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.
|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)
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.