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Current Season Winter Chilling: What’s Going On ???


Scott NeSmith

Dept. of Horticulture

1109 Experiment Street

Griffin, GA 30223

Old man winter has been slow to visit the Southeast this season, and we desperately need winter cold to stimulate suitable bloom and leaf bud development of blueberries in the upcoming spring. While many of our friends to the North are typically concerned very little with the amount of cold they get during the winter (they get plenty), here in the Southeast it is actually very important.

Just to review quickly, “chill hours” is a horticultural term used to describe the number of cold hours required for many fruit crops to bloom “normally”. The conventional method of calculating chill hours is to sum up the actual number of hours that temperatures are below 45 F. In Georgia, we typically declare the period from October 1 thru February 15 as the effective chill hour accumulation period. However, neither the temperature nor accumulation time are exact, they are simply general guidelines. Varieties of fruit plants such as blueberries are typically classified by the number of chill hours they need in order to readily break bud and bloom.   If a high chill cultivar is planted in a low chill environment, then spring bud break may be erratic and prolonged due to insufficient chilling. This can lead to poor pollination, especially for rabbiteye blueberries. Likewise, if a low chill plant is grown in a high chill area, then it is very likely the plant will begin to bloom prematurely since it receives its chill requirement earlier in the season. This can lead to freeze damage to blooms and severe crop loss. Therefore, growers of blueberries in the Southeast need to be mindful of the chill hour requirements of the varieties they select, and it is helpful to monitor chill hours each year to help in crop management.

For the 2015-2016 season, we have accumulated an unusually low amount of chill hours to date.   Table 1 depicts the chill hours for this season and the previous 4 years as of this writing (December 22, 2015) from UGA’s Georgia Automated Environmental Monitoring Network ( for various locations in south Georgia. The data clearly show we have an extremely low number of chill hours for the date. So, what does all of this mean and how could it affect the 2016 crop?

A major influence of this year’s low chill hour accumulation is to expect slow and protracted flowering to occur. To illustrate, Figure 1 shows a conceptual model for ‘Tifblue’ rabbiteye blueberry flowering in response to different chill hours from a greenhouse study of ours published years ago. The cumulative bloom data are depicted as per heat unit or growing degree day (GDD) after chilling, but again it is a conceptual model. The main emphasis for this discussion is to demonstrate that as the amount of chilling received decreases, note how delayed the onset of flowering is after warming begins. Also, note that flowering is not very concentrated, but can be spread out over many days and possibly even weeks. Since ‘Tifblue’ is one of the higher chilling rabbiteye cultivars, we might expect a less dramatic response for lower chill cultivars.  However, at the chilling hour accumulation rate we have at this time, the outlook is for most varieties to have erratic flowering habits. These odd flowering patterns will of course be reflected in ripening times as well.

Table 1. Chill hour accumulation from Oct. 15 through Dec. 22 for the current season and previous two seasons for select locations in south Georgia. Data are from UGA’s Georgia Automated Environmental Monitoring Network (

Location Chill hours (< 45 F)
2015 2014 2013 2012 2011
Alapaha 116 439 258 215 310
Alma 84 335 203 193 234
Baxley 151 408 277 206 276
Homerville 111 405 227 227 333
McCrae 209 502 331 324 433
Valdosta 52 253 144 106 128
Woodbine 58 308 176 189 174

Figure 1. Schematic of Tifblue rabbiteye blueberry flowering after different amounts of winter chilling.

Coupled with this potential for slow, protracted flowering of a particular variety in response to the low number of chill hours we have received for the 2016 season, we can also expect problems with overlap of flowering between some varieties as well.   To help illustrate this point, Table 2 depicts the average number of chill hours received at the UGA Research Farm near Alapaha during a 5 year period several years ago, along with bloom dates of 7 rabbiteye blueberry cultivars for each year. As we study these data we see that 1999 was the year most similar to the current year with regards to the amount of chilling received. In 1999, only 363 chill hours accumulated by Feb. 15 at this location. Note that all varieties in that year tended to flower much later and there were huge gaps in timing of extreme varieties like Climax (lower chill variety) and Tifblue (higher chill variety). There was a great difference in the timing and pattern of flowering as compared to the very high chill year of 2001. One can infer that the low chilling for 2016 should result in a similar response to 1999, and we should thus expect protracted flowering and possible pollination problems where variety bloom overlap is not timed well.   Growers might prepare themselves by having plenty of bees on hand, especially for rabbiteye varieties by the end of February. Also, there may be need for gibberellic acid sprays for some varieties as the flowering time progresses.

