The Growing Point | September 2019

What are growing degree days?

As we head into harvest season in central and northern parts of Alberta, there is one statement that is becoming common to hear; “crop progress is delayed by at least a week”. This is easily understood when looking at the Growing Degree Day (GDD) accumulation in the western and northern parts of the province. GDD is a scientific method to roughly determine how much heat, and therefore plant growth, has occurred through the season. Plants require a certain amount of temperature to progress through its life stages. If temperatures are lower, plant rate of growth is slowed until temperatures are too low to allow any growth. If temperatures are higher, plant growth rate increases and will increase until temperatures are too high to allow for proper plant cell function. GDD is calculated as follows:

The base value, which is the assumed temperature that plant growth progress essentially stops, is typically 5°C in Alberta. So, if the daytime high is 23°C and the night low is 8°C, your calculated GDD would be:



Using the Alberta Climate Information Centre and selecting the weather station closest to you, you can determine how many GDDs your area has accumulated, and from this, how many potential days you are behind the average. Set the date range as the crops estimated germination date until today. Select the options of ‘Observed’ and ‘Normal’ ‘Growing Degree Days (5°C)’. Then click ‘Graph’ and you can will see a graph estimate of how many GDDs you have accumulated in your region. If you click ‘View’ you will see accumulation for each day.