Growing Degree Days (GDD) are a measurement of the growth and development of plants and insects during the growing season. Development does not occur at this time unless the temperature is above a minimum threshold value, or what is also referred to as the base temperature. This base temperature can vary for different organisms and is determined through research and experimentation.
The actual temperature experienced by an organism is influenced by several factors and these factors will ultimately affect that organisms growth and development. We can probably all agree that depending on the weather, an organism's temperature may be a few degrees more or less than that recorded. For example, an organism in direct sunlight will likely experience higher temperatures, than those in full shade, and of course somewhere in the middle if the organism is located in dabbled shade, or both sun and shade at some point throughout the day. What is comes down to is the actual location can result in those temperature differences.
Fertility and nutrient levels in the soil can also affect the growth rate of insects and plants. The presence of weeds and precipitation may indirectly influence development as well. Due to these factors and some other scientific considerations, a base temperature of 50 degrees Fahrenheit is considered acceptable for all plants and insects, and what is used on the Ohio website.
GDD is a tool that should be in each green industry professional's "tool-box", and can be beneficial for consumers too. In Ohio, we are very lucky to have a GDD website that was developed as a result of work that Daniel Herms, Denise Ellsworth, Ashley Kulhanek and other contributors including Ohio Master Gardener Volunteers over the years. Check out the website for more information: https://www.oardc.ohio-state.edu/gdd/
The website uses GDD that ultimately provides a biological calendar that 'marries' a list of plants at their first and full bloom, and insect activity. This calendar is a sequence of events that includes both plants and insects and ties to each organisms to the GDD.
It is important to say that while the actual number associated with GDD is based on weather stations across Ohio, there can be some differences based on microclimates, but the sequence of activity is always in the same order. As you use GDD, it is always recommended to get outdoors and compare what the website is telling you what should be happening, and what you are seeing. For example, the first plant on the Ohio list is first bloom of silver maple at 34 GDD. Check out the website, type in your Ohio zip code, see what the website says your GDD is, and then head out to the field and make the seasonal observations that is included in the list. Are you seeing silver maples blooming in your area?