Texas is home to 471 wineries and more than 5,000 acres of grapes. It has five different grape growing regions, stretching from the Gulf Coast to the High Plains and North Texas, as well as the Hill Country around Fredericksburg and West Texas north of the Rio Grande. Weather systems that hit one region don’t usually affect all regions; for example, hurricanes often follow along the Gulf Coast and frost events stay in the High Plains and North Texas.
However, the cold weather that descended on Texas beginning on February 9 was somewhat unique in that it affected the entire state over the course of the next week to ten days. The last time such a deep freeze had hit the state was in 1989. Because the damage for vineyards was potentially so extensive, the viticulture extension team at Texas A&M Agrilife put together a two-hour webinar, called the “Texas Winegrowers Freeze Recovery,” on Friday, February 26 that was attended by more than 300 people.
Dr. Justin Scheiner, assistant professor of viticulture at Texas A&M University in College Station, TX and viticulture specialist in the Gulf Coast and North Texas regions, noted in his introduction that “the Gulf Coast may have more damage than the High Plains, even though the temperatures were colder in the High Plains – it’s not just the varieties growing, but the state of the vines.”
For the most southern parts of Texas, the February freeze was a very cold Spring frost event, as vines on the Gulf Coast had started to set buds and in the Rio Grande River valley had already had bud break. For West Texas and the High Plains where the grapes were fully dormant, it was a mid- to late-Winter freeze. Farther to the east and south, vines may have been closer to bud break and therefore had more potential for damage from really cold temperatures during the week of February 10 to 17.
Monte Nesbitt, an extension specialist at Texas A&M for fruit trees, began his talk by noting that he may not know much about grapevines, but he is quite familiar with procedures to protect crops such as pecans, fruit and citrus trees during a frost/freeze event. He commented that if a grower uses a sprinkler system, the water must go on everything simultaneously and not stop. If the sprinklers do stop, “You’re worse off than if you didn’t sprinkle at all,” he said. When water is converted from liquid to solid, it gives off heat, and that heat is transferred through the ice to the plant. Snow, on the other hand, is more of an insulator, as it is a mixture of a complex lattice with air pockets.
One disadvantage of a sprinkler system is the quantity of water it requires to keep plants from freezing. Nesbitt sited one example of an eight-acre vineyard that operated 224 sprinklers for 34 hours. The good news is that two days after the event, the vines looked good. The bad news: the sprinkler system had applied 1,142,400 gallons of water.
Jim Kamas, viticulture and fruit specialist in Fredericksburg, reviewed statistics from past cold events in the Hill Country region. In the winter of 2009-2010, cold temperatures arrived in the region on December 25, 2009. Low temperatures remained in the 20s in January and the mid-teens in early February. On February 8, 2010 there was a frost event, but afterwards there was no damage. The following winter, temperatures were at 78° F on February 1, 2010, and the next day, the temperature dropped to 14° F. The result was “massive trunk injury, and grapevines were frozen to the ground,” according to Kamas.
This winter, Fredericksburg recorded 83.67° F on February 4, and 74.53° F on February 5. On the night of February 16-17, the temperature dropped to 3.17° F, So far, growers have not seen damage on vines like what occurred in 2011. Why not? Kamas posted a temperature chart that showed the temperature slowly falling between February 9 and February 15. Temperatures were at 32° F by February 11 and stayed in the 20s until February 14. This, Kamas said, was an example of re-acclimation of vines, which helped them handle the very low temperatures in mid-February.
What can a grower expect to see if the vines have been damaged? The first clue, Kamas said, is “lethargic or sporadic bud break, and damaged tissues may start to collapse. Frost injury is confirmed when sucker growth is more vigorous and healthy than the old cordon, and in summer, high transpiration rates may lead to vine collapse. This year, he has seen more damage on older Syrah vines than on Syrah planted in 2020.
In summary, Kamas noted that bud cold hardiness varies among growing regions, by variety and across the dormant period, and It can also depend on the amount of accumulated carbohydrate reserves in the vine.
Brianna Crowley, viticulture program specialist also based in Fredericksburg, had assessed the primary bud damage in the Hill Country region at numerous different vineyards. She found that there were big differences for different sites, different varieties, and for different cropping levels. She also saw differences within the same vineyard. Overall, she found that Cabernet Sauvignon, Merlot and Montepulciano had fared better than some of the other varieties. In looking at the season ahead, Crowley said, “I think we can expect a strange year this year.”
Dr. Pierre Helwi, viticulture program specialist in Lubbock commented that “winters in Texas are unpredictable and can be extreme.” He looked at data from three of the last four winters:
2017-2018: Winter had a lot of fluctuations. It was relatively warm from October 15 to December 31; then there was a rapid drop in temperature.
2019-2020: It was a long harvest, by Texas standards, and lasted into October. Two weeks later, on November 5, the temperature dropped to 15° F. Vines had not had enough time to go dormant, and a lot of varieties suffered frost damage.
2020-2021: An early harvest ended in September, so vines had some time to acclimate before frost hit on October 29. It was a cold winter from December through January 20, 2021, when low temperatures rose. In early February, the low temperatures again went down, and the vines had the chance to re-acclimatize. When the severe cold hit on February 16, the vines had maximum hardiness from the re-acclimation process, plus there was high humidity.
