flowering at the same time that they are mean’t to feed on and pollinate, and when they are not present, they struggle to survive and parts of the system collapse.  As Dr Gegear eloquently states, these native insects may go to non-native plants to feed, but it is ‘fast food’ and does not support a healthy system that allows pollinators to feed on good pollen and pollinate important native plants to produce fruit and seeds.

National Phenology Network – Is a great site that provides broad information on the study of the relationship between animals, insects and climate.

Mass Landscape Message – Seasonal newsletter that comes out of UMass that provides local information including GDDs for different regions in MA.

Climate Smart Farming at Cornell University – Best tools around for tracking this information and Cornell is such a great resource on many levels.

Ohio State Plant/Insect List with GDD – This is a great combined list of ornamental plants, native plants, insects and weeds. It provides the GDDs for flowers or insects to emerge.  With a weather forecast and this list you can become Nostradamus and predict when flowers will bloom. So valuable just having this list help me to remember the order in which plants flower. I am looking for a detailed native plant list.

This phenological process allows plants insects and animals to evolve each season in stride based upon the weather, and this year it looks like everything is going to wake up early.