Growing degree-day


Growing degree days, also called growing degree units, are a heuristic tool in phenology. GDD are a measure of heat accumulation used by horticulturists, gardeners, and farmers to predict plant and animal development rates such as the date that a flower will bloom, an insect will emerge from dormancy, or a crop will reach maturity.

Introduction

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 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 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, 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 and the maximum temperatures on growth are called crop heat units.

GDD calculation

GDD are calculated by taking the integral of warmth above a base temperature, Tbase :
A simpler, approximately equivalent formulation uses the average of the daily maximum and minimum temperatures compared to a Tbase. As an equation:
If the minimum temperature Tmin is below the Tbase one, there exist two variants:
GDDs are typically measured from the winter low. Any temperature below Tbase is set to Tbase before calculating the average. Likewise, the maximum temperature is usually capped at 30 °C because most plants and insects 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.

Example of GDD calculation

For example, a day with a high of 23 °C and a low of 12 °C would contribute 7.5 GDDs.
As a second example, a day with a high of 13 °C and a low of 5 °C would contribute:
Common nameLatin nameNumber of growing degree days baseline 10 °C
Witch-hazelHamamelis spp.begins flowering at <1 GDD
Red mapleAcer rubrumbegins flowering at 1-27 GDD
ForsythiaForsythia spp.begin flowering at 1-27 GDD
Sugar mapleAcer saccharumbegin flowering at 1-27 GDD
Norway mapleAcer platanoidesbegins flowering at 30-50 GDD
White ashFraxinus americanabegins flowering at 30-50 GDD
CrabappleMalus spp.begins flowering at 50-80 GDD
Common broomCytisus scopariusbegins flowering at 50-80 GDD
HorsechestnutAesculus hippocastanumbegin flowering at 80-110 GDD
Common lilacSyringa vulgarisbegin flowering at 80-110 GDD
Beach plumPrunus maritimafull bloom at 80-110 GDD
Black locustRobinia pseudoacaciabegins flowering at 140-160 GDD
CatalpaCatalpa speciosabegins flowering at 250-330 GDD
PrivetLigustrum spp.begins flowering at 330-400 GDD
ElderberrySambucus canadensisbegins flowering at 330-400 GDD
Purple loosestrifeLythrum salicariabegins flowering at 400-450 GDD
SumacRhus typhinabegins flowering at 450-500 GDD
Butterfly bushBuddleia davidiibegins flowering at 550-650 GDD
Corn Zea mays800 to 2700 GDD to crop maturity
Dry beansPhaseolus vulgaris1100-1300 GDD to maturity depending on cultivar and soil conditions
Sugar beetBeta vulgaris130 GDD to emergence and 1400-1500 GDD to maturity
BarleyHordeum vulgare125-162 GDD to emergence and 1290-1540 GDD to maturity
Wheat Triticum aestivum143-178 GDD to emergence and 1550-1680 GDD to maturity
OatsAvena sativa1500-1750 GDD to maturity
European corn borerOstrinia nubilalis207 - Emergence of first spring moths

Pest control

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. For example:
Several beekeepers are now researching the correlation between growing degree-days and the life cycle of a honeybee colony.

Baselines

The optimal base temperature is often determined experimentally based on the life cycle of the plant or insect in question. Common baselines for crops are either 5 °C for cool-season plants and 10 °C for warm-season plants and most insect development.

Crops

In the cases of some plants, not only do they require a certain minimum temperature to grow, but they will also stop growing above a warmer threshold temperature. In such cases, a modified growing degree day is used: the growing degree days are calculated at the lower baseline, then at the higher baseline, which is subtracted. Corn is an example of this: it starts growing at 10 °C and stops at 30 °C, meaning any growing degree-days above 30 °C do not count.

Units

GDDs may be calculated in either Celsius or Fahrenheit, though they must be converted appropriately; for every 9 GDDF there are 5 GDDC, or in conversion calculation:
GDDC = 5/9 * GDDF
The equivalent unit compliant with the International System of Units is the kelvin-second. A quantity of kelvin-seconds is four orders of magnitude higher than the corresponding degree day.