By D. Scott NeSmith
University of Georgia
What and why?
Chill hours is a term for the number of hours required (below 45 degrees from Oct. 1 through Feb. 15 in Georgia) for many fruit crops to bloom "normally." Neither the temperature nor the accumulation time are exact. They're just guidelines.
Blueberry varieties are classified by the number of chill hours they need to readily bloom. If you plant a high-chill cultivar in a low- chill environment, then spring bud break may be erratic and prolonged. This can lead to poor pollination, especially for rabbiteye blueberries.
Likewise, if you grow a low-chill plant in a high-chill area, it's very likely to bloom prematurely, which can lead to freeze damage to blooms and severe crop losses.
In a study of seven rabbiteye blueberry varieties over the past five years near Alapaha, Ga., the five-year average bloom dates were: "Climax," March 7; "Premier," March 13; "Austin," March 16; "Alapaha," March 18; "Brightwell," March 21; "Tifblue," March 23; and "Ochlockonee," March 27.
A few things are evident from the study.
First, the varieties bloomed on different dates. So they respond differently to the same amount of chilling. This is due to some varieties having a lower chilling requirement than others.
It's critical to recognize the differences in blueberry variety bloom dates and to plant a mixture that will bloom near the same time.
Overlapping bloom times are crucial in growing rabbiteye blueberries, since the species requires cross pollination to successfully set fruit.
Tifblue, for example, would likely perform much better, in terms of fruit set, if planted with Brightwell than if planted with Climax. The new releases Alapaha and Austin have compatible bloom dates, so they would be good varieties to plant together.
If you want three varieties, it might be useful to plant a mix of Alapaha, Brightwell and Premier to provide the most overlap in bloom time.
The study shows, too, how greatly the bloom date can vary from year to year, depending on the number of chill hours.
Interestingly, the test site had a near-record low number of chill hours (363) in 1999 and a near-record high number (916) in 2001. These two years should represent the extremes for bloom dates for these varieties.
The bloom dates in the frigid winter of '01 and the warm winter of '99 were: Climax, March 1 and 16; Premier, March 3 and 28; Brightwell, March 5 and 29; Austin, March 6 and 24; Alapaha, March 8 and 23; Tifblue, March 11 and 30; and Ochlockonee, March 14 and April 4.
Use these dates to help assess the risks of growing a variety in your area. Those that consistently bloom after your average last frost date would be less at risk than those that bloom earlier.
If you still want to plant the risky varieties, try to use higher sites that are less prone to frost damage. Or, if possible, use frost- protection sprinklers on these varieties.
Getting the information
In Georgia, you can get up-to-date weather data, including chill hours, for more than 50 sites across the state from the Georgia Automated Environmental Monitoring Network (www.griffin.peachnet.edu/bae/).
You can get a general idea of when your blueberries might bloom, based on the chill hours received for the year. (Again, this isn't exact).
It's hard to do anything about the weather. But this shouldn't keep us from being learning the effects of year-round weather on the growth of crops like blueberries.
It may look like little is going on in those leafless blueberry plants during the winter, but don't be fooled. Those plants are chilling out, getting prepared to bloom.
(Scott NeSmith is a horticulturist with the University of Georgia College of Agricultural and Environmental Sciences.)