Tech Notes Issue 6

Growing Degree Days and How You Can Use Them

Growing degree-daymodels are used topredict pest or plant lifecycles. Growing degreedays are basedon temperature or heatunits.

A growing degree day is a measureof heat above a threshold forone day. A growing degree day(GDD) is = (maximum temperaturefor the day + minimum temperaturefor the day) divided by 2 giving the average daily temperature minus a base temperature.The base temperature is the minimum temperature needed forgrowth and is commonly 10 C, but can vary depending on the use, plant or pest.

For example, if we had a day witha maximum temperature of 20 C and a minimum temperature of 14C, and were using a 10 C base temperature the calculation would look like:

GDD = {(20 + 14)/2 - 10} = 7

GDD have no units associated with them. A GDD is calculated daily and a running total or total accumulated growing degree days would be the result of adding consecutive daily values. For example, if on the second day theGDD was 4, the total accumulated GDD for the two days would be (7 + 4) = 11.

A negative growing degree daydoes not exist. For example, if you were to calculate a - 2 GDD, the value would be set to 0 fort hat day.

Predicting Poa annua Seedhead Emergence

In the northern United States(temperate region) growing degree-day models are used to predict Poa annua inflorescence (seedhead) appearance. The value of a seedhead emergence model is to more accurately timea plant growth regulator application that would suppress seed head emergence. These GDD models specifically are best used on a regional basis because the start date at which they begin is variable.

For example, a commonly used model that was published 25 years ago uses an April 1 start date because that is when snow cover typically ended in Michigan where the model was developed. This GDD model would not to applicable for Australia and South Africa. However, the ease at which the models can be developed - the association between temperature (GDD) and a biological event like seed head appearance can be easily developed no matter where you are in the world.