We experienced some really cold temperatures from December 23 – 27. It is not the first time that it’s been cold in Georgia, and we have even seen similar temperatures in past freeze events over the last ten years. However, the Christmas freeze of 2022 is somewhat unique; these freezing temperatures arrived relatively earlier in the winter, dropped rapidly, and were prolonged over several days. That combination could potentially spell trouble for wine grapes and muscadines.
How much trouble? It’s hard to say exactly, and only time will tell for sure. There are a couple of factors involved. One is the level of cold hardiness that plants had achieved when the freezing temperatures occurred. Plants that were 100% dormant may be able to handle temperatures down to 0°F. However, keep in mind that the minimum temperature that varieties can handle varies. Most vinifera are less cold hardy than hybrids. Muscadines have a minimum temperature limit to 10 degrees Fahrenheit. Fluctuations in temperatures reduce cold hardiness. Considering that temperatures were in the 40s and 50s before the freeze came through, we have seen fluctuations.
Healthy vines will have better cold hardiness. This means vines that had optimal crop load in the previous year (low stress), effective spray programs (good leaf maintenance into the fall and adequate carbohydrate storage), and proper fertility (not overly succulent going into the winter) will fare better. Vines that are unhealthy will develop cold hardiness more slowly as the season progresses, and they are more susceptible to cold injury and even winter kill.
Freeze damage is not always readily apparent. It will take a few weeks for the symptoms to show. If your vines have experienced freeze damage, you may see cracks in the cordons or trunks. These cracks create opportunity for trunk-infecting pathogens (e.g. Botryosphaeria and crown gall) to establish disease. If damage from disease is severe enough, affected plants will need to be replaced. If the damage is not too severe, plants should be pruned back to cut out the damaged tissue. Some vines may have sustained vascular damage that does not become apparent until summer, and ‘vine collapse’ may be observed at that time and beyond. In the coming weeks and throughout the growing season, scout vines for damage to assess what type of action needs to be taken.
Dr. Mark Hoffmann from North Carolina State Extension recommends to assess damage using a razor blade and a hand lens. Check buds along un-pruned wood that will be retained. Use the razor to slowly trim away the bud. This will expose necrotic tissue, if it is present. As you trim down you will first expose the primary bud, then secondary, and finally the tertiary bud. If you have 0 – 15% dead primary buds prune normally. 15 – 80% dead primary buds, increase the number of buds retained. 80%+ dead primary buds, do minimal pruning and regulate vegetative growth later in the season.
This pdf is from Dr. Mark Hoffmann that shows where to make the cuts on buds.
Below are some links to more resources that provide more information on freeze damage and its mitigation.
https://aggie-horticulture.tamu.edu/vitwine/2018/05/10/cold-hardiness/
