3 November 2023
El Niño seasons are known for clear cloudless days, lower rainfall, higher day temperatures and lower night temperatures, which together increase the chance of frosts. The AWRI helpdesk has recently received reports of frost damage to grapevines from regions across SA, WA, Victoria, NSW and the ACT. This eBulletin outlines key factors to consider when deciding what action to take after a frost and points to some useful resources to learn more about frosts and how to mitigate against them.
Due to the variable nature of frosts (temperature and duration) combined with the variability of vineyards (growth-stage, variety, pruning system, target yield, value and quality etc) and the variable growing season duration between regions, there is no single or simple answer to the question of what to do after a frost. The key factors to consider are outlined below when deciding how to respond to a frosted vineyard situation by assessing a few different scenarios.
The desire to act quickly after a frost is strong because you are seeing your vines in trouble, and you want to do something. The best thing you can do is to wait at least a week to assess the extent of the damage. Frost symptoms on leaves can show after a few days, while the impact on inflorescences can take a week or more to appear. It is also sensible to wait until the risk of frost has gone before acting, because another frost could change your plans. When the risk of frost has passed, conduct a careful vineyard assessment of where the damaged vines are and the extent and severity of the damage. If you decide you are going to take action, it is then recommended not to delay because the vine is expending energy to adjust to what has occurred.
Scenario 1. Light frost
If the assessment reveals that the damage to the crop is light overall, no immediate action is required. The damaged tissue may be susceptible to Botrytis if wet weather follows the frost, so this should be considered in future disease management strategies.
Scenario 2. Severe frost early in the season (up to E-L 11)
If the shoots were small and subjected to a severe frost (up to E-L 11, four leaves separated), there is no need to act because the vine will replace the lost primary shoots with a secondary shoot and the damaged shoot will shrivel up over time. The replacement shoot will not be as fruitful as the primary shoot, but there should be a crop and fruit development should be relatively uniform. The secondary shoots should have time to lignify at the end of the season and provide good pruning options. Once new growth emerges and has reached E-L 12, a nitrogen application can assist the new growth as the vines used up their early season reserves.
Scenario 3. Frost damage post E-L 11
The most difficult situation to manage is where a moderate frost occurs in a vineyard at a growth stage later than E-L 12. When well-developed primary shoots are only partially damaged and the shoot tip is killed, lateral shoots will be strongly stimulated to grow from the leaf axis on the healthy parts of the primary shoot. In this situation, there is the possibility of fruit setting from inflorescences that survived on the primary shoots as well as those coming from secondary shoots that burst plus any fruit from laterals. At harvest, this leads to variable ripeness and poor fruit quality. This lateral growth can also lead to a crowded canopy and those shoots provide poor pruning material for the following season.
Trials that have assessed post-frost options (largely ‘do nothing’ vs removing damaged primary shoots) have had variable outcomes. In terms of yield the following season, neither approach is consistently better. A ‘do nothing’ strategy is cost-effective in the season in which the frost occurred, but it can come at the expense of the following season when pruning options are poorer and yield is impacted as a result. Action to remove the damaged primary shoots down to the compound bud to encourage a secondary shoot is an expense that needs to be considered in terms of the impact that it has on fruit quality in the current season as well as at pruning time. For cane-pruned vineyards, having good replacement canes is critical and the ‘do nothing’ strategy may not be conducive to this.
Considerations when deciding what action to take:
- Length of the growing season: In a cool region, is there time to fully ripen a new crop? If not, the only option for a ripe crop this season is to focus on the fruit remaining on primary shoots after the frost.
- Wine quality expectations: Can you tolerate a lower grade of fruit caused by uneven ripening? If not, removing green bunches at veraison may be necessary.
- Pruning: Can you tolerate poor-quality replacement canes and spurs? If not, then intervening post-frost may be necessary.
- Assess the cost of any proposed action: A small trial can tell you the time it takes to undertake a post-frost intervention. Assign a value to this time and assess against the value of the crop or other potential benefits.
- Inputs: Is there an opportunity to reduce some inputs to reflect the lighter crop? The cost/benefit ratio of managing a lower yielding crop needs to be considered. The season has already started and now is potentially starting over. It is difficult to spend more than what was intended for the year, so consider if some management operations and vineyard passes are still necessary.
Designed for agriculture, the Bureau of Meteorology (BOM) provides Frost potential maps, which show forecast low temperature thresholds for various locations across Australia. The maps are updated each day and show forecasts for the next 48 hours. More information about the Bureau of Meteorology’s frost risk outlook and frost warning service can be found in this AWRI webinar (5 September 2019). Some regions and associations have additional weather stations and/or frost warning services.
Ways to reduce the risk of frost damage
Passive methods include site selection, soil management, trellis design, variety choice and pruning method/timing. For established vineyards, active methods require energy to reduce longwave radiation loss to the sky, or to directly warm the air near to the soil. These include sprinklers, air mixing, heating and foliar sprays. More information on these methods can be found in the following resources:
- Frost management in vineyards (AWRI factsheet)
- Protecting your vineyard from frost (AWRI demonstration video)
- Managing frost (AWRI webinar 29 September 2020)
- AWRI frost information pack
- Frost control in New Zealand vineyards. New Zealand Wine booklet (2020).
For assistance with frost or any other technical viticulture or winemaking topic, contact the AWRI helpdesk on (08) 8313 6600 or firstname.lastname@example.org.
Liz Riley (Vitibit) and Dr Peter Dry are thanked for their contributions to this eBulletin.
This work is supported by Wine Australia, with levies from Australia’s grapegrowers and winemakers and matching funds from the Australian Government. The AWRI is a member of the Wine Innovation Cluster in Adelaide, SA.