In the absence of extreme conditions such as
unseasonal drought or disease, plants grow in a
cumulative stepwise manner which is strongly influenced by the ambient
temperature. Growing degree days take aspects of local weather into account and allow
gardeners to predict (or, in greenhouses, even to control) the
plants’ pace toward maturity.
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. Growing degrees
(GDs) is defined as the number of temperature degrees above a certain threshold
base temperature, which varies among crop species. The base temperature is that
temperature below which plant growth is zero. GDs are calculated each day as
maximum temperature plus the minimum temperature divided by 2 (or the mean
temperature), minus the base temperature. GDUs are accumulated by adding each
day’s GDs contribution as the season progresses.
GDUs 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. Crop specific indices that employ separate equations for the
influence of the daily minimum (nighttime) and the maximum (daytime)
temperatures on growth are called crop heat units (CHUs).
GDD calculation[edit]
GDD are calculated by taking the average of the daily maximum and minimum temperatures compared to a base temperature, Tbase, (usually 10 °C). As an equation:
If the mean daily temperature is lower than the base temperature then GDD=0.