A misconception, the amount of seasonal accumulated growing degree days using the Fahrenheit scale is the same as the Celsius scale. They are not the same. 9 GDD calculated using Fahrenheit are equal to 5 GDD using Celsius in the calculation. For instance if an area in California has an accumulated amount of 2520 heat units (GDD) for the season calculated using the degrees Fahrenheit, calculated in Celsius that would be 1400 heat units. So there you have it, Celsius not working for you, use Fahrenheit.
Growing Degree Days are used to estimate the growth and development of plants and insects during the growing season. GDD needs to be calculated daily using the daily maximum and minimum temperature to get a daily mean (average) temperature for your area, usually measured daily from April 1 to October 31.
There are additional things to consider beside GGD.
In addition you need a certain amount of Frost Free Days to be able to accumulate enough GDD and rain and soil type will have influence too.
For grapes the heat unit of GDD starts at base of 10 Celsius. Anything up to and including this temperature will not add to the amount needed. It is capped at 30 C and once the temperature reaches above 30 C it no longer adds to the daily GDD count.
It can be used as an indicator for someone planning to establish a vineyard to find the most suitable grape variety for planting. Grapes require a certain amount of growing degree days to fully develop and knowing the amount of degree days in your area would give you some indication of what type of grape would be more suitable for planting in your vineyard. It goes without saying that if there are not enough GDD your grapes would have a hard time to ripen. Growing Degree Days can be used to predict plant stages providing a more accurate count than using calendar days. It gives a good estimation of when to spray or schedule other management activities.
There are multiple ways to calculate GGD and the following two methods are not the most accurate but they are two simplest methods for calculating these heat units.
In method 1 temperature below the lower threshold are used as they are but if this produces a negative result then this will count as a 0.daily min temp 12C
In order to minimize the underestimation of heat accumulation days both daily minimum and maximum temperatures are assumed to be at least the lower threshold (10 C) and upper threshold (30 C) numbers in the method 2 calculation.daily min temp 12C
GDD calculation using Fahrenheit units is done similar to calculating using Celsius units
The lower threshold for growing grapes is 50 degrees Fahrenheit (10 for Celsius) and the upper threshold is set at 86 degrees Fahrenheit (30 for Celsius)
GDD can be calculated in Celsius or Fahrenheit, but GDD accumulations will vary significantly depending on whether they are being tracked in Celsius or Fahrenheit. They cannot be interchanged without making conversions. A Celsius degree-day is not the same as a Fahrenheit degree-day because a Fahrenheit degree is smaller than a Celsius degree. It takes nine Fahrenheit degree-days to make five Celsius degree-days.
Converting GDD Celsius to GDD Fahrenheit.