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Growing Degree DayGrowing 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. Absent 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 sum of the averages of the daily high and low temperature each day compared to a baseline (usually 10°C). GDDs are typically measured from the winter low. For example, a day with a high of 23°C and a low of 12°C would contribute 5 GDDs. Any temperature below the baseline is set to the baseline before calculating the average. For example, a day with a high of 13°C and a low of 7°C would still contribute two GDDs. 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. Plant development Selected example GDDs (all in °C base 10°C): - Witch-hazel (Hamamelis spp.) - begins flowering at <1 GDD
- Red maple (Acer rubrum), Forsythia spp., Sugar maple (Acer saccharum) - begin flowering at 1-27 GDD
- Norway maple (Acer platanoides), White ash (Fraxinus americana) - begins flowering at 30-50 GDD
- Crabapple (Malus spp.), Common Broom (Cytissus scoparius) - begins flowering at 50-80 GDD
- Horsechestnut (Aesculus hippocastanum), Common lilac (Syringa vulgaris) - begin flowering at 80-110 GDD
- Beach plum (Prunus maritima) - full bloom at 80-110 GDD
- Black locust (Robinia pseudoacacia) - begins flowering at 140-160 GDD
- Catalpa (Catalpa speciosa) - begins flowering at 250-330 GDD
- Privet (Ligustrum spp.), Elderberry (Sambucus canadensis) - begin flowering at 330-400 GDD
- Purple loosestrife (Lythrum salicaria) - begins flowering at at 400-450 GDD
- Sumac (Rhus typhina) - begins flowering at at 450-500 GDD
- Butterfly bush (Buddleia davidii) - begins flowering at at 550-650 GDD
- Corn (maize) - 1360 GDD to crop maturity
- Dry beans - 1100-1300 GDD to maturity depending on cultivar and soil conditions
- Sugar Beets - 130 GDD to emergence and 1400-1500 GDD to maturity
- Barley - 125-162 GDD to emergence and 1290-1540 GDD to maturity
- Wheat (Hard Red) - 143-178 GDD to emergence and 1550-1680 GDD to maturity
- Oats - 1500-1750 GDD to 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
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 GDDC = 9 GDDF External references * University of Massachusetts Amherst Extension, Integrated Pest managment Tools, web site accessed Jan 2005
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