Growing Degree Days

A Growing Degree Day is any day in which the average temperature is above a specified base temperature, which is given for a particular subject matter. Growing Degree Days is an accumulation of these days and their values which are above the base temperature, providing an accumulated heat value.

An example might be the average temperature of a day, in say April, being 10°C, and if the base temperature is given as 5°C, then for that day it is 5 GDD. The starting period for monitoring GDD might typically be March or April, through to the end of October or November. The value of recording GDD over the winter period for plants is doubtful as values would be minimal and any growth is typically weak forced growth, particularly with reduced light levels over the winter. However, where supplementary lighting and heating is used, knowing the difference between the natural and artificially enhanced GDD can aide turf management decisions.

Values for GDD do not automatically convert to growth rates and should not be used for this because of many other variables influencing plant response to environmental conditions, in particular: light levels and quality of light, available soil water, soil nutrition, soil pH, wind and relative humidity, as well as different species of plant having different requirements and growth patterns.

An accumulation of mean temperature figures to aid estimating when grasses might come into flower and produce seed heads and for the application of a plant growth regulator, if deemed appropriate. GDD can also be used for estimating different stages of pest development to help in determining suitable control methods. The maximum and minimum daily temperatures are summed and then divided by two, with the base temperature being deducted to produce the required figure.

The formula for GDD = Max {(Tmax+Tmin)/2-Tbase,0}.
If the mean daily temperature is lower than the base temperature, then GDD=0.

The base, or reference, temperature is specific to a species and is the temperature at which growth and development is zero, or effectively zero. For C3 turfgrasses, in general, this might be taken as being 5°C or 41°F. For wheat, rye, barley and oats the base temperature is normally 5.5°C or 42°F, which is the temperature the UK Met Office uses.
The 2001-2020 GDD baseline median values for the UK range from 627.6 GDD within the Cairngorms (high altitude) to 2,544.7 in central London (with a urban heat island no doubt having an effect here), with an average UK value of 1,735 GDD (n=2074).
(https://climatedataportal.metoffice.gov.uk/datasets/TheMetOffice::annual-growing-degree-days-projections-12km/about)

The maximum GDD per day is usually limited to 26-30°C because above this grass plants will tend to be under heat stress conditions.

If you change between temperature units then carry out correct conversions; GDDC = 5/9 x GDDF. For example, 5/9 x 60°F = 33 GDDC.

A worked example is therefore:

GDD = Max {(25+15)/2-5,0} = 15; therefore 15 is added to the accumulated GDD total.