Tree Fruit: What are chill hours?
Mississippi's winter chilling information
Deciduous fruits and nuts stop growing in late summer or fall, drop their leaves, go dormant during the winter, and then resume growth in the spring. This relationship between plant and environment is important to the survival of the plant. Growing plants that are non-hardy and incapable of becoming hardy need dormancy during winter for survival. In areas where winters may fluctuate between cold and mild temperatures, species have developed long chilling requirements, so they will not begin to grow in midwinter even though it may warm up to growing temperatures for several days.
Endodormancy (rest) is defined as that period when buds are dormant because of internal physiological blocks that prevent growth even under ideal external conditions for growth. Chilling termperatures above freezing terminate endodormancy. Chilling hours are defined as period of time between 32º F and 45º F. Plants are assigned a certain chilling requirement based on the amount of cold needed to cause 50 percent of the buds to break and flower in the spring. Most blueberries have a chilling requirement of 400-600 hours. Peaches are planted using the chilling requirement as a criteria for variety selection and range from a low of 400 to a high of 1250. Average chilling hours during the winter in Mississippi are: Hattisburg - 400-600; Jackson - 600-800; Mississippi State University - 800-1000; and Holly Springs - 1000-1200.
In 1999, middle to lower Mississippi experienced a low chilling hour accumulation. There is a material (Dormex) that can be sprayed on the commercial plantings that will substitute for about 200 hours. This material is a restricted-use pesticide and can damage the plant if used improperly. Dr. John Braswell, Dr. Frank Matta, and I have asked for, and received, a Mississippi Label for this material.
Dr. Arlie Powell (fruit specialist in Alabama) has performed research and demonstrations with Dormex in Alabama for over 10 years. Information concerning Dormex can be found on their web site, Alabama Winter Chilling
At this time, we feel that we will accumulate sufficient chilling hours in Mississippi. However, some growers may be interested in discussing the use of Dormex in late Febuary.
Mississippi’s recent bout of bad weather came at a critical time for producers of blueberries, the state’s largest commercial fruit crop. Blueberries can be easily damaged by cold weather, but the timing of mid-February’s icy weather limited the potential damage.
Despite weather challenges combined with a decreased production year for most pecan varieties, Mississippi’s 2020 crop will be decent.
The COVID-19 pandemic presented a new obstacle for Mississippi blueberry growers in 2020, impacting the labor force for the early-season varieties.
The invasive species of fruit fly, Spotted Wing Drosophila, can wreak havoc on the state’s largest commercial fruit crop – blueberries. But homeowners likely won’t find it to be a significant problem.
Regional agriculture advisory groups will meet across the state next month to provide input on educational programing and research conducted by Mississippi State University.