By Kathryn Carter, Fruit Specialist OMAFRA
Spring of 2021 has been a roller coaster ride of temperatures, and the cool temperatures from this past weekend have resulted in frost damage in some grape growing regions of the province (Ottawa and portions of Prince Edward County). The recent frost event has made 2021 even more memorable as it is one of the later frosts in the Ottawa area (Table 1).
Harrow | London | Goderich | Collingwood | Guelph | Vineland | Oshawa | Ottawa | |
2020 | 13-May | 13-May | 12-May | 13-May | 13-May | 13-May | 13-May | 12-May |
2019 | 01-Apr | 29-Apr | 11-May | 29-Apr | 29-Apr | 29-Apr | 29-Apr | 29-Apr |
2018 | 29-Apr | 30-Apr | 30-Apr | 30-Apr | 30-Apr | 30-Apr | 30-Apr | 23-Apr |
2017 | 23-Apr | 9-May | 8-May | 9-May | 9-May | 9-May | 9-May | 9-May |
2016 | 15-Apr | 14-Apr | 8-May | 10-May | 16-May | 15-Apr | 9-May | 25-Apr |
2015 | 24-Apr | 23-May | 23-May | 23-May | 23-May | 23-May | 23-May | 25-Apr |
2012 | 29-Apr | 29-Apr | 29-Apr | 29-Apr | 29-Apr | 29-Apr | 29-Apr | 30-Apr |
Grapevines are sensitive to freezing temperatures during the growing season and spring frost can damage buds and young shoots. Temperatures below -1º C (30 º F) can damage vines. Frost damage varies between vineyards, within a vineyard, and even within a single vine as often buds are at different stages. Spring frosts often lead to the loss of fruitful buds and subsequent decreased yield and fruit quality.
Assessing frost damage
One of the first things to do after a frost has occurred in the vineyard is to assess the level of frost damage. Initially frost damage may not be obvious, and it may take several days of clear, sunny weather to get a better idea of the extent of damage. In general two types of tissue are likely to be injured during a spring frost event: buds and shoots.
Frost affected shoots may wilt, turn black and take on a water-soaked appearance (Figure 1). Frost damage may not be immediately noticeable on shoots but symptoms may appear more clearly after a few days. Damage is observed more quickly on young succulent shoots, while older more hardened shoots may take a few days to show symptoms. Frost damage to the flower head may not be immediately apparent, but after several days, flowers may begin to dry out and start to fall off, especially when touched.
Damage to the trunk, head, cordon and spurs of the vine are less common from spring frost events, but it can occur. In some cases trunk or cane splitting may be obvious. However, often trunk injury is not apparent until later in the season, or even in subsequent years when weak growth becomes more apparent. Vascular damage to trunks can cause vines to collapse suddenly after the onset of hot temperatures. If trunk- or cane-splitting remains is unaddressed, crown gall and grapevine trunk disease will result, leading to vine loss in subsequent years. Avoid taking actions immediately to address frost as Dr Jim Willwerth (CCOVI) says “In many cases it is best for the grower to just not look at it for a week and see how the vines respond with secondary growth etc.” During the week after the frost, maintain pest management programs accordingly. Personally, I have seen growers walk away from crops assuming there wasn’t any crop, only to wish later, that they had maintained their pest management programs.
Figure 1 Freeze damage after budbreak in spring 2012, Vineland (Willwerth et al., 2014)
Impacts of frost damage
Frost damage of shoots can greatly reduce yield and lead to uneven ripening, depending on the severity of wilting.
Where frost injury has occurred, secondary buds will develop and, depending on the type and variety of grape, a partial crop may be produced. V. vinifera grapes are considered to be relatively unfruitful on secondary buds. With them, expect about a 25% crop or less. French-American hybrids tend to be more fruitful on secondary buds than other types of grapes and may produce up to 50% crop.
Determining what to do after the frost will depend on the severity of the damage, the timing of the damage and variety of grape. If the frost damage occurred before shoots were at the 5-leaf stage (Modified Eichhorn Lorennz system (EL) 12), there should be minimal impact on bud fruitfulness in the following year as shoot growth will have adequate time to produce fruitful buds during the current growing season. However, if the frost occurred after the shoots are at the 8-leaf stage (EL 15), reduced bud fruitfulness may be observed in the following season.
Dealing with Frost Damage
Once frost injury has occurred, the options for managing the damage vines include: taking no actions, removing only the frost damaged tissue, or removing all shoots back to the cordon. The timing of the frost determines the best option to take. The following are some general guidelines of how to manage frost damage in vineyards based on (Lockwood, 2020).
