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What is GDD
Flash Video PowerPoint
Growing degree days (GDD) are a heuristic tool in phenology. GDD are used by horticlturists 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.
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
Calculate GDD
GDD are calculated by taking the average of the daily high and low temperature each day compared to a baseline (usually 10 °C). As an equation: GDD = (High+Low)/2 - Baseline. GDDs are typically measured from the winter low. Any temperature below the baseline is set to the baseline 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.
How to use the GDD Report
Wind Rose
A wind rose gives a very succinct but information-laden view of how wind speed and direction are typically distributed at a particular location. Presented in a circular format, the wind rose shows the frequency of winds blowing FROM particular directions. The length of each "spoke" around the circle is related to the frequency of time that the wind blows from a particular direction. Each concentric circle represents a different frequency, emanating from zero at the center to increasing frequencies at the outer circles.
Evapotranspiration
The combined processes of evaporation and transpiration.
EVAPORATION - Evaporation is the physical process by which a liquid is transformed to a gaseous state.
TRANSPIRATION - The process by which water in plants is transferred to the atmosphere as water vapor.
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