Growing Degree Days

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Growing Degree Days (GDD) is a heuristic used by horticulturists and gardeners to predict the date that a flower will bloom or a crop reach maturity. Below a certain temperature, plants grow very slowly if at all and above a certain temperature, their growth is "maxed out" but between those two ranges, the relationship between plant growth and temperature is close to a straight line. 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. Of course, this assumes no extreme conditions such as drought or disease.

BeeCulture published an excellent primer on GDD in Feb 2002. GDD is an effective way to predict what will be blooming in your immediate area and can be invaluable in deciding when to add supers to your colonies.

GDD Calculation

GDD is calculated by taking sum of the average of the high and low temperature each day compared to a baseline.

(Tmax + Tmin)/2 - Tbase

The baseline varies by plant type but for many temperate plants and even insects, a good baseline is 50° F (10°C). For example, a day with a high of 70°F and a low of 54°F would contribute 12 GDDs. (70 + 54)/2 - 50 = 12 Cumulative GDD is generally measured from the winter low.

Any temperature below the baseline is set to the baseline before calculating the average. For example, a day with a high of 56°F and a low of 44°F would still contribute three GDDs. Likewise, the maximum temperature is usually capped at 86°F (30°C) because most temperate plants and insects do not grow any faster above that temperature.

Bloom duration

Bloom duration is harder to measure. In some species, it too is a function of GDD. In others, bloom duration is a function solely of time. And it's always subject to weather. No matter what GDD says, the apple bloom is over after the spring storm rips all the petals off the trees.

GDD Chart

OSU has a dynamic GDD calculator based on area code. Click here].

GDDs of common plants in NE Ohio

Note 1: All GDDs calculated in this table are calculated based on the (86/50) baseline and expressed in Farenheit.
Note 2: There are a lot of missing values in this chart. Please help us fill it in.

Plant Type Bloom Starts Bloom Duration Comments
Witch Hazel < 1 GDD    
Maple trees (inc Red and Sugar) 1 - 50 GDD    
Crocus ~ 70 GDD    
Daffodil ~ 110 GDD   not a significant nectar source but valuable for early pollen
Crabapple 80 - 150 GDD    
Dandelion ~ 140 GDD   An excellent source of nectar. Strong pollen producer but missing 2 amino acids necessary for bees
Hyacinth ~ 150 GDD    
Black Locust 250 - 290 GDD    
Tulips ~ 275 GDD   An abundant pollen source but no significant nectar
Bleeding Heart ~ 280 GDD several months  
Cherry tree ~ 300 GDD    
Apple tree ~ 300 GDD 1-2 weeks Moderate to good nectar producer. Lots of pollen. Honey tends to granulate quickly.
Catalpa 415 - 600 GDD    
Blueberry ~ 550 GDD   Bees will work blueberries but rarely build up surplus honey
Mustard ~ 550 GDD    
Tulip Poplan tree ~ 550 GDD    
Purple Loostrife 720 - 810 GDD   Excellent nectar source
Sumac 810 - 900 GDD    
Basswood tree ~ 900 GDD    
Blackberry ~ 900 GDD    
Box Elder ~ 900 GDD    
Milkweed ~ 900 GDD    
Clover ~ 1,600 GDD    
Queen Ann's Lace ~ 1,800 GDD    
Yellow clover ~ 1,800 GDD    
Goldenrod ~ 2,200 GDD several months Excellent source of fall nectar. The hive will have a very distinctive beer-like odor when the bees are working goldenrod. Honey crystallizes quickly.
Thistle ~ 2,500 GDD    
Aster  ? GDD thru 1st frost One of the leading sources of fall nectar. Honey crystallizes very quickly.
Red clover  ? GDD    
White clover  ? GDD    
Sunflower  ? GDD   An excellent source of nectar. Pollen depends on the variety.
Raspberry  ? GDD   A honey crop from raspberry is almost water-white and has a mild flavor.
Honeysuckle  ? GDD    
Chives & Onions  ? GDD    
Buckwheat  ? GDD   Produces a very dark, distinctive honey
Peonie  ? GDD   Excellent nectar source
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