degree-day

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 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.

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 that employ separate equations for the influence of the daily minimum (nighttime) and the maximum (daytime) temperatures on growth are call crop heat units (CHUs).

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 10 °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

References

 This article incorporates public domain material from the Congressional Research Service document "Report for Congress: Agriculture: A Glossary of Terms, Programs, and Laws, 2005 Edition" by Jasper Womach.

Notes

Bibliography

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