Helwi noted that it’s important for growers to pick the right varieties for their site, as they have different behaviors, and then growers need to manage their vineyards properly.
In 2019 Helwi started a project he called “Bud Cold Hardiness Real-Time Monitoring.” He had realized that information on cold hardiness came from colder climates, such as in New York, and he wanted to determine what was the lethal temperature (LT) where 50% of the buds would be killed on grapes grown in Texas. Buds were collected from canes of five varieties: Aglianico, Cabernet Sauvignon, Marsanne, Montepulciano, and Roussanne. They were then placed in separate cell containers, each one of which had a temperature sensor. The cells were put in a freezer and the temperature was dropped from 40° F to -40° F. When a bud group would freeze, it would release heat, which was registered by the sensor as the LT for that variety. The project developed a graph that showed on a given day the minimum outside temperature for the day and the LT for each variety on that day. Results showed that Roussanne was most cold hardy and Aglianico the least cold hardy.
“Winter injury can be expected in Texas,” Helwi said. Growers need to know what’s happened in the vineyard before pruning, and he encouraged growers to collect sample canes and check the buds.
The good news for growers this winter is that those growers who have looked at buds have not seen as much damage as in 2019, and the Roussanne buds were still alive, even though temperatures had been recorded at the LT for that variety. While the potential damage to vines is not yet known, there is hope that it won’t be as bad as in 2019.
Daniel Hillin, program specialist in Lubbock, stated that the High Plains does not yet have a clear picture of the damage to grapevines, although site, varieties and stress load are all factors. He presented a list of different ways that growers could determine the amount of damage in their vineyards:
Take 100 buds of a given variety and cut them open to see if there is brown phloem and dead tissue;
See if there are cracked trunks or cordons;
Look for suckers that are longer than shoots;
Watch for the premature pushing out of clusters on a new shoot;
Later in the season, vine collapse is a certain sign of damage.
Michael Cook, viticulture program specialist in Grapevine, said, “We’re going to see differences between regions in the weather we saw, the varieties grown and the stages of acclimation. In the storm from February 14 to 17, there were different amounts of snow and ice in different parts of Texas, with the most in the east or northwest Texas. Much of North Texas got about 5” of snow – dry, powdery, with lots of air pockets that hopefully trapped some ground heat. Ice was more to the southeast of Dallas/Fort Worth.”
Cook reported that the North Texas region was probably at its most protected point – vines were dormant, sap was not flowing, buds were not pushing or swelling, and there was zero bud break.
Temperatures fell gradually and stayed cold; snowfall occurred before temperatures fell to the lowest level; there was minimal ice formation; and many growers had saturated their soils beforehand. Young vines seem to have fared well, and mature vines overall had less that 30% bud mortality. His conclusion: “Time will tell.”
Fran Pontasch, viticulture program specialist in College Station, commented that the Gulf Coast region is known for growing hot climate grapes, and they don’t do so well in cold weather. Temperatures on February 16 went down to as low as 4° F, and that weather was consistent throughout the region. She checked with numerous vineyards and found similar results from the cold, with one exception: a low-lying vineyard of Black Spanish (Lenoir) and Blanc du Bois grapes that is usually subject to late Spring frosts. During the February cold spell, the low-lying area kept the buds tight and dormant, while higher up there was significant damage, including vine death. She thinks that the level of dormancy had a lot to do with the lack of damage on those vines.
Management of cold injured vines
Kamas commented first that rootstocks can influence the cold hardiness of the vine. He then continued, noting that how a vine responds to freeze injury depends on the health of the vine and its carbohydrate reserves. He suggested developing the concept of a “carbohydrate bank” so that the carbohydrates can act as antifreeze for the vine.
The only way to make deposits into this bank is to put in carbohydrates through photosynthesis. In Texas, grapes can be harvested in July and August and still have leaves for another two or three months. Those leaves in a healthy canopy are the best source of carbohydrates for the vine going into the Winter season. Withdrawals from carbohydrate bank can come initially from new foliage (which then becomes a source for more deposits), metabolic functions, cane maturation, respiration, and most importantly, crop load.
What can grape growers do if they find their vines have been injured during winter cold events?
If the vines simply have bud damage, prune as you normally would, or leave extra buds. “Instead of leaving two bud spurs, leave three bud spurs, and you can make up for as much as 50% bud mortality,” Kamas said. Dormant pruning is an invigorating action for the vine.
Don’t be in a hurry to cut out affected tissue. If there is green tissue, that indicates that there is some functional conductive tissue.
If major tissue shows no signs of life, go ahead and prune it back to areas with green tissue.
Severely injured vines need substantive care and handling to return to health. Leave several shoots and train them to an available trellis wire.
Consider using cane pruning for the recovery year until there is some lateral bud break on some canes, and some replacement cordons can be laid down.
Greatly reduce nitrogen inputs if severe injury presents itself.
Do not water stress the vines!
In summary, growers should do an adequate, but not excessive, job with water and nitrogen, and a good job of weed control. And most important, keep the canopy healthy for as long as possible after harvest.