Growing degree days (GDD), also called growing degree units (GDUs), 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. GDD is credited to be first defined by Réaumur in 1735.
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 (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, 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 GDD are calculated by taking the integral of warmth above a base temperature Tbase (dependent on the plant type; see baseline section):
GDD = T m a x + T m i n 2 − T b a s e . {\displaystyle {\text{GDD}}={\frac {T_{\mathrm {max} }+T_{\mathrm {min} }}{2}}-T_{\mathrm {base} }.}
If the minimum temperature Tmin is below Tbase, there exist two variants:
Variant A: Do not change T m i n {\displaystyle T_{\mathrm {min} }} . Only if ( T m a x + T m i n ) / 2 < T b a s e {\displaystyle (T_{\mathrm {max} }+T_{\mathrm {min} })/2<T_{\mathrm {base} }} , set ( T m a x + T m i n ) / 2 = T b a s e {\displaystyle (T_{\mathrm {max} }+T_{\mathrm {min} })/2=T_{\mathrm {base} }} . The resulting GDD is 0. This can be written more compactly as GDD = max { T m a x + T m i n 2 − T b a s e , 0 } {\displaystyle {\text{GDD}}=\max \left\{{\frac {T_{\mathrm {max} }+T_{\mathrm {min} }}{2}}-T_{\mathrm {base} },0\right\}}
Variant B: Change T m i n < T b a s e {\displaystyle T_{\mathrm {min} }<T_{\mathrm {base} }} to T m i n = T b a s e {\displaystyle T_{\mathrm {min} }=T_{\mathrm {base} }} . 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 (and a base of 10 °C) would contribute 7.5 GDDs.
23 + 12 2 − 10 = 7.5 {\displaystyle {\frac {23+12}{2}}-10=7.5}
As a second example, a day with a high of 13 °C and a low of 5 °C (and a base of 10 °C) would contribute:
Version A: 0 GDD, as max { ( 13 + 5 ) / 2 − 10 , 0 } = 0 {\displaystyle \max\{(13+5)/2-10,0\}=0}
Version B: 1.5 GDDs, as ( 13 + 10 ) / 2 − 10 = 1.5 {\displaystyle (13+10)/2-10=1.5}
Plant development
Pest control 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. For example, when using a baseline of 10 °C:
Black cutworm larvae have grown large enough to start causing economic damage at 166 GDD Boxwood leafminer adults emerge at about 139 GDD The first annual generation of birch leafminer adults emerge between 48 and 203 GDD and the second annual generation between 319 and 524 GDD The first annual generation of fall webworm larvae emerge between 350 and 749 GDD, the second between 791 and 1540 GDD, and the third between 1580 and 1790 Mimosa webworm larvae's first appearance averages 543 GDD Oak lace bug's first annual egg hatch occurs between 311 and 652 GDD, the second between 869 and 1048 GDD, and the third between 1260 and 1306 GDD Pine needle scale's first annual egg hatch occurs between 35 and 526 GDD and the second between 1110 and 1511 GDD
Honeybees 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 4.5 °C wheat, barley, rye, oats, flaxseed, lettuce, asparagus,"canning purposes" 8 °C sunflower, potato 10 °C maize (including sweet corn), sorghum, rice, soybeans, tomato, coffee, grapes, snap beans, lima beans 30 °C the USDA measure heat zones in GDD above 30 °C; for many plants this is significant for seed maturation, e.g. reed (Phragmites) requires at least some days reaching this temperature to mature viable seeds
Pests 6 °C Stalk borer 7 °C Corn rootworm 9 °C Alfalfa weevil 10 °C Black cutworm, European corn borer, standard baseline for insect and mite pests of woody plants 11 °C Green cloverworm
Modified growing degree days 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:
GDD C = 5 9 GDD F {\displaystyle {\text{GDD}}_{\text{C}}={\frac {5}{9}}{\text{GDD}}_{\text{F}}}
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 (1 Celsius degree-day is 8.64×104 K·s; 1 Fahrenheit degree-day is 4.8×104 K·s).
See also Degree day Growing season Heating degree day Weather derivative Winkler scale Phenology
References This article incorporates public domain material from Jasper Womach. Report for Congress: Agriculture: A Glossary of Terms, Programs, and Laws, 2005 Edition. Congressional Research Service.
Notes
External links Good explanation of Growing Degree Day calculations with examples Current year-to-date GDDs for selected US cities a table of GDDs necessary for grapes GDDs to various stages of maturity for selected crops Online GDD calculator Turf Growing Degree Days Calculator for sports stadia and golf courses