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.


Pecan Budbreak: The Last Holdout

Pecan trees are really attuned to the continental climate we experience in the middle of the U.S.  They are almost always the last tree to break bud in the spring.  Why is this? Dr. Darrell Sparks from the University of Georgia published a study back in 1993 that suggested both chilling and heat accumulation were responsible for budbreak timing in pecan (The link to the paper is here).  Below is a short description of pecan budbreak that I wrote for a conference back in 2007:

“Pecan trees, like most temperate fruit species, exist under a physiologically mandated rest period.  This rest period, also called dormancy, helps to regulate the timing of budbreak. The start of dormancy generally begins in late summer when shoot growth stops and apical dominance ceases.  Amling and Amling (1980) stated that rest is a growth-inhibiting physiological condition that can develop internally in buds.  The rest period can be satisfied by the exposure of buds to periods of cold temperatures.  Temperature, as well as hormones such as abscissic acid largely control the activity of the buds along with light intensity and day length (Nesbitt, 2002).  The hormone levels that induce dormancy dissipate during the process of chilling accumulation which generally occurs when temperatures are below 45 F, but above 32 F.  After the required number of chilling hours have been met (and this varies among cultivars and genotypes), an accumulation of heat over time will activate the buds and growth will begin again.  The in-between period when chilling has been satisfied, but heat accumulation has not been met, is known as quiescence.

Budbreak during the spring is closely associated with the chilling requirement.  Trees with a long chilling requirement will normally begin growth later than trees with a short chilling requirement. Budbreak regulation by heating and chilling is an evolutionary survival mechanism  derived through adaptation resulting in pecan being native throughout a large area of the United States.  In cold winter regions the high chilling received in the winter enables buds to break with minimum heating in the springtime.  Growth commences within a short period of time, thus increasing the probability that the fruiting cycle will be completed within the abbreviated growing season associated with cold areas.

Conversely, pecans are one of the most adapted plants to the southern U.S. because they have a relatively low chill hour requirement, but a high heat unit requirement. However, there are some cultivars that break bud very early which increases the danger of bud damage to spring frost (Nesbitt 2002).  The lack of a mandated chilling requirement contributes to pecan’s survival in regions with little or no chilling (Sparks, 2003). In these cases, the dormant period is prolonged in the absence of chilling temperatures; however, a deficiency of chilling temperatures can delay foliation, increase fruit drop, and reduce yield when pecans are grown in warm climates that lack sufficient chilling hours (Smith, 1994). The need for greater heat unit accumulating temperatures delays budbreak and minimizes the chance of damage from late spring freezes (Smith et al. 1992).  The mechanism of increased heat unit accumulation is evident in the southern U.S., where pecan is one of the last deciduous tree species to breakbud in the spring (Sparks 2005).

Budbreak in pecans is described as being under the interaction of chilling and heat accumulation.  Problems begin when sufficient heat is accumulated for the re-initiation of growth, leading to budbreak, in early spring when the chance of cold weather and damaging frost conditions has not yet passed.  The typical continental climate that exists in the Southwest, with wildly fluctuating winter temperatures, can pose a threat to those pecan trees that awaken from their quiescent phase and initiate budbreak because the heat requirement has been satisfied.

 Literature cited

Amling, H.J. and K.A. Amling. 1980. Onset, intensity, and dissipation of rest in several pecan cultivars. J. Amer. Soc. Hort. Sci. 105:536-540.    

Nesbitt, M. 2002. The pecan tree in winter. Pecan South 34(12):4-5.

Smith, M.W. 1994. Freeze injury to pecans. Proc. 28th Western Pecan Conf. pp. 155-157.

Smith, M.W., B.L. Carroll, and B.S. Cheary. 1992. Chilling requirement of pecan. J. Amer. Soc. Hort. Sci. 117:745-748.

Sparks, D. 1993. Chilling and heating model for pecan budbreak. J. Amer. Soc. Hort Sci. 118:29-35.

Sparks, D. 2005. Adaptability of pecan as a species. HortScience 40:1175-1189.

One aspect of this I do not hit upon is tree response to day length (photoperiod). Although timing of budbreak is highly heritable, other factors can influence it, such as shorter photoperiods.  Pecan trees are great survivors in our highly variable climate because they are native to the region.  In some ways they are very conservative in the spring, with late budbreak, but dangerously reckless in the fall (with late fruit ripening).  It is an interesting crop to observe and work with.  Right now (at the end of March) they are just beginning to show some green whereas other native and non-native trees have long since broke bud.  I wonder if such late budbreak is in some way a contributing factor to alternate bearing.

Pecan tree in south MS, just beginning budbreak at the end of March

Pecan tree in south MS, just beginning budbreak at the end of March

Buds begin to show green on pecan tree  at the end of March in south MS

Buds begin to show green on pecan tree at the end of March in south MS


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.

Blueberries and Chilling Hours

One of the last things one might consider when choosing a blueberry cultivar is chilling requirement.  A chill hour can vary depending on the model used, but the most common model in this region defines it as the number of hours below 45 F.  This is a requirement for the plant to satisfy its dormancy and thus to grow and fruit normally the following year.  In regions where cold temperatures are more common, plants can remain in a quiescent phase even after their chilling hour requirement has been met.  However, in warmer climates this may not be the case, so early flowering is a problem in low-chill cultivars.  Today, I have an example of that with a Florida-based, Southern Highbush cultivar, Springhigh.  As defined by the Florida Foundation Seed Producers, Inc., “‘Springhigh’ is a very vigorous, upright bush with excellent survival in the field.  It ripens about 9 days earlier than Star.  Berries are very large and medium dark, with good scar and flavor and medium firmness.  Sets fruit well even when pollination conditions are poor.”

Springhigh blueberry

Springhigh blueberry

‘Springhigh’ is reported to have a chilling requirement of around 200 hours, which is considered to be low-chill.  Today is December 18, 2012 and the place is Poplarville, MS.  We have gotten past the 200 chill hour mark for this year, although we have not gotten very cold at all (barely below 32F).  Let’s take a look at the state of  ‘Spring High’ in mid-December.

Buds are popping in mid-December.

Buds are popping in mid-December.

Flowers are showing in mid-December.

Flowers are showing in mid-December.

Fruit set in mid-December.

Fruit set in mid-December.

Large, developed fruit in mid-December.

Large, developed fruit in mid-December.

I was surprised at how much fruit this plant had on it.  As you can see it has all stages of reproductive growth on it right now.  Now, it is not all over the plant, many buds have not pushed yet, but if the weather continues as it has….who knows?  If we get a hard freeze then the fruit would certainly be a loss, but in the absence of that the productivity could be poor because no one is prepared to harvest plants so early in the season.  There is a reason we don’t really recommend very low chill types here.  Even along the coast they could get zapped.

So, chilling requirement is an important consideration in your growing region.  There are many cultivars out there that “fit”, so finding one is not a problem — it is knowing the chilling requirement before planting and pairing the cultivar with the site that is important.