Article written by Heather Kolich, Forsyth County Extension Agent
With an October freeze and weeks of balmy December days followed by several consecutive days of sub-freezing temperatures, winter has tossed Georgia some unusual surprises so far. Both temperature conditions can affect the development of fruit crops, including blueberries and peaches, crops that are economically important to Georgia.
From apples to strawberries, perennial fruits and nuts need a certain number of cold hours, called chill hours or chilling hours, during the winter to convince them to break dormancy for fruiting. The number of required chill hours varies by the type of fruit as well as by the different cultivars within a fruit species. For example, citrus fruits require 0-100 chill hours to produce fruit. An apple cultivar with a low chilling hours requirement, such as Anna, will begin budding with as little as 200 hours, but an apple variety with a high chilling hours requirement, such as Honey Crisp, will remain dormant until it has accumulated 800-1,000 hours of cold between 32-45 degrees Fahrenheit.
If the area of planting does not accumulate the requisite number of chilling hours between October and the end of February, fruit plants produce fewer blossoms, resulting in a lower yield of fruit. Alternatively, if the area receives more than the required amount of chilling, the plants may break bud before winter ends. That is one of the reasons that oranges are grown in Florida and apples and blueberries are grown from Georgia to Michigan.
Checking the Chilling Hours Calculator on the University of Georgia Weather Network (www.georgiaweather.net) shows that Elijay, the Apple Capital of Georgia, has accumulated over 700 chilling hours between October 1 and December 30, 2022. In the southern half of the state, Vidalia, home of the Vidalia Onions, has accumulated 334 chilling hours.
Subfreezing temperatures, especially when prolonged, can injure or kill fruit buds. Although chill hours are historically counted at temperatures between 32 and 45 degrees Fahrenheit, in an interview for Specialty Crop Industry trade journal, UGA Cooperative Extension Fruit Specialist Phil Brannen said the deep-freeze weekend could be beneficial to Georgia’s peach and grape crops by adding chill hours. Extreme cold also helps to kill the bacteria that causes Pierce’s Disease in grapes as well as the pest insect that spreads the disease.
The killing cold also benefited the southern half of the state, where cotton, corn, and peanuts are major economic crops. According to Bob Kemerait, UGA Professor and Extension Specialist in Plant Pathology, the extreme cold contributed to the control of several crop pests, including rust fungi, root-feeding nematodes, and kudzu.
On the home landscape front, many plants are showing signs of cold injury. Time and patience are the best treatment now and as temperatures return to normal for bedding plants, woody perennials, and turfgrasses. Wait and see is the advice from UGA Extension horticulture and turfgrass specialists Dr. Bodie Pennisi and Dr. Clint Waltz.
Pansies and violas planted in beds have a good chance of recovering, but other seasonal color plants, including perennials like heuchera and dusty miller, may not prove as hardy. Plants in containers and raised beds may not recover either, as the roots were more exposed to freezing than those of plants in the ground. Turfgrasses are also likely to recover, according to Dr. Waltz. Recovery is evidence of green tissue, but don’t apply fertilizer until plants begin active growth.
Perennial shrubs and trees were also affected, especially on young twigs at the edges of the canopy. Resist pruning until closer to spring, when the damage can be assessed. Pennisi says the damaged twigs could provide protection to the rest of the plant if we have another freeze event.
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