Calculating GDD’s and Why They May Be Important in Predicting First Bloom (Feb. 27, 2014)

— Written By Barclay Poling
en Español

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Dear Growers, Agents, Agronomists and Others

I am actually on the road back to Raleigh today (by way of Pungo and Va Beach), but I received so many emails last night regarding the advisory posted last evening on how we are running behind on “heat units” this year relative to other seasons. And, I received a couple of questions about  Growing Degree Day units and how these can be related to different plant developmental stages, such as 1st bloom. This is an important stage for a number of reasons, including the fact that certain sprays cannot be used once bloom begins. Practically speaking, I look at first bloom as the stage when about 5% of the total blossoms on a strawberry plant are actually in bloom. Thus, on a plant with the potential to produce 60 good blossoms, then  5% bloom would represent justt 3 good blossoms  (5% x 60 = 3 blossoms). Over the last 2 years I have observed an interesting trend on a local farm near Raleigh where the number of GDD units associated with 5% bloom is about 90 Growing Degree Day units (GDDs). Yes, lots more research on this is needed, but this may be a good start today to just show you how I calculate GDDs using a 30 day forecast to get an idea when we might hit 90 GDDs at this particular farm location. Basically, this farm is now at 44 GDDs (Jan 1-Feb 25), and I simply then calculated how many units will be accumulated during the next 30 day period from Feb 26 to Mar 27. But, let’s pause and just discuss GDDs, as one question I received had to do with how GDD and chilling units, are possibly related. They are not!

So, to be sure we are all on the same page, here is a good definition of GDD from Ohio State (

“Growing Degree Days are a measurement of the growth and development of plants and insects during the growing season. Development does not occur at this time unless the temperature is above a minimum threshold value (base temperature). The base temperature varies for different organisms. It is determined through research and experimentation.”

However, the authors go on to state, “A base temperature of 50 degrees Fahrenheit is considered acceptable for all plants and insects.”

In strawberry, that is the base temperature we have elected to use, and though SkyBit actually allows me to use other base temperatures for the Clayton, NC, location (e.g. 45 F, 55 F, etc). I feel the 50 F base temperature for strawberry.

There are several  methods of calculating the GDD, but we are using the Simple Method described here in the OSU bulletin:

a) Simple Method:

This method compares the daily mean temperature (Tmean) to the base (Tbase) or threshold temperature. The daily mean temperature is the average of the daily maximum and minimum temperatures. If this average is greater than the threshold temperature the GDD accumulated for that day is the threshold temperature subtracted from the daily mean temperature. If the daily mean temperature is less than the base temperature, then the GDD for that day is zero. Adding the GDD of all the days previously considered and the GDD of that day calculated the GDD of a particular day.


  • For my Nashville, NC, grower cooperator (referenced in last nights advisory) I used the simple method for calculating GDD and Base 50. If AccuWeather is close to being right about the their forecast for daily highs and lows over this 30 day period from 26-Feb to 27-Mar, this location will only accumulate 58.5 GDDs
Table 1. Utilizing AccuWeather 30 Day Forecast to Generate GDDs
Hi Temp Lo Temp Ave Base 50 GDD units
26-Feb 46 22 34 50
27-Feb 51 21 36 50
28-Feb 39 27 33 50
1-Mar 50 35 42.5 50
2-Mar 66 47 56.5 50 6.5
3-Mar 69 44 56.5 50 6.5
4-Mar 52 38 45 50
5-Mar 53 43 48 50
6-Mar 61 45 53 50 3
7-Mar 65 43 54 50 4
8-Mar 68 40 54 50 4
9-Mar 71 32 51.5 50 1.5
10-Mar 57 42 49.5 50
11-Mar 58 44 51 50 1
12-Mar 61 38 49.5 50
13-Mar 60 38 49 50
14-Mar 58 37 47.5 50
15-Mar 65 42 53.5 50 3.5
16-Mar 66 43 54.5 50 4.5
17-Mar 67 44 55.5 50 5.5
18-Mar 68 37 52.5 50 2.5
19-Mar 56 33 44.5 50
20-Mar 58 42 50 50
21-Mar 60 45 52.5 50 2.5
22-Mar 63 38 50.5 50 0.5
23-Mar 57 35 46 50
24-Mar 60 40 50 50
25-Mar 65 45 55 50 5
26-Mar 67 45 56 50 6
27-Mar 60 44 52 50 2

So, recalling that we had 44 GDDs from Jan 1-Feb 25 and that we may accumulate 58.5 more units in the next 30 days, then my projected total GDD unit accumulation for the period from Jan 1 – Mar 27 is estimated at 102.5 (44 + 58.5). As I said earlier, we figure about 90-92 GDDs for 5% bloom so that would likely be around the 22nd of March at this location (102.5-13 = 89.5).

I will continue with this exercise later on this weekend, but for now I have some appointments to keep!

Dr. E. Barclay Poling
Retired Small Fruit Extension Specialist
& Professor Emeritus
Department of Horticultural Science
Campus Box 7609, 162A Kilgore Hall
NC State University
Raleigh, NC 27695-7609
A pessimist sees the difficulty in every opportunity, an optimist sees
the opportunity in every difficulty.
Winston Churchill, Prime Minister of England

Posted on Feb 27, 2014
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