Growing Degree Hours

By Dane Castle, Fall Creek® Grower Support South Africa

In short, Growing Degree Hours (GDH) are a measure of heat accumulation that we use to predict plant development. It is calculated as a cumulative sum of hourly temperature above a threshold for each hour of the day, excluding high temperatures that cause plant stress. 
How does temperature affect plant development?

GDH can be one of the main factors that control the blueberry plant’s rate of cell development and division, as well as its growth. The accumulation of GDH will determine the onset of each developmental stage, for example, once the flower buds have broken and begin to swell.
Another important factor is the Diurnal Temperature Variation (DTV), which is the difference between day and night temperatures. DTV affects stem elongation in some plants, either increasing or decreasing internode elongation, and can play a role in fruit flavor. Generally, larger DTV leads to a better flavor balance of the fruit.

How do you record GDH?

1. Calculate the average temperature over 24 hours by using the daily minimum and maximum temperatures.
2. Ensure more accurate results by using a logger in field that records temperature readings hourly. 
3. Record temperatures throughout the growing season, so you can calculate accumulated degree hours and chill. Remember to:

  • Record every hour.
  • Have no sunlight on logger.
  • Position logger at plant height. 


Below 4.5 degrees = little or no growth (no GDHs) 
Between 4.6 to 26 degrees = linear increase in growth
How do you calculate GDH?

GDH = Average 24-hour temperature – 4.5 degrees x 24 hours

For example:
13.5 degrees – 4.5 degrees = 9 growing degrees
9 growing degrees x 24 hours / day = 216 GDH / day

The average weekly temperature can be used similarly – multiplying the growing degrees by 168 hours/week. A 3-degree difference can have a 25% difference in GDH. 

How to use GDH

GDH can help you to determine various scheduling in crop:

  • Sprays
  • Flower timing and ordering of beehives
  • Timing of harvest labor
  • Market planning

You can also use GDH to estimate the effects of microclimates, such as greenhouses or high tunnels, and the impact on plant growth. For example, a 1-degree daily difference may not seem like much, but it can affect the GDH significantly.

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