Growing degree day
Growing degree days are a heuristic concept in phenology . They are used by farmers and gardeners to predict the date when a plant will flower or when grain will ripen.
Unless extreme conditions such as out-of-season drought or disease occur, plants grow in a cumulative manner that is heavily influenced by ambient temperature. Growing degree days take into account aspects of the local weather and enable gardeners to predict when the plants will be ready or even to control it in greenhouses.
Calculation methods according to application
There is no standardized way of calculating growing degree days (see ¹).
The most common are three calculation methods:
- Calculating the simple average.
- The modified calculation of the simple average.
- The Baskerville-Emin (BE) calculation method.
The period of summation is not standardized either. Different time periods and calculation methods are used depending on the application of the growth degree days.
Use case: growth and maturity of annual plants
If you want to predict the course of growth and maturity of annual plants with the degree of growth days, then the time of sowing is usually used as the start of the summation. The last day of summation is usually the first day of frost. The "modified calculation of the simple average" or the "Baskerville-Emin method" are mostly used here because the spring and early summer days with lower minimum temperatures result in higher heat sums.
Use case: growth and maturity of perennial plants; usually wine, oranges, peaches, etc.
When predicting the full maturity of perennial plants or fruits with the degree of growth days, the beginning of the summation is usually the first month of the year with a monthly average temperature of min. +10 ° C used. Alternatively, the first three consecutive days with an average daily temperature of +10 ° C are used as the start of the totalization. The accumulation ends on the last day of the last month with a monthly average temperature of min. +10 ° C. Alternatively, it is added up to the first day of frost. The “calculation of the simple average” calculation method is mostly used here because the spring and early summer days, which are less important for fruit ripening, are rated lower.
Calculation formulas
Modified calculation of the simple average
Growing degree days (WGT) according to the "modified calculation of simple average" calculation method are calculated by taking the average of the daily minimum ( T min ) and maximum ( T max ) temperatures and comparing it to a threshold value ( T z ) (usually 10 ° C). The equation is:
Before calculating the average, any temperature that falls below the threshold is changed to the threshold. Likewise, maximum temperatures are usually capped at 30 ° C, because most plants and insects do not grow or multiply faster if this temperature is exceeded. However, some warm temperate and tropical plants need days above 30 ° C in order to allow fruits or seeds to ripen.
For example, a day with a maximum temperature of 23 ° C and a minimum temperature of 12 ° C makes a contribution of 7.5 growing degree days. A day with a maximum temperature of 13 ° C and a minimum temperature of 7 ° C contributes 1.5 growth degree days.
Calculating the simple average
Growing degree days (WGT) using the Simple Average Calculation method are calculated by taking the average of the daily minimum ( T min ) and maximum ( T max ) temperatures and comparing it to a threshold ( T z ) (typically 10 ° C) ). The equation is:
Maximum temperatures usually capped here at 30 ° C.
Here an example day with a maximum temperature of 23 ° C and a minimum temperature of 12 ° C also provides a contribution of 7.5 growth degree days. However, a day with a maximum temperature of 13 ° C and a minimum temperature of 7 ° C contributes 0 growth degree days.
Growing degree days (WGT) according to the Baskerville-Emin calculation method (BE)
WGT = ((W * Cos (A)) - ((Threshold daily average) * ((3.14 / 2) -A))) / 3.14
(see ¹)