Hi everyone,

I was waiting to send this post until it seemed that we were reaching the end of our chill accumulation for this year. The weather has been very variable the last few days from warm, to wet, and to cold. The nice thing about the temperatures going down is that we have accumulated more chill hours/chill portions. Currently, if we look to our accumulation using the Weinberger model (# hours below 45F) and modified Weinberger model (# hours between  32F and 45F) from Oct 1 to now, we are below of what we got last year. However, we are above of what we got in 2016-2017 . There is a good summary table for all stations at UGA in https://www.georgiaweather.net/?content=ch.

Here are the values by Feb 8, 2020

Chilling Hours Under 45 ℉ (Weinberger model)
Station Name 2019 TO 2020 2018 TO 2019 2017 TO 2018 2016 TO 2017
Attapulgus 464 567 752 341
Butler 697 759 918 467
Byron 764 795 954 497
Fort Valley 771 813 986 470


Chilling Hours Between 32 ℉ and 45 ℉ (Modified Weinberger model)
Station Name 2019 TO 2020 2018 TO 2019 2017 TO 2018 2016 TO 2017
Attapulgus 434 525 576 309
Butler 637 682 691 401
Byron 695 696 733 426
Fort Valley 702 686 737 416

These past few days of cold weather have put us close to the same chill level as last year when we start looking at the chill portions using the dynamic model. This model is better suited for our temperature variations – in summary it accounts for gain and losses of chill. Currently, there are two sites  that we can observe the chill portions. The first site is https://getchill.azurewebsites.net/. To be able to pull your data for this season you need to know the station ID. One way to find the weather station ID nearby your location is to go to Weather Underground website https://www.wunderground.com/. Here you can search for your location and it will give you the data for a station nearby. To see the ID for the station(s), click on change under the name of the location and it will show you the stations around the location. Here is a list of the IDs for some of the locations that data can be pulled to the website for peach growing areas:

Attapulgus, GA: KPAM

Buttler, GA: KPXE

Byron, GA: KGVL

Fort Valley, GA: KPDK

When using the GetChill website, make sure that the data is being pulled from the station. You can try few stations to make sure that the data is consistent.

Another option for chill portion accumulation is a website that I have been checking for few months now. It tends to differ by a few chill portions with the GetChill website (Getchill uses the data from weather stations). I talked with Dr. Pam Knox (UGA Agricultural Climatologist) and she that: “the data source for the calculator appeared to be a grid based on NWS data supplemented with some other networks (but not the UGA network) and I believe it should be a reasonable source of data. Keep in mind that there are going to be some variations in local microclimate at your sites due to slope, soil type, and other factors which may make the values different than what the grid is predicting, but it’s the only place I know that you can find a chill portion calculator now”.

To access the website go to https://climatetoolbox.org/tool/historical-climate-dashboard. You will need to add your location. So, click on choose location. Next, you can choose your data option. There are four panels. You need to add in two of those panels the chill calculation options. Click on the drop down menu and you will be able to see an option for the Modified Weinberger model and one for chill portion(Dynamic model). The nice thing about this tool is that you don’t have to look for station IDs, but again is important to remember that it would be recommended to check both websites as Getchill is based on weather station data, Climatetoolbox is based on NWS (NOAA National Weather Service data).

Chilling portions (Dynamic model) – GetChill
Location (Station) 2019 TO 2020 2018 TO 2019 2017 TO 2018 2016 TO 2017
Attapulgus (KPAM) No data 29 29 13
Butler (KPXE) 44 50 47 21
Byron (KGVL) 58 – Incorrect 66 – incorrect 62 – Incorrect 48 – Incorrect
Fort Valley (KPDK) No data 64 – Incorrect 59 – Incorrect 42 – Incorrect


Chilling portions (Dynamic model) – ClimateToolBox
Location Current Data
Attapulgus 33
Buttler 48
Byron 47
Fort Valley 50

Comparing the GetChill and ClimateToolbox

Last year, using both the GetCHill data, in middle GA area we had about 50 chill portions by now. Currently using the GetChill data and ClimatetoolBox data, we are with 44 and 48 chill portions, respectively. We are below what we accumulated last year and previous years (except 2016-2017 season where we got extremely low chill). Last year, bloom came on normally with an overall good fruit set. Some higher chill varieties had an extended bloom season which created some issues. This year, we expect similar situations with some lack of chill symptoms (delayed bloom, extended bloom, etc.) present in some higher chill varieties.