- Situations where no action is needed: If severe frost damage has occurred and all the green shoots have been killed back to the cordon or cane, no action is needed and vines should be allowed to grow out new shoots. Secondary shoot growth should be adequate to establish spurs and new canes for the subsequent year. 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. Keep vine health in mind when determining your crop load size for this year. Maintaining too many clusters in an attempt to maximize crop loads after frost injury can have a negative impact on next years crop depending on the severity and type of damage. V. vinifera grapes are considered to be relatively unfruitful on secondary buds.
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. Keep vine health in mind when determining your crop load size for this year. Maintaining too many clusters in an attempt to maximize crop loads after frost injury can have a negative impact on next years crop depending on the severity and type of damage. V. vinifera grapes are considered to be relatively unfruitful on secondary buds.
If the frost occurs after the 8 leaf stage (E-L 15) it is generally best not to remove shoots as this time, as it can result in damaged secondary buds and reduce the current seasons yields. Be aware that retaining dead tissue will result in an increased risk of disease (botrytis), so keep this in mind with your pest management program. Do not remove the tops off damaged shoots when frost occurs at this stage, as it will stimulate the bursting of secondary buds below, increasing lateral growth and resulting in increased shading which can increase shading and a late-ripening secondary crop can cause issues at harvest.
- Removing damaged tissue only: Literature does not indicate that there are any benefits to removing only the damaged tissue. Cutting the frost damaged tops off green shoots will stimulate secondary bud break lower on the shoots increasing lateral growth and resulting in increased shading that can delay ripening at harvest, and a secondary crop that can cause challenges at harvest. Differences in yields and growth are no different than where no action was taken. As a result minimizing damaged tissue only will increase labour costs, with no additional benefits, so this strategy is not recommended at any time.
- Removing all shoots back to the cordon: If the frost occurred before the 5 leaf stage (E-L 12) ( shoots are 10 cm in length or larger), and shoots were not killed completely back to the cordon, consider removing the shoots to the cordon. Removing the shoots at this timing will help keep good quality pruning material for next year. Keep in mind that this action may reduce the current season’s yields, but next season yields should be better as a result. The young shoots can be removed by rubbing out buds or breaking off shoots from spurs or canes to force dormant secondary buds at the base of the shoot. If damaged shoots are not removed, numerous poor quality poor placed lateral shoots may develop affecting the quality of canes for winter pruning. The later in the season that the shoot removal is conducted, the greater the reduction in bud fruitfulness for the following season, so it is better to remove the shoots sooner, rather than later. Removing all the dead tissue will help to reduce disease pressure in the vineyard.
- In marginal frost damage: The effects of frost are not always clear cut, and in some circumstances where not all the primary buds were killed, secondary buds may still develop and form clusters. This can cause complications at harvest as the optimal time to harvest the primary bud crop will come well before the secondary bud crop is ready to be harvested. In some cases it may be necessary to cut off either the primary bud crop or the secondary bud crop before harvest time or multiple harvests. Before deciding removing the primary bud crop take into consideration that secondary bud crops often have a delayed harvest, so in cool climate viticulture areas in some seasons these secondary crops may not have enough time to develop fully.
- Trunk and cane damage: Damaged trunks and canes should be removed at the end of the season. Maintaining injured trunks and canes in the vineyard will create issues with crown gall and trunk diseases.
Other considerations after a frost
Regardless of actions taken after a frost, it is important to focus on keeping the vines health throughout the season. Frost impacted vines have partially depleted their energy reserves in producing new growth that is not viable. Subsequent regrowth of the vine will be weaker than with the first flush of growth. Therefore, it will be critical to keep the vines healthy. With low /or no crop, some of the vines may be prone to more vigorous growth. It will be important to try to manage the growth and ensure light penetration into the canopy to ensure adequate sunlight exposure of shoots to develop fruit buds for next years crops. Buds that develop on shoots in dense canopies with low light exposure have fewer inflorescences (flower clusters) per shoot than those canopies that are better exposed.
Make sure to pay close attention to insect and disease control programs as maintaining healthy vines this year will have a significant impact on the health and well being of next years vines.
Conclusion
Frost damage of shoots can greatly reduce yield and lead to uneven ripening, depending on the severity of wilting. The options for managing frost damaged vines include: taking no actions, removing only the frost damaged tissue, or removing all shoots back to the cordon. The timing of the frost determines the best option to take. It is important to focus on keeping the vines health throughout the season.
Resources
Grapes.-frost-damaged-vines-4.15.20.pdf (uga.edu)
Cold_Injury_Grapes_Hoffmann_DRAFT.pdf (ncsu.edu)
Best_Practices_Manual__Winter_Injury_Sept_14_(5).pdf (brocku.ca)