Growing Degree Days
Growing degree days (GDD) is a weather-based indicator for assessing crop development. It is a calculation used by crop producers that is a measure of heat accumulation used to predict plant and pest development rates such as the date that a crop reaches maturity.
In the absence of extreme conditions such as drought or disease, plants grow in a cumulative stepwise manner which is strongly influenced by the ambient temperature. The Growing Degree Days calculation allows producers to predict the plants’ pace toward maturity. Daily growing degree day values are added together from the beginning of the season, providing an indication of the energy available for plant growth. Growing degree day totals are used for comparing the progress of a growing season to the long-term average and are useful for estimating crop development stages and maturity dates.
Growing degrees (GDs) is defined as the mean daily temperature (average of daily maximum and minimum temperatures) above a certain threshold base temperature accumulated on a daily basis over a period of time. Negative values are treated as zeros and ignored. The base temperature varies among crops and the value is derived from the growth habits of each specific crop. The base temperature is that temperature below which plant growth is zero. For example, cereal and forage crops show little growth or development when average temperatures are below 5°C.
Unless stressed by other environmental factors like moisture, the development rate from emergence to maturity for many plants depends upon the daily air temperature. Because many developmental events of plants and insects depend on the accumulation of specific quantities of heat, it is possible to predict when these events should occur during a growing season regardless of differences in temperatures from year to year.
GDD units can be used to: assess the suitability of a region for production of a particular crop; estimate the growth-stages of crops, weeds or even life stages of insects; predict maturity and cutting dates of forage crops; predict best timing of fertilizer or pesticide application; estimate the heat stress on crops; plan spacing of planting dates to produce separate harvest dates.
GDs are calculated each day as maximum temperature plus the minimum temperature divided by 2 (or the mean temperature), minus the base temperature. GDDs are accumulated by adding each day’s GDs contribution as the season progresses. If the average temperature is below the base temperature, the growing degree day value for that day is zero.
GDD are calculated by taking the average of the daily maximum and minimum temperatures compared to a base temperature, Tbase, (usually 10 °C for grapes; 5 C for cereals and many grasses.). The GDD calculation is not usually used for corn, as researchers have developed a more accurate temperature index that considers day and night temperature separately (see Advanced Silage Corn Managment book – on www.farmwest.com).
GDD = (Tmax + Tmin) / 2 - Tbase
GDDs are typically measured from the start of growth in spring for a particular crop. Any daily mean temperature below Tbase is set to Tbase before calculating the average giving a zero value. Likewise, the maximum temperature is usually capped at 30 °C because most plants do not grow any faster above that temperature. However, some warm temperate and tropical plants do have significant requirements for days above 30 °C to mature fruit or seeds.
Insect development and growing degree days are also used by some farmers and horticulturalists to time their use of organic or biological pest control or other pest control methods so they are applying the procedure or treatment at the point that the pest is most vulnerable.