Late-spring frost events can cause severe injury to grapevines, often leading to the loss of fruitful buds and subsequent decreased yield and fruit quality. Severe frost injury has the potential to destroy a whole vintage. In areas such as the Southeast, where spring frosts are common, both passive and active frost control techniques are essential to maintain the longevity and economic sustainability of a vineyard. However, while active control techniques such as wind machines and sprinkler systems can save a crop during a frost event, implementing those methods is costly and technically challenging. Here we discuss recommended methods of prevention and frost protection, management, and implications of frost injury in grape vines. Please refer to Spring Frost Control, chapter 11 of the North Carolina Winegrape Grower’s Guide for a broader discussion of spring frost control measures.

Late spring frosts are common in the southeastern United States, capable of costing several million dollars (Poling, 2008; Zabadal et al., 2007) in damage. In contrast to many other high-value crops, frost protection methods for wine grapes are less frequently deployed, due to high costs and management challenges. However, the damage that can occur as a result of severe frost events can be devastating. Large investments, such as the purchase of wind machines, are economically sustainable if frequent frosts (every five to six years) occur (Poling, 2008). For example, after two frost events in late April and one in mid-May of 2020, more than 70% of North Carolina wine grape vineyards suffered significant crop loss, with some losing close to all fruit for the year. Such losses are devastating if experienced frequently. An integrated approach of preventative measures, management, and pruning can help to mitigate short- and long-term implications of late-spring frost damage.

It is crucial to understand the differences among cold events that can occur in spring. Frost events are caused by radiation, while freeze events are caused by moving air masses that cool surfaces (advection). Generally, cold events can be characterized into three different categories (Table 1): Frost events, frost/freeze events, and freeze events. Frost events are by far the most common during spring in the Southeast, characterized by clear nights and wind speeds less than 5 mph. Hoar frosts are accountable for over 90% of all frost events in the Southeast, often indicated by water crystals forming on surfaces. Another common frost type is black frost that occurs on days with low humidity and therefore the typical ice crystals are missing. Frost generally occurs when the temperature of a surface is equal to or lower than a dew point of 32°F. This is often the case on cloudless, cooler nights that promote radiation (Poling, 2008; Centinari, 2018). Radiation occurs when warmer temperatures during daylight hours heat plants and the soil. Cloud-free skies and cooler temperatures at night cause heat to reradiate from surfaces. Frost then can build up on plant surfaces (shoots, buds, etc.), causing injury through dehydration and physical damage. Clear nights with air temperatures of 40°F or lower, and dew points of 32°F or lower, are at high risk for frost. While the National Weather Service might issue a frost warning at temperatures above 32°F, that does not mean that a radiation frost will occur and damage a specific vineyard.

Frost / freeze events are typically longer in duration (more than 10 hours), caused by a mix of radiation and persistent winds. Crop protection is made very difficult by sustained winds greater than 5 mph.

Windborne (advective) freeze events are caused by moving air masses, bringing cold air from northern weather systems into the Southeast. While this happens frequently in winter, spring freeze events are rare. However, they can be very dangerous, last for several days, and bring significantly colder temperatures at a time when the crop is most vulnerable. One of the most memorable spring freeze events was in April of 2007, when five days of freezing weather had a significant impact on farming all across the East Coast, including Florida (Poling, 2008). On grapevines, windborne freezes may not just cause the loss of buds and shoots, but may also lead to damage on trunks and cordons. Vines suffering severe cold injury can die, or, if they survive, often show deep cracks in the permanent wood structure and have to be removed later in the year.

In this article, we will mainly talk about prevention and management of spring frost events in established vineyards. Most of the aforementioned mitigation and prevention measures will not help in the case of windborne freezes. The prevention of frost injury in an established vineyard can be partly achieved through the use of active measures, such as vineyard wind machines or sprinkler systems, and passive measures, such as delayed pruning. However, the best practice to avoid frost and cold damage in a vineyard begins before planting, with the selection of the correct site, suitable cultivars, and the best training systems.

While this article focuses on the prevention and management of frost injury in established vineyards, the best prevention is to select a proper field site and the correct cultivars.

