Plant Development and Growing Degree Days

Forsythia Northern Lights blooms at 58 GDD. CONTRIBUTED
Forsythia Northern Lights blooms at 58 GDD. CONTRIBUTED

If you are a regular reader of my column, you have perhaps heard me talk about growing degree days or GDD in the past. GDD is used to measure growth and development of plants and insects during the growing season.

There is a direct correlation between plant development (plant phenology) and insect emergence. This has been researched for many years and is consistent each season.

GDD can be calculated by three main methods, but I like to use the simple method. Determine the average of the daily maximum and daily minimum temperatures. If this number is over the threshold of 50 degrees Fahrenheit, GDDs are accumulated. If the number is under 50 degrees, zero GDDs are accumulated.

For instance if the high is 63 and the low is 43, the average is 53. Three GDDs would be accumulated that day. If the average is 45, no GDDs would be accumulated.

Keep in mind that plants don’t go backward in development. They just halt and hold where they are until additional GDDs are accumulated.

For instance, about the time that the Northern Lights forsythia is in full bloom (94 GDD), the eastern tent caterpillar has hatched (92 GDD). Another common insect problem, the European pine sawfly hatches (144 GDD) about the time that the Bradford callery pear first starts blooming (142 GDD).

For someone wanting to control a pest, it’s a lot easier watching the trees that correlate to emergence of the insect than it is hunting for the insect itself.

Last Friday we were at 31 GDD for Springfield, Ohio, zip code 45502. On Feb. 21 we were at 45 GDD. Silver maples should be in bloom at 42 GDD and red maples bloom at 44 GDD.

If the weather stays warm and we continue to accumulate GDDs, we will see Northern Lights forsythia blooming at 58 GDD.

Looking over past years in terms of plant development, on this date, February 21st, we were at the following GDDs: 2010 - 0; 2011 - 11; 2012 - 12; 2013 - 23; 2014 - 4; 2015 - 4; 2016 - 22; and 2017 - 45.

Going back 10 years, I found that we were at 44 GDD on this date in 2009 and 34 in 2008.

To some this is boring information but I find it fascinating. If you want to learn more about GDD and to check out the GDDs for your zip code, go to:

The website is a great tool for plant and insect development as well as for timing of any pesticide applications that you might make.

People are a little concerned about plant development and injury from cold right now and we may see some injury. The key component is in the transition phase from warm temperatures to cold temperatures.

If we have a prolonged warm spell followed by a dramatic drop in temperatures, plant tissues will be tender and likely damaged.