Chill Hours

peach bloomsAg Biz News Column
Chad Gulley
County Extension Agent –Ag/NR
Smith County

 Chill Hours for Fruit Trees

Chill hours are important to many wanting to grow fruit trees around the state of Texas.  The dormant buds of many plants require a period of cold weather to grow, flower, and develop properly, but requirements vary widely by species. For dormant buds of fruit trees, this is commonly referred to as the chilling requirement.

Chilling hours are calculated as a tool for fruit producers to gauge whether their crop has been exposed to cold temperatures for a long enough time period.  Temperatures can fluctuate from one location to another.   Chilling and dormancy is complicated, but the old standard measure of air temperature at locations is as accurate as any.

Several methods for calculating chill hours exist, however two methods listed next are used locally at the Overton Research and Extension Center.  Method one , chilling hours are calculated by counting the number of hours between 32 and 45 degrees F, from October 1st through February 28th/29th.  Method two, chilling hours are calculated by counting the number of hours at or below 45 degrees F, from October 1st through February 28th/29th.

The Texas A&M AgriLife Research and Extension Center in Overton, Texas has a website that keeps up with chilling hours at http://etweather.tamu.edu/chill/ .   From the Overton site as of February 19, using method one we have received 972 chill hours.  Using method two, we have received 1346 chill hours.  Again, this may vary from location to location but it gives us a number to go off of for planning our fruit orchard.

With peach trees, for example, if varieties are chosen that have a chilling requirement that is too low, there is a greater probability that they will bloom early and be more subject to frost.  If the chilling requirement is too high, they may be very slow to break dormancy and abort fruit.  Plant varieties which have the correct chilling requirement for your area. It is generally recommended that prospective grower choose varieties that have a chilling requirement 100 hours above or below what is received on average at an orchard location.

Peach varieties vary in chill hour requirements.  Flavorich and Regal are examples of peaches with chill hour requirements in the 700 hour range.  Derby, Dixieland, Fairtime, Loring, and Redskin are examples of peaches with chill hour requirements in the 750 hour range.  Fireprince, Redglobe, Ouchita Gold, Parade, and Ruston Red are examples of peaches with chill hour requirements in the 850 hour range.

Blueberry varieties also require chill hours and will vary by variety.  Prince and Woodard blueberry varieties have chill hour requirements in the 350 hour range. Climax and Brightwell blueberry varieties have chill hour requirements in the 400-450 hour range.  Blueberry varieties requiring longer chill hours include Premier (550 hours), Powderblue (600 hours), Tifblue (650 hours), and Ochlockonee (700 hours).

For a complete list of fruit and nut varieties for the Smith County area access the following website at http://easttexasgardening.tamu.edu/  or contact our office at (903) 590-2980.  This list contains information on peaches, apples, blueberries, blackberries, strawberries, plum, pears, pecans, and more.

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