Table 2. Chill hours and bloom dates for a 5 year period (1998-2002) for several rabbiteye blueberry cultivars at the UGA Blueberry Research Farm near Alapaha, GA. Dates are for time of 50% bloom from our historical records. Chill hours are those accumulated from Oct. 1 through Feb. 15 each season.




Chill   hours

Alapaha Austin Brightwell Climax Premier Ochlockonee Tifblue
1998 620 Mar. 15 Mar. 15 Mar. 26 Mar. 4 Mar. 8 Mar. 29 Mar. 28
1999 363 Mar. 23 Mar. 24 Mar. 29 Mar. 16 Mar. 28 April 4 Mar. 30
2000 697 Mar. 17 Mar. 17 Mar. 18 Mar. 8 Mar. 13 Mar. 24 Mar. 20
2001 916 Mar. 8 Mar. 6 Mar. 5 Mar. 1 Mar. 3 Mar. 14 Mar. 11
2002 574 Mar. 25 Mar. 25 Mar. 8 Mar. 31 Mar. 26
avg. 634 Mar. 18 Mar. 16 Mar. 21 Mar. 7 Mar. 13 Mar. 27 Mar. 23

A final “prediction” for the 2016 blueberry season in response to this year’s high amount of winter chilling is to expect poor leaf emergence, much of it beginning after bloom instead of at flowering.  This may affect fruit development as well, causing slow ripening. Also, the slowed leaf and flower emergence might affect our pathology spray programs and their timing. So, we need to be alert to info coming from our Pathologists with regard to pest management practices.

In summary, the chill hours for the 2016 blueberry season are almost historically low to date. This lack of chilling should be expected to influence rate of flower and leaf development over the next few weeks for both highbush and rabbiteye blueberries. Growers can expect slow, extended flowering once warm-up begins, especially for higher chill cultivars. They should also plan for a challenging pollination period, with potential for poor overlap of flowering between some varieties. Finally, growers should be prepared for erratic leaf bud break, which could affect their disease control strategies. Of course all of these “forecasted” responses to low chilling depend greatly on weather patterns over the next few weeks, which are unpredictable. However, having some estimation for what’s ahead can help with crop management.

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About Ash Sial

Dr. Ash Sial is Associate Professor in Department of Entomology at the University of Georgia. He has had extensive training in agricultural entomology from various institutions. He earned his Ph.D. in Entomology from Washington State University where he worked with apple growers to develop sustainable IPM programs for major pests of tree fruits. After graduation, he accepted a Post-Doctoral Research Scientist position at University of California, Berkeley and worked with winegrape growers to develop sustainable IPM programs aimed at managing exotic and emerging arthropod pests such as vine mealybug, and the diseases transmitted by mealybugs such as grapevine leafroll disease. He then joined Cornell University to investigate various aspects of biology and ecology of an invasive insect pest – spotted wing drosophila, which has recently emerged as a major threat to fruit production in the United States. Currently, he serves as the blueberry entomologist and IPM Coordinator for Georgia. At the University of Georgia, the goals of his research program are to investigate biology and ecology of major arthropod pests of blueberries in order to develop sustainable IPM programs, and disseminate that information to all stakeholders including commercial blueberry producers in a timely and convenient manner. He has published numerous peer-reviewed papers, delivered research and Extension presentations including invited guest lectures and a keynote address. He has also served professional societies including Entomological Society of America (ESA) in a leadership role at the regional and national levels. He has been recognized for excellence in research productivity and professional leadership at the regional and national level with several prestigious awards including the John Henry Comstock Award.