The choice of vineyard site and cultivar has a large impact on the occurrence of frost injury. Sites that frequently show frost on grass, vegetation, or other structures should be avoided, as they usually do not experience very good airflow (Figure 1). Vineyards located at higher elevations (relative to surrounding topography) will be affected by fewer radiational frosts. In addition, better air movement in higher-elevation vineyards helps to lessen disease pressure. Vineyard floor management programs comprised of a vegetation-free strip under the vines, along with close mowing of vegetation between rows and around the perimeter of the vineyard, may provide a slight increase in air temperatures within the vineyard as compared to tall, un-mowed vegetation or freshly cultivated soil. Promoting good air drainage out of the vineyard by limiting or removing any possible obstructions to air flow below the vineyard will likewise help to limit frost damage.

Early bud-breaking cultivars should also be avoided. However, market demands often determine the choice of cultivars, with the early-breaking ‘Chardonnay’ and ‘Merlot’ at the top of consumers’ lists. We highly recommend double-pruning or delayed pruning techniques (see below) on those cultivars.

Figure 1. Effect of vineyard site and topography on air flow (Poling, 2008).

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Figure 1. Effect of vineyard site and topography on air flow (Poling, 2008).

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Wind Machines

Vineyard or orchard wind machines (Figure 2) may be effective tools to prevent or minimize frost injury during radiant frost events. Wind machines work on the principle that during the night, warm air will be radiated into higher air layers and become “trapped,” preferably at a height between 15 and 75 feet. This weather condition is called inversion and is fairly common during spring frost events. The height of the inversion layer is referred to as the ceiling and the difference in temperatures at ground level versus the ceiling is called the strength of the inversion.

Wind machines mix the warmer air below the ceiling with the colder air near the ground and remove the coldest air close to leaves, replacing it with warmer ambient air. The amount of protection achieved is dependent on the strength of the inversion. The use of a wind machine during a frost/freeze or an advective freeze should be avoided. Usually, wind speeds of 5 mph or greater will disturb inversions and make wind machines less effective. The operation of a wind machine also depends on the microclimate in your vineyard, the terrain, reliable weather data, and good weather predictions. A machine can cover as much as 10 acres of vineyard, depending on the terrain and vineyard layout. In the best-case scenario, a wind machine should be planned as an integral part of vineyard establishment.

A weather station should be installed in the vineyard and online agricultural weather services (e.g., AWIS Weather Services) should consulted frequently to better predict the use of wind machines. Wind machines can be either portable or permanently installed, and should not be confused with wind turbines that are used to generate electricity. Wind machines have tall fans that are driven by propane, gasoline, or electric engines. The cost of a wind machine can easily exceed $25,000. However, if frost injury frequently leads to loss of yield, the installation of a wind machine is a sustainable investment. The operation of a wind machine creates a lot of noise; we highly recommend talking to your neighbors before purchasing and installing a wind machine.

Overhead Sprinkler Systems

Frost protection with overhead sprinklers requires maintaining a continuous water/ice interface, and an undisturbed tissue/ice connection. To achieve this, water needs to be supplied constantly starting at dusk until temperatures are high enough for the ice to thaw (Figure 3). This will build up a layer of ice over the tissue. Constant application of water will keep the ice temperature around 31°F or 32°F, slightly above critical levels for damage to buds and shoots.

Using sprinkler systems to protect leaf and bud tissue from frost injury requires consistent droplet size and coverage. The water-ice layer on top of the tissue should always be clear (not cloudy or white), and water droplets should be seen running down the sides.

Vineyard overhead sprinkler systems require large water sources, high capacity pumps, reliable operation in cold temperatures, fast sprinkler head rotation times, and high uniformity in droplet size and distribution patterns (Table 2).

The choice of equipment is therefore highly important. Sprinklers with a minimum rotation time of 30 seconds are recommended to be installed in a vineyard. Conventional or hybrid impact sprinklers with a mechanical braking system are recommended to handle differences in water pressure. If sprinkler systems use silicone, they might slow down in colder temperatures. Water needs to be distributed uniformly, and droplet size should be consistent over the area that is covered. Differences in droplet size can lead to areas in which plant tissue is not covered sufficiently, leading to subsequent cold injury. Hybrid impact sprinklers with an integrated nozzle and deflector design are recommended to achieve higher uniformity. Systems with nozzles that are easy to assemble and replace are highly recommended. The frost protection success of a sprinkler system relies on the frequent and uniform distribution of water throughout the night (Figure 3).

