By Lindsey Rice
ISA Certified Arborist® OH-6739A
January 9, 2025
If you are a client of Russell Tree Experts and have us care for your trees, you’ve likely heard us use the term “growing degree day” or GDD. The concept of a growing degree day is basically how many units of heat have been accumulated within a 24-hour period. Any day with a mean temperature (average of the maximum and minimum temperature) over the “threshold temperature” of 50 degrees, growing degree days are accumulated. A threshold temperature is a temperature at which a specific plant or insect can progress in its development or life cycle. Different organisms have different base temperatures; however, 50 degrees Fahrenheit is considered acceptable for all plants and insects.
Calculating growing degree days can be complex to varying degrees. The simplest method is to subtract the base temperature from the mean temperature for the day and then add it to the previous day's growing degree day total.
Example:
It is April 11th. The average temperature is 61 degrees. Yesterday, the running total of GDD was 214. Subtracting the base temperature of 50 from the mean temperature of 61 degrees, we would arrive at 11 degrees. We would add that to yesterday's running total (214+11), and today’s current GDD is 225.
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What information can we learn from knowing the growing degree day? Plant experts and entomologists have observed that the accumulation of heat causes development to proceed. For any given insect, its eggs will hatch, or an adult will emerge at precisely the same growing degree day every year. The date can vary from year to year since the temperature is not the same every day of the year. Below is a graphic of the growing degree day on the same date, April 11th, for the past fourteen years.
(https://weather.cfaes.osu.edu/gdd/summary.asp):
Since we know that plants and insects do the same thing on the same approximate GDD every year (not necessarily on the same date), we also know when to apply certain treatments for optimal effectiveness. Additionally, we can use plants as a “phenological indicator” to understand when certain insects will be active. Using our example of April 11th and GDD of 225, Serviceberry (GDD 219) and Common Chokecherry (GDD 221) should both be in full bloom. Various adult leafminers will also be emerging around this time (birch leafminer – GDD 215, elm leafminer – GDD 219, alder leafminer – GDD 224).
Some people have even taken it further and planted phenological gardens! Farmers have used this practice for centuries to know when to plant certain crops and vegetables and when to look for specific pests. Here are a few rules of thumb:
Plant potatoes when the first dandelion blooms
Plant peas when forsythia blooms
Plant radishes and spinach when crocus is in bloom
Plant carrots and beets when daffodils bloom
Plant bush beans when apple trees bloom
Plant annual flowers and squashes when lilacs are in full bloom
These were developed as a way of measuring growing degree days before the formal calculation of growing degree days existed, but they achieved similar results.
Entomologists work hard daily to figure out the growing degree day of certain active insects and pests to improve “best practices” observed in horticulture and arboriculture. One insect we are watching closely is the nitidulid beetle. These beetles have been known to carry a fungus that causes oak wilt, a deadly fungal disease of oak trees. As we learn more about these beetles, we can implement more effective management practices to ensure the safety of everyone’s oak trees! For now, we go off of the best information available to us. You can learn more how Russell Tree Experts handles oak wilt by reading here!
I am proud to be part of the arboriculture industry. The industry is very receptive to better and new information as knowledge is developed and discovered. The world is evolving and I’m happy that Russell Tree Experts prides itself on keeping up with the times and staying well-informed on essential tree issues. As we learn more, we will share with you all as well. As always, thank you for your time, and I hope you all do your best to be a tree advocate in your community. To request a free tree work quote by one of our 20+ ISA Certified Arborists, give us a call at (614) 895-7000 or visit RussellTreeExperts.com/Quote!
ADDITIONAL RESOURCES ON GROWING DEGREE DAYS
The Ohio State University: https://weather.cfaes.osu.edu/gdd/glossary.asp
Smithsonian Gardens: https://gardens.si.edu/learn/blog/gardening-guided-by-phenology/
ADDITIONAL ARBOR ED ARTICLES!
Lindsey Rice | Special Projects Manager, Russell Tree Experts
Lindsey joined Russell Tree Experts in 2015 with a B.S. in Agribusiness and a minor in Horticulture from The Ohio State University. Growing up in Northwest Ohio, she participated in various sports, band, and FFA which ultimately inspired her love for the tree industry. In her free time she loves to spend it outdoors with her husband and daughters.
ISA Certified Arborist® OH-6739A