As a rule of thumb, we use the following correspondence between chill hour requirement and chill portions (this was adapted from Dr. Reighard’s presentation in Savannah last month)

To have a potentially good crop – at the minimum — by Feb 15th varieties requiring

  • 650 chill hours need ~ 30-35 Chill portions
  • 750 chill hours need ~ 35-40 Chill portions
  • 800 chill hours need ~ 40-45 Chill portions
  • 850 chill hours need ~ 45-50 Chill portions
  • 950 chill hours need ~ 50-55 Chill portions

Currently, we are close to 50 chill portions in middle Georgia. This means that overall we are quite similar to last season. At this point, if a specific variety(ies)  from last season had issues during bloom (either delayed or extended), then more likely will be seeing similar situations this year. This can all change of course if we receive additional chill from now to mid- February (Currently, the NOAA forecast shows 50% probability above average temperatures. This outlook means that probably we accumulated as much chill as we are going to get at this point).

Management techniques to induce bud break with lack of chill

There are few management techniques that could be used to induce bud break during lack of chill. However, results from these different techniques are variable based on weather conditions, flower bud development, varieties, etc. Some of the techniques that we currently use for orchard  management such as dormant oil application, have been also shown to help with flower bud break dormancy and do not have any deleterious effects in plants.

Hereafter, I am going to summarize some of the tools and results out there.


The following information is for discussion and not intended to be considered as recommendations. Results have been variable and some information may be incomplete or partly based on personal opinion, and product labels need to state if these compounds can be used in GA for dormancy release.


Dr. Ali Sarkhosh (sarkhosha@ufl.edu) established some trials with Edger, a breaking dormancy compound in 2019. He used foliar application of Erger at different timings compared to a control. Erger is a plant biostimulant contains diterpenes, polysaccharids, calcium and nitrogen (nitric, ammonium, and ureic forms). Additional information for this compound can be found here. Dr. Sarkhosh presentation can be found here. Dr. Sarkhosh doesn’t believe the use of Erger should be recommended at this stage as results represent only one year of data. In addition, data needs to be accessed more thoroughly before being made available. Current presentation shows a graphical development timeline of each treatment vs. the control.


Dr. Edgar Vinson (vinsoed@auburn.edu) shared a presentation of some of the studies with dormancy breaking compounds that they were made in Auburn. The presentation is available here. Variable results were observed. In this experiment, potassium nitrate, hydrogen cyanamide, and armobreak (Class Act) were evaluated. Applications were made in plants after assessment of pollen development. Flower buds were dissected and anther/pollen development/coloration were assessed. The optimal stage were white/transparent anthers for optimal treatment (especially with hydrogen cyanamide) without damage to the flowers. Flower bud development was studied throughout the experiment. At the end, potassium nitrate was reported to be as effective as budpro (commercial form of hydrogen cyanamide) in flower bud development.


We evaluated the effect of dormancy breaking compounds for two seasons. A total of four different trials were done. Two in 2016 and two in 2017. Different varieties were used. Yield data and bloom progression data were evaluated. In general, it was determined that the hydrogen cyanamide effect was variable. A summary of the different experiments can be found here. A comment about the anther/pollen development is described in the text.



As we stand, I cannot recommend the use of hydrogen cyanamide due to the variable results shown by different studies and possible bloom thinning effect in trees if applied too late . However, it is still a tool that can be used and its use should be considered. Other compounds can also have an effect, such as potassium nitrate, which may be good alternative options. I would recommend if you have additional questions about a specific trial or compound to please contact me at 3528713981 or dchavez@uga.edu. We can talk about your options and as I mentioned before would like to foster discussion to come up with the best options for your farm.

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