It’s not even St. Patty’s day and the growing degree day models are starting to light up all over Southern Wisconsin. Because the season has been so unseasonably warm, many have been asking about the accuracy of the models. I don’t know for sure, but I’ll take a guess and say that I think they are going to be off this year in many places.
Growing degree days are a simple way to estimate the progression of the season, similar to but much easier than tracking soil temperatures. Growing degree days don’t start accumulating until the average temperatures get above freezing (sometimes above 50F). Normally this coincides with when the snow and frost leave. However, due to the quick onset of really warm temperatures, many places may still have had frost in the ground when the growing degree days were skyrocketing. In these situations, the models will be ahead of reality, in situations where the frost was out before the warm streak, the models may be pretty accurate. Either way, I recommend erring on the upper end of the target ranges this year. Also, for the Primo/Proxy users, I recommend going with split applications this year. Check out the USGA research article on our Primo/Proxy GDD page by Randy Kane and Lee Miller for application ideas.
One way to check how the models correlate to soil temperatures is to measure soil moisture and pay attention to the crabgrass pre-emergent GDD model. The optimum timing for applying crabgrass pre-emergent herbicide is when soil temperatures are 50-55 F. The optimum growing degree days for pre-emergent application is 250-500 GDD. So if soil temperatures (two inch) are 50-55 degrees F when the model is between 250 and 500 GDD, I’d say things are on track. However if soil temperatures are still delayed significantly when we hit 350 GDD, then you can bet that the GDD models for the rest of the year will be off.