Overhead sprinkler systems only can be used during low-wind speeds. We recommend installation of sprinkler systems in blocks with early bud-breaking cultivars. It is critical to have a water supply large enough to allow continuous irrigation over several hours and possibly for more than one night back-to-back. If pulling water from a stream, consider the stream flow during periods when frost control might be needed. If pulling from a pond, consider pond size and the recharge capacity of the pond. Sprinkler systems can cause flooding and saturated conditions on not well drained soils, leading to problems with disease and die-back later in the season. Overhead sprinkler systems for frost protection are recommended only if the water supply is adequate and soils have good internal and surface water drainage characteristics.

Delayed pruning

Delayed pruning and double-pruning are widely used passive methods to delay bud break and “trick” late spring frosts on spur-pruning systems. With double pruning, one-year-old wood of early bud-breaking cultivars trained on a bi-lateral cordon system (such as a Vertical Shoot Positioning or a high-wire cordon system) can be mechanically pre-pruned at the catch wire leaving about 8 to 12 buds per shoot. After removal of the wood, the vines will remain like this until bud break. Cutting to fewer than seven nodes above the desired points for bud break may result in an inadequate delay in the time of bud break. A final pruning to the desired number of spurs and buds per vine should then be performed manually in early spring, usually at growth stage E-L 4-5. The apical buds on the pre-pruned one-year-old wood will break first, and in the case of a late spring frost, will be “sacrificed” for the still-dormant basal buds, which will be retained after the final manual pruning step.

With delayed pruning, no pruning is done until bud break, at which time selected shoots are cut back to the desired bud count and unneeded shoots are removed entirely. As a result, bud break may be set back by two to three weeks as compared to shoots pruned earlier. If the final manual pruning step with either delayed pruning or double-pruning is not done until a later growth stage, crop load reduction and/or delayed fruit maturation may result (Hickey & Hatch, 2019).

Other Methods of Frost Protection

There are several other active measures of frost prevention, including the use of helicopters to move air from the inversion zone to the vineyard, the use of hot stones or heaters to supply hot air in a vineyard, and the use of fans in lower elevations to move warmer air into the vineyard (Poling, 2008). The use of vineyard fires is only recommended if users are fully aware of the risks. Vineyard fires are very labor-intensive and can cause severe damage to a vineyard if not handled properly. All active frost protection measures rely on the presence of a temperature inversion. In the case of windborne freeze events, a vineyard is almost defenseless and the methods that are described are less effective, technically not workable, or outright dangerous.

Figure 2. A stationary wind machine in North Carolina.

Eric Case

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Figure 2. A stationary wind machine in North Carolina.

Eric Case

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Figure 3. The effects of water coverage and droplet size on leaf and bud tissues.

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Figure 3. The effects of water coverage and droplet size on leaf and bud tissues.

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How to assess injury

Usually two types of tissue are likely to be injured during a spring frost event: buds and shoots. Damage to the permanent structure of the vine resulting from a spring frost event, while not common, can occur. However, injuries to the trunk, head, cordon, or spurs are more likely if cold-sensitive cultivars are planted in locations that lack good airflow, or after a strong freeze event.

Assessing bud injury: The assessment of bud injury is very common in the Southeast and we recommend it as a routine practice in early bud-breaking cultivars every year. This technique requires the sacrifice of 60 to 100 buds/acre/cultivar either before pruning or after a late spring frost to assess viability of primary and secondary buds. These assessments will either determine the pruning strategy (e.g., how many buds to retain on the final cut if pruning was delayed) or will help to assess potential damage and estimated crop load after a late-spring frost. Please refer to section 6, “Resources,” for more guidance on bud injury assessment.

Assessing shoot injury: Frost damage on shoots causes browning and wilting. Frost damage may not be immediately noticeable on shoots but symptoms may appear more clearly after a few days (Figure 4). Young succulent shoots will wilt once the frost thaws, but older, more hardened shoots will take a few days to show symptoms. Frost damage of shoots can greatly reduce yield and lead to uneven ripening, depending on the severity of wilting. The management of shoot injury depends on the severity of the injury and the growth stage of the plant. For more information on shoot frost injury, please refer to section 6, “Resources,” below.

Assessing trunk injury: Trunk injury can occur without directly visible symptoms, and results in weak growth during the following seasons. Phloem tissue can be injured even though no splitting may have occurred. Trunk- or cane-splitting is a very severe cold injury and requires attention. If trunk- or cane-splitting remains unattended, crown gall and grapevine trunk disease are often the results, leading to expensive losses and decay in subsequent years.

