Chill update

Hi everyone,

I hope that everyone is doing well. It has been an interesting month or two for chill accumulation. I was worried last year seeing the seasonal forecast and how things were panning out. However, we were able to get some uniform chill accumulation including some additional hours even this week.

Generally speaking, we try to close our number for accumulation to Feb 10th. So if we look at the traditional models: 1) The Weinberger model (# hours below 45F) and 2) The modified Weinberger model (# hours between  32F and 45F) from Oct 1 to Feb 10, we are just below of what we got last year. However, we are above of what we got in 2018-2019 and 2019-2020 . There is a good summary table for all stations at UGA in https://www.georgiaweather.net/?content=ch.

Here are the values by Feb. 10, 2022

Overall, we are at the same chill level as previous years. Same story is for the Dynamic Model (chill portions) – model better suited for our temperature variations accounting for gain and losses of chill. Currently, there is just one site that can be used to obtain this data. Sadly, it only provides data to the current date – no information specific to other years or a cumulative comparison throughout a season. I talked with Dr. Pam Knox (UGA Agricultural Climatologist) about the principle used for this site and she said 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 not have to look for station IDs. Climatetoolbox is based on NWS (NOAA National Weather Service data).

Chill portion data accumulation from last year compared to this year are similar for all the locations. The last two seasons, 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.

We use the following correspondence table between chill hour requirement and chill portions (adapted from Dr. Reighard’s presentation few years back).

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 in 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 the end of 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).

Please if you have any questions, comments, and suggestions, or if you would like to have a conversation about our current situation please feel free to contact me at 352-871-3981 or dchavez@uga.edu.