By D. Scott NeSmith
University of Georgia
Volume XXVIII |
Our friends to the north may not worry over the amount
of winter “chill hours”
they get. They get plenty. But here in the Southeast, it’s
critical in growing
blueberries.
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.
Study results
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.
