Mitigating frost injury in grapes

By Kathryn Carter, Fruit Specialist, OMAFRA

Globally we are seeing milder winters, and earlier springs, which are resulting in earlier budbreak on grape vines, increasing the risk of spring frost injury. Mitigating frost injury will be a significant challenge for cool climate grape production in the future. 

Impact of spring frost on grapes

Late winter cold snaps can result in winter injury to hybrid and vinifera grapes. Winter injury occurs when ice crystals form within plant tissues damaging dormant buds on the vine. As the grape vines begin to de-acclimate in the spring they gradually lose their maximum cold hardiness and become more vulnerable to cold temperatures (Figure 1).   Winter injury can cause significant damage to the grape vine, resulting in crop loss, damaged canes and even trunk injury.

Figure 1 Profile of bud cold hardiness during the dormant season (CCOVI VineAlert Website)

Frost injury occurs when the vines have started to break bud, and air temperatures dip below 0° C resulting in damage to tender green shoots and young leaves. Frost injury impacts buds, green shoots and leaves and can result in a significant reduction yields and uneven ripening. The risk of early frost injury to grapevines is greatest from budbreak to flowering. Early season frosts can also affect vine structure, potentially impacting the pruning strategy used in subsequent years.

Susceptibility to winter and frost injury

Hybrid grapes lose their cold hardiness and begin budbreak earlier than most Vitis vinifera making them more susceptible to winter injury and frost in the spring (Kovaleski et al., 2018). Similarly grape varieties vary in their susceptibility to frost injury depending on their budbreak. Vines are reasonably hardy until sap flow begins in the spring and then hardiness levels can be lost at a rate of up to 4 °C or more in a week. Be aware that phloem and xylem tissue in trunks and canes are less hardy by a few degrees or more than buds, especially as sap flow resumes.

Bud break is often the most common time when frost damage occurs in vineyards. Vines at bud break are vulnerable due to the high water content of growing organs in the buds, which makes them more vulnerable to cold temperatures. Air temperatures of -2 or -3° C can permanently damage green tissue.   

Types of frosts

The options available to mitigate a freeze/frost event depend on the type of frost that occurs.

Advective frosts are a widespread weather event resulting in strong, cold winds that sweep into a region in the day or night bring cold temperatures. Unfortunately, protective measures like wind machines are ineffective against advective frosts.

Radiation frosts occur when a dry, cold air mass settles over an area with minimal cloud cover and wind at night. Wind machines and other active strategies in managing frost are effective in helping safeguard crops from radiation frosts.

Mitigating frost

Passive strategies

Passive methods used well in advance of the actual freeze danger are probably the most economical and effective way of mitigating frost. Some strategies are common sense and already widely used. Following are some examples of passive methods which can be used.

Strategic site selection: Selecting a vineyard site with optimal air drainage can help avoid cold air settling in low lying areas. Prior to planting a vineyard, research the maximum and minimum temperatures at the site, and date of last frost to determine the risk of freeze/frost events.

Cultivar selection: Planting cultivars that are appropriate for the vineyard climatic conditions can help reduce the risk of cold injury. Consider planting varieties that have a delayed budbreak in low lying areas to minimize susceptibility to frost.  (Table 1). 

Table 1. Ranking grapevine varieties with budbreak timing versus cold hardiness loss rate. Table by Kovaleski et al, 2018.

Prudent training system selection: Frost risk decreases with height above soil, as a result using training systems that position buds higher on the trellis can help to reduce frost injury.

Strategic pruning practices: Vines that are pruned earlier will lose dormancy sooner resulting in an earlier budbreak.  Delaying pruning can help push back bud break reducing the risk of frost injury.  Double pruning can also be used to delay budbreak. Double pruning of cane pruned vines, involves leaving long canes after the initial pruning, and then cutting them back to the desired number of buds once the risk of frost injury has passed and bud damage has been assessed.  Leaving extra buds during pruning results in the apical buds bursting earlier and supresses budbreak of the basal buds which produce fruitful shoots. Additionally, growers can leave an extra “kicker” cane as an insurance measure that can be removed later.

Delaying bud break chemically: Currently there aren’t any products registered for frost protection in grape vines in Canada. Research in other areas has shown the application of vegetable-based oils at non-toxic rates can delay bud break by two to 20 days, helping to reduce the risk of frost. Researchers have been investigating the use of fall applications of plant growth regulators such Abscisic acid (ABA) and its analogs for maintaining cold hardiness and delaying bud break in the spring. Commercialization of these ABA analogs are ongoing.

Optimized middle-row management: Maintaining short ground cover can increase soil warming during the day, leading to enhanced heat release at night, which may provide some frost protection.

Active strategies for managing frost

Active frost protection takes place just before and during the occurrence of the frost after a warning has been issued in the weather forecast. They are usually only effective under radiative frost conditions when winds are light or calm, and are most suitable in low-lying, frost prone areas. Advective freezes usually cannot be prevented by active strategies.

Wind machines: These machines blend warmer air from above the inversion layer with cooler air near the vines, potentially raising temperatures by 1-4 °C.  This method can be effective when there are large temperature differences between air layers near the surface and those up higher. Currently this is the most commonly adopted measure of mitigating cold injury in orchards. Wind machines can not be used when wind speeds are above 13 km/h, due to risk of damage the machines.  The use of wind machines can result in noise complaints from neighbours.  For information on best management practices when using wind machines refer to OMAFRA Factsheet 10-045 Wind Machines for Minimizing Cold Injury to Horticultural Crops . Research indicates that wind machines should be set to start 2 to 3 degrees warmer than the critical temperatures to for the growth stages (Table 2) to ensure optimal frost protection.

Table 2 Critical Temperatures and wind machine use to prevent frost injury in grapes (CCOVI VineAlert)

Sprinkler systems: Overhead sprinklers create an insulating layer of ice around buds and shoots, harnessing the heat released when freezing occurs to protect the buds. Protection from freezing temperatures as low as -6°C have been reported for low growing berry and vine crops, when 1.5 to 2.5 mm per hour of water was applied.

Plant-based products: Researchers in Washington state and other areas have shown that the application of cellulose nanocrystals (CNC) to fruit buds forms a thermal insulation layer, resulting in buds that are more resistant to freezing temperatures. Some research suggests that cold hardiness was improved by 2–4 °C with CNC treatment. Research continues on the use of cellulose nanocrystals on grapes.

Micro-organisms and proteins: Researchers in BC are currently developing a frost protection tool for plants made of naturally occurring micro-organisms and protective proteins that directly inhibits molecular ice nucleation and arrests ice crystal growth.

How can grape growers to do prepare for a frost?

Growers should continue monitoring crop development, bud hardiness (see Egrape from Grape Growers of Ontario), and local temperatures to assess the potential risk of frost/freeze injury. When frost or freeze risk is imminent use mitigation strategies wherever possible.

After a frost/freeze event assess the damage to get a better understanding of the crop loss and contact crop insurance immediately.