Growing Degree Days (GDD) - What Is Your #?

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While we are living in a world with cancellations, postponements, and social distancing, the spring season has not been canceled, and as a matter of fact the season is progressing. A great way to track that progression is through Growing Degree Day (GDD) Accumulations and the Plant Phenology Network.


While many of you might be familiar with GDDs and Plant Phenology, this initial alert will serve as an introduction. It might be new-news for some, or a refresher for others. Additional BYGL Alerts will follow as we track the progression of spring, and ultimately summer, in the buckeye state.


GDD are a measurement of the growth and development of plants and insects during the growing season. This development does not occur unless the temperature is above a minimum threshold value, or a base temperature. The base temperature varies for different organisms and is determined through research and experimentation. The actual temperature experienced by an organism is influenced by several factors. These factors will ultimately affect the growth and development.


Here are some examples:


*If you take a walk outside, you will notice there are areas in your own landscape that are warmer than others. Exposure to the sun, or lack of sun, wind, or protection from the wind, can create a microclimate that could accelerate plant and insect development, or reversely, slow that process of development down. Simply put, a plant in full sun will likely begin flowering sooner than that same plant in a location that is shaded. Proximity to buildings and driveways can also be factors. 


*Fertility and nutrient content of the soil can have a direct affect on the growth rate of plants. While we want healthy plants, too much of a good thing is not good. There has been some research that has shown that plants that are overly fertilized and pushing a lot of vegetative growth can have an increase of pest pressure in some cases.


*There are other factors that can also have an impact on plant and insect development including the presence of weeds or unwanted plants in a given area, or too much or too little water either through irrigation, hand watering or mother-nature.



Because of these factors and the variability within even a landscape and some other scientific considerations, a base temperature of 50 degrees Fahrenheit is considered acceptable for all plants and insects. If you are curious about the base temperature and math behind how GDDs are calculated, check out this webpage on the OSU Growing Degree Days site at:


Here is where you can dig in and how this site can helpful at the local level. All you need is an Ohio zip code. With that said, remember the examples described earlier and how micro-climate conditions can have an affect on the rate of development. Check out the information online, but ground truth what is happening locally. What I find fascinating is that whether you might be a little head of the curve, right on, or a little lagging, the sequence of events is the calendar remains the same.


Calendar? What calendar you say? The calendar is a chronological list of plants – first bloom and full-bloom; and insect activity. A HUGE shout out to Dr. Dan Herms for his work on this during his time at OSU. If it wasn’t for Dan and others in the Department of Entomology and Master Gardener Volunteers who volunteered in phenology gardens across Ohio to record first and full bloom, this resource wouldn’t be available for use to access today.


So how does this work?  


*Click on this website:


*The date will default to today’s date. Type in your Ohio zip code.


*Click on “Show me the Calendar” rectangle.


*You will be shown a snapshot of what is happening in your zip code with the GDD accumulation and upcoming phenological events. The snapshot includes the GDD number, the species, and what is happening with that species (i.e., first bloom, full bloom, emergence). If you would like to see the calendar in its entirety along with where the current GDD number is, click on “View Full Calendar”.


*You will have a lot of information at your fingertips. It is important to do some ground truthing to make sure the information that is on the website matches what you are seeing locally.


*So today, in Toledo, we are at 23 GDDs.


*With that information, I headed outdoors. Just outside my office is a row of corneliancherry dogwoods (Cornus mas). I observed buds swelling and you guessed it – first bloom! It is important to note that these shrubs are planted alongside a blacktop driveway, but most importantly that can be a slight variation for the website to what is happening on the ground. But again, no matter the variation in progression, the order will remain the same.   


Corneliancherry Dogwood
Corneliancherry Dogwood in First Bloom at Toledo Botanical Garden
Photo Credit: Amy Stone


Corneliancherry Dogwood Blossoms
Corneliancherry Dogwood in First Bloom at Toledo Botanical Garden
Photo Credit: Amy Stone


I am hopeful that everyone will click-on the website and find out what their GDD is. Tomorrow we will continue this conversation and will highlight GDD accumulations across the state to illustrate the differences from north to south and east to west, and I will be highlighting a feature that will allow you to compare where we are today, to last year, and to several years ago. It is something that I am sure you will love to explore more, and I promise you will find very interesting – at least I do!