A Chilling Tale of Three Blueberry Cultivars

The number of chill hours we received this year has been lower than usual (see Chill Hour link on right side of this page). I am in the process of accumulating chill hour data from the last 15 years or so to see how (or if) it has changed over that time period. At any rate, this year was somewhere between 500 and 600 hours here in Poplarville.  Enough for most blueberry cultivars, but not all. To this point, see the photo below.

From left to right: Springhigh, Jewel, and O'Neal southern highbush blueberries. Notice that O'Neal has no leaves compared to the other two.

From left to right: Springhigh, Jewel, and O’Neal southern highbush blueberries. Notice that O’Neal has no leaves compared to the other two.

Springhigh has a low chilling requirement of 200 hours. It is also very early blooming which causes problems in our location most years. This cultivar was released in 2005 out of Florida.  Jewel has a similar chilling requirement of about 250 hours. It too is from Florida. O’Neal is much different from the other two.  For flowering it requires about 400-500 hours of chilling; however the leaves have a higher requirement. Although I would not consider the flowering good on this bush it has some. Leaf development is known to be slow and sporadic on O’Neal anyway in the spring. This cultivar is from North Carolina.

I suspect that the amount of chilling we received fell in the area of enough for O’Neal flowers but not quite enough for O’Neal leaves. Leaves will continue to emerge but may take longer than normal. That delay in full leafing may affect the quantity and quality of the crop on O’Neal, whereas both look fine on Springhigh and Jewel.

This publication from Georgia has good information on these cultivars and chill hours.

Blackberry Flowers in January

Up until very recently, we have not had much cold weather in south Mississippi. In fact, as of last week we had accumulated fewer than 200 chill hours (that will be much higher this week after the recent cold snap).  A couple of days before the cold came sweeping through, I wandered among the ‘Chickasaw’ and ‘Kiowa’ blackberries here on the station in Poplarville.  At first I thought what I saw was an anomaly — a single instance; however, as I walked the field I saw several buds popping open like those in the photos below.

Fully open flower on 'Chickasaw' blackberry in early January 2016 in south Mississippi

Fully open flower on ‘Chickasaw’ blackberry in early January 2016 in south Mississippi

More buds just starting to pop open. They will wish they hadn't when the cold weather sets in.

More buds just starting to pop open. They will wish they hadn’t when the cold weather sets in.

So what is the overall outlook here? The exposed flowers will certainly be damaged in the cold.  We reached about 25 degrees here in Poplarville on Monday morning. That should be cold enough to damage open flowers. What else? Any tissue that was actively growing will get zapped.  Is it enough to worry about in terms of crop reduction? I would say no. Even though there were numerous plants exhibiting these symptoms of budbreak, there were still plenty of dormant buds to pick up the slack later on.

Since this study is used for disease expression, we won’t spray anything to control any fungi that may come about on dying tissue. However, it would be a good idea to keep a close eye on plants and see if evidence of botrytis comes it. Anthracnose, another problem disease, is something else to consider controlling when the bushes are dormant.

Mississippi Chill Hour Accumulation

I have previously wrote on the topic of chill hours, but I also get a lot of requests for what the accumulated hours are for the season.  This year I will be posting them on this site on the page entitled Chill Hours (on the right hand side of your screen).  By visiting this page, you will be able to keep up to date on the accumulated chill hours as reported by locations in five counties in Mississippi — Copiah, George, Jones, Lee, and Wayne.  The recordings are reported by volunteers, so they may or may not be available for each week.  In the future I hope to put together data from previous years (at least those I have) and also make them available on the site.

As of today, the first posting is up.  Each recording season runs from October 1 to April 1 of the following year.

Mississippi Vaccinium Journal April-June 2013

The latest issue of the Mississippi Vaccinium Journal is now available.  Topics covered are chill hours, freeze damage, SWD, and a notice for the Food Safety Modernization Act.  You can get the newsletter by clicking on the link below or you can go here to get all issues (http://msucares.com/newsletters/vaccinium/index.html).

MSVJ Volume 2 Issue 2

MSVJ Volume 2 Issue 2

MSVJ Volume 2 Number 2

Blueberry Chilling Hours Information from NC

I subscribe to the excellent North Carolina Blueberry Journal blog written by Bill Cline.  Below is an excellent description of chilling hours and how they relate to blueberries.  Even though this information is specific to North Carolina, there is a lot to be learned for other locales.

The NC Blueberry Journal


Chill Hour Requirements

Posted: 17 Feb 2012 09:59 PM PST

Most blueberries require a certain amount of cold weather during winter in order to leaf and flower normally the following spring.  Generally chill hours refers to hours below 45 degrees Fahrenheit, however for blueberry chill hour calculations in North Carolina we use a model developed by Dr. Mike Mainland to account for both (i) partial chilling at moderate temperatures and (ii) the negation of chill that occurs with warm temperatures in winter.

The Mainland Model accumulates 1 point for average hourly temperatures below 45° F; 0.5 points for temperatures 45-55° F; -0.25 points from 55-65° F; -1 point for temperatures 65° F and above. The model begins each fall when there is a positive balance that is not negated by warmer weather. Once 800+ chill hours have accumulated, points are no longer subtracted for temperatures 55° and above. The model ends February 28 at midnight.

Chill requirements vary a lot by blueberry cultivar and species.  Gerard Krewer and Scott NeSmith at UGA wrote a nice article summarizing chilling hour requirements for cultivars in GA, many of which are also grown in NC.  For cultivars grown in the southeastern US, approximate ranges (chill hour requirements) would be something like this:

0 to 250 hours = Very low-chill southern highbush cultivars for Florida and other low-chill areas (Emerald, Snowchaser and others)
250 to 400 hours = Moderate low-chill southern highbush cultivars for southeastern NC, SC and GA (Star, Rebel and others)
350  to 800 hours = Rabbiteye cultivars for most of NC, such as Premier, Powderblue, Tifblue and others
400 to 900 hours = higher-chill southern highbush cultivars (Legacy, Reveille, O’Neal and others)
900 to 1200 hours = Most Northern highbush cultivars (Duke, Jersey, Bluecrop and others)

Some cultivars have different chill requirements for leaves compared to flowers.  O’Neal flower buds break dormancy readily after 400 hours or so, but the leaf buds do not seem adequately chilled until about 700 hours are reached.

Chill hours vary tremendously depending on where you are in NC.  To see current chill hour accumulation at a site near you, go to the excellent blueberry chill calculator hosted by the State Climate Office. For instance, today (18 Feb 2012) accumulated chill  hours at a mountain location (Asheville) are1578 hours, compared to only 772 in the coastal plain (Elizabethtown).  At these levels, blueberries in westen NC should leaf and flower normally, while in the coastal plain, higher-chill cultivars like Duke, Jersey or Reveille will experience reduced or delayed flower development.