Managing frost injury

After frost injury has occurred, taking no action may be the best management option. Some grape cultivars, especially French-American hybrids, may have fruitful secondary buds which will be able to produce 40% to 70% of a full crop. Trying to maximize crop loads after cold injury may lead to a reduced crop during the coming year, depending on the severity and type of damage.

No action:

• In the case of bud damage after the final pruning, taking no action may still be best.
• If incomplete kill of shoots occurs, no action might be the best approach. Once the vines reach a certain stage (E-L 15; see the E-L scale), taking no action is the best approach. However, taking no action means there is increased risk of disease due to the dead material retained.

Removal:

• Before E-L 12, buds can be rubbed off to force the growth of secondary buds. However, it is unclear if this will have an advantage over taking no action.
• Moderate shoot injury: Cutting the tops off the green shoots stimulates the bursting of secondary buds below. However, this is not recommended; additional lateral growth can be an issue, and a late-ripening secondary crop can cause issues at harvest.
• Severe shoot injury: Removing all shoots back to the cordon may be considered if at E-L 12 or lower and if higher quality pruning material for next season is desired. This action will force dormant buds to break. Yields in this season will be less than if no action is taken! The later the shoot removal is conducted, the greater the reduction in bud fruitfulness during the following season.
• If visibly damaged, trunks and canes need to be removed. They can be removed at the end of season, depending on overall health of the trunk. Keeping injured trunks and canes in the vineyard over several seasons will cause large problems with crown gall and trunk diseases.

Figure 4. Shoots affected by wilting and browning after a spring frost in 2020.

Eric Case

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Figure 4. Shoots affected by wilting and browning after a spring frost in 2020.

Eric Case

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Late-spring frosts are a challenge for viticulture in the Southeast. Injury caused by such frost events can lead to uneven ripening and yield loss. However, several methods are available to prevent and manage late-spring frost damage, many of which are expensive and / or labor intensive. A grower must consider a number of economic as well as practical considerations when deciding whether to invest in an active cold-protection system or method, and difficult tradeoffs are generally involved. Those methods are the only tools currently available to mitigate frost damage in vineyards, however, and should be deployed in locations with higher risks for late-spring frosts.

Poling, B. 2007. "Spring Frost Control." In North Carolina Winegrape Growers Guide, 159–177. North Carolina State University.

Assessment of bud damage:

Hoffmann, M. 2019. Grape Harvest in the Southeast: Complex Decision Making. (presentation), (PDF, 2.4 MB), North Carolina State University.

Checking Bud Mortality in Your Vineyard (video), Cornell Extension.

Martinson, T. and Pool, R. 2011. Assessing Winter Cold Injury to Grape Buds (fact sheet) (PDF, 1.6 MB), Cornell University.

Assessment of shoot damage and frost protection:

Dami, I. 2014. Guide to Assess Freeze Damage in Grapevines in Early Summer, (PDF, 2.3 MB), The Ohio State University.

Hellman, E. 2019. Frost Injury, Frost Avoidance and Frost Protection in the Vineyard. Texas A&M.

Jones et al. 2010. “Effect of Frost Damage and Pruning on Current Crop and Return Crop of Pinot Noir”, (PDF, 403 KB), New Zealand Journal of Crop and Horticultural Sciences 38(3): 209–216.

Striegler et al. 2012. Understanding and Preventing Freeze Damage in Vineyards (workshop proceedings) (PDF, 4.5 MB), Dec. 4–5, 2007, University of Missouri–Columbia.

Walton et al. 2009. Grapevine Growth Distortions, (PDF, 1.5 MB), Oregon State University.

Centinari, M. 2018. Understanding and Preventing Spring Frost and Freeze Damage to Grapes. Extension Bul. ART-5334, Pennsylvania State University.

Hickey, C. and Tremain, H. 2019. Dormant Spur and Cane Pruning Bunch Grapevines. (PDF, 6.3 MB), Extension Bul. 1505, University of Georgia.

Poling, B. 2008. “Spring Cold Injury in Winegrapes and Protection Strategies and Methods.” HortScience 43(6):1652–1662.

Rain Bird. 2011. Vineyard Frost Protection – 3 key requirements for effective protection with overhead sprinklers. (PDF, 1.0 MB)

Zabadal, T.J., Dami, I.E., Goffinet, M.C., Martinson, T.E. & Chein, M.K. 2007. Winter Injury to Grapevines and Methods of Protection. Extension Bul. E2980, Michigan State University.