Learning More About Frost Damage

After the frost event in our vineyard on May 18th, my husband forwarded me a link to an article Managing Grape Vines After a Frost written for Ontario Fruit Growers but appropriate for us. We keep on telling ourselves how odd it is to have a frost event in mid-May but even odder, how the south half of our vineyard was affected so much more than the north half of our vineyard.

The article mentions that in spring, temperatures below -1º C (30 º F) can damage vines. Frost damage can vary within a vineyard and even within a single vine as buds are often at different stages of growth.
Assessing frost damage

• 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
• damage to the trunk, head, cordon and spurs of the vine are less common from spring frost events, but it can occur
• often trunk injury is not apparent until later in the season, or even in subsequent years when weak growth becomes more apparent
• 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"

Impacts of frost damage

• determining what to do after the frost will depend on the severity of the damage, the timing of the damage and variety of grape
• where frost injury has occurred, secondary buds will develop and, depending on the type and variety of grape, a partial crop may be produced
• 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
• 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

• if 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
• 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
• 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
• removing damaged tissue only can stimulate secondary bud break lower on the shoots increasing lateral growth and resulting in increased shading that can delay ripening at harvest
• removing all shoots back to the cordon if the frost occurred before the 5 leaf stage and shoots were not killed completely back to the cordon, consider removing the shoots to the cordon
• 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
• 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 if the secondary buds are fruitful, it may present a problem at harvest since the secondary bud crop may ripen later

Other considerations after a frost damage

• keep vines healthy throughout the season
• with low or no fruit, the vines may be very vigorous, manage the growth and ensure light penetration
• 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

You can be sure that we will be keeping all of these recommendations in mind as we manage our frost damanged vineyard.
References:
1. W. Reichheld, Managing Grape Vines After a Frost, June 4, 2021, ONfruit.