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About Growing Degree Hours

References

 Growing Degree Hours (GDH) are the result of a calculation which combines the passage of time with the temperature experienced during that time period. It is similar to a degree-day calculation in describing plant development because growth and development of peaches, plums, and nectarines are strongly influenced by temperature and time.

 Recent research (Minoun and DeJong, 1999) has documented that accumulated temperatures (Growing Degree Hours, GDH) for the first thirty days after bloom are highly correlated with yearly differences in harvest date for a given peach, plum, or nectarine cultivar. This Harvest Prediction Module facilitates comparisons of accumulated Growing Degree Hours (GDH) during the first 30 days after bloom for the current year with GDH data for the previous 5 years. The module can be used in either a general or specific way.

General use would be to simply use the module to get an idea of how warm (or cold) the current spring (up to 30 days after bloom) is relative to the previous years. If you have records of bloom date (the estimated time when 50% of the flowers on a tree or in an orchard are fully open) and harvest date for previous years, then you can simply look for a year with a comparable GDH accumulation (at 30 days after bloom) and expect that the number of days from bloom to harvest will be similar to that year.

A more specific use would be to use bloom and harvest date data for several previous years along with GDH accumulation data from the table for the corresponding years to develop regression equations for a specific cultivar in a manner similar to what was done in the research paper by Ben Mimoun and DeJong, 1999. This allows you to judge the accuracy of the GDH model for a specific cultivar and provides a method for interpolation in areas of the Days to Harvest vs. GDH Accumulation relationship where comparable data are missing. Regressions of this type can be calculated with standard computer spreadsheet programs or on a standard calculator that has statistical functions.

As can be seen from the paper, Mimoun and DeJong (1999) , GDH accumulation differences of 140 – 170 GDH (during the first 30 days after bloom) cause a one day difference in days from bloom to harvest in many cultivars. But it is also clear that the developmental response to temperature is substantially different in some cultivars. However, unless one has bloom to harvest data for several years on which to develop a regression model for a specific cultivar it is probably more useful to use the GDH data in the more general way.

 GDH=IF(C2<=25,21/2*(1+COS(3.14+3.14*(C2-4)/(25-4))),21*(1+COS(3.14/2+3.14/2*(C2-25)/(36-25))))

Possibly easier to understand equation:
 GDH =
   if t <= 25:
            21/2 * (1 + cos(3.14 + 3.14*(t-4)/(25-4)))
   if t > 25:
            21 * (1 + cos(3.14/2 + 3.14/2*(t-25)/(36-25)))
where t = temperature in Celsius