Growing Degree Days
Growing degree days (GDD), an estimate of accumulated heat, are used to predict the growth and development of plants, insects and diseases during the growing season. Insect, disease and plant development are very dependent on temperature and the daily accumulation of heat. The amount of heat required to advance a plant or pest to the next development stage remains constant from year to year, however, the actual amount of time (days) can vary considerably because of weather conditions.
Each crop, insect and disease species has a minimum base temperature or threshold below which development does not occur. These base temperatures have been determined experimentally and are different for each organism. GDD information can be very useful for predicting plant, insect and disease development. Some Ontario crops still use the GDD system while others have moved to the Ontario Crop Heat Unit (CHU) system described in the next section. Field crops that use the GDD system are cereals which have a Base: 0 (plant development occurs at 0°C or higher), and alfalfa and canola which have a Base: 5 (plant development occurs at 5°C or higher). To calculate GDD, first determine the mean temperature for the day. This is usually done by taking the maximum and minimum temperatures for the day, adding them together and dividing by two. The base temperature is then subtracted from the mean temperature to give a daily GDD. If the daily GDD calculates to a negative number, it is recorded as zero. Each daily GDD is then added up (accumulated) over the growing season.
GDD are sometimes referred to as “degree days” or the “degree days averaging method.” Some jurisdictions also use the term “heat units” interchangeably with “degree days.” In Ontario, the terms “growing degree days” (GDD) and “crop heat units” (CHU) are used independently since they represent two very different, temperature-dependent, development models.
Growing Degree Day Equation:
The GDD equation used by OMAFRA is calculated as follows:
Daily GDD = ((T max + T min) ÷ 2) – T base
T max = the daily maximum air temperature
T min = the daily minimum air temperature
T base = the GDD base temperature for the organism being monitored
Example:
Maximum Temperature: 28°C
Minimum Temperature: 15°C
Pest: European corn borer (ECB)
Base Temperature for ECB:10°C
Calculation:
Daily GDD = ((28 + 15) ÷ 2) – 10 = 11.5
Therefore: 11.5 GDDs were accumulated for that day for the European corn borer GDD model.
There are four factors to consider when comparing GDD accumulations from various sources or regions.
- Are the base temperatures used in the equations the same?
Different organisms have different base temperatures used to calculate GDD: 150 GDD at Base 10 does not equal 150 GDD at Base 0. - Are the start dates for the accumulations the same?
Generally, GDD accumulations start on April 1 each year, but some insect GDD models start at the emergence of a specific life stage. This is referred to as a biofix. - Are the equations used to calculate the daily GDD the same?
Many modifications to the simple GDD calculation have been developed over the years and may be referred to generally as degree days. - Are the temperatures used in degrees Celsius or Fahrenheit?
GDD accumulations will vary significantly, depending on whether they are being tracked in Celsius or Fahrenheit. GDD models have been designed specifically for use in one or the other and cannot be interchanged without making conversions. The ECB GDD model was based on measurements in Celsius.