Growing degree day
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Growing degree days (GDD) are a heuristic tool in phenology. GDD are used by horticulturists and gardeners to predict the date that a flower will bloom or a crop reach maturity.

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.

GDD calculation

GDD are calculated by taking the average of the daily maximum and minimum temperatures compared to a base temperature, T_base, (usually 10 °C). As an equation:

GDDs are typically measured from the winter low. Any temperature below T_base is set to T_base 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.

For example, a day with a high of 23 °C and a low of 12 °C would contribute 7.5 GDDs.

A day with a high of 13 °C and a low of 7 °C would contribute 1.5 GDDs.

Plant development

Insect development and pest control

Growing degree days are also used by some farmers to time their use of pest controls so they are applying the treatment at the point that the pest is most vulnerable. For example:

• Black cutworm larvae have grown large enough to start causing economic damage at 165 GDD
• Azalea Lace Bug emerges at about 130 GDD
• Boxwood leaf miner emerges at about 250 GDD

Several beekeepers are now researching the correlation between GDD and the lifecycle of a honeybee colony.

Baselines

10 °C is the most common base for GDD calculations, however, the optimal base is often determined experimentally based on the lifecycle of the plant or insect in question.

• 5.5 °C wheat, barley, rye, oats, flaxseed, lettuce, asparagus
• 6 °C stalk borer moth
• 7 °C Corn Rootworm
• 8 °C sunflower, potato
• 9 °C Alfalfa weevil
• 10 °C maize (including sweet corn), sorghum, rice, soybeans, tomato, Black cutworm, European Corn Borer, standard baseline for insect and mite pests of woody plants
• 11 °C Green Cloverworm
• 12 °C many other crop calculations
• 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

GDDs may be calculated using either Celsius or Fahrenheit, though they must be converted appropriately; 5 GDD^C = 9 GDD^F

See also

• Degree day
• Growing season
• Heating degree day
• Weather derivatives

External links

• Growing Degree Days as a Method of Rating the Maturity of Corn Hybrids
• 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
• University of Massachusetts Amherst Extension, Integrated Pest management Tools, web site accessed Jan 2005
• Online GDD calculator