Written by Victoria Rose
Opening the WineGB members’ webinar entitled “Frost – what to do next” on 20 May, Jo Cowderoy, operations manager, remarked that it seemed surreal to be opening a discussion on frost while sat in the baking hot sun. Chaired by Stephen Skelton MW, consultant viticulturist and chair of WineGB viticulture working group, the panel, including Professor Steve Dorling, Dr Alistair Nesbitt, Alex Valsecchi and Matt Strugnell, had been brought together to review the horrific frost event that occurred in the week beginning 11 May 2020.
Stephen shared results from the recent frost survey which had been organised by Pete Wain of MapMan, who is also conducting surveys on flowering, véraison and harvest. “We had 198 separate frost reports from UK vineyards and, as 60% of those said they had no frost protection, it’s perhaps not surprising that people have got damage,” said Stephen. “Of the 86 vineyards that reported 30% damage or more, 66% had no protection, so this really is a wakeup call for those people who don’t have protection.”
The most common methods of protection were bougies, or bougies plus other methods, at 12%, with various types of fans being used by 10%, and just 7% using sprays. “It appears that bougies did hold back frost and seemed to work quite well,” said Stephen. “58% said they had less than 10% damage while using bougies, 17% had 10-20%, and 25% had 20% or more damage. From those that used no protection, 43% had 40% damage or more. There’s no doubt that frost protection works – it’s just a question of gauging to what extent you’re going to get frost and whether you need to protect the vines.”
Assessing the weather maps
Explaining what had caused the frost event was Professor Steve Dorling of University of East Anglia and CEO of WeatherQuest, who opened with a map summarising which morning of the week 11-15 May was the coldest around the country. “There is quite a strong pattern, but it did depend on where you were in the country as to which morning was the most challenging, from a temperature perspective,” said Steve. “While southern England experienced the coldest morning on Tuesday, it was the Friday morning that was the biggest challenge for others.”
On Monday night and Tuesday morning ground temperatures reached as low as -9C in some places, while the air temperature, which is measured at a height of 1.5m, was typically -1C, dropping to -3C in a few areas.
“The frost events happened because there was a combination of light winds, clear skies, a very dry ground, which meant that the ground cooled quickly overnight, and a chilly air mass that led to these radiative thrusts happening between Tuesday and Friday,” said Steve. “Those events are what we call radiative frosts, whereas the Monday morning event, which was mainly confined to Northern Britain was an advective frost.”
Steve ran through a series of maps explaining how growers could view information in real time and what they should be looking for when trying to predict frost events in the future. Looking back at historic data from the MetOffice, Steve highlighted that some areas of the country had not experienced a May night as cold as this in the last 30 or 40 years, and that there had been a very strong variability in the frequency of spring air frosts from 1989 to 2018.
Frost protection equipment
As frost is something that most growers in the UK will have to deal with at some point, Dr Alistair Nesbitt from Vinescapes spoke about the various protection methods available. “It’s really important to be prepared for frost, and that doesn’t just mean having equipment in place, it means understanding what the risks are within your vineyard,” said Alistair. “Growers need to know the different types of frosts and have strategies in place to deal with those. Temperature, inversion strength, topography and soil moisture all have significant impacts on how effective frost protection is, and that means that what works in one year, in one location, may not work in another location, under different conditions.”
After pointing out there is no magic solution or ‘one size fits all’, Alistair suggested that growers adopt a layering approach and have one system in place for classic radiation frost down to -1C and another for colder temperatures.
Static wind fans, rarely seen in the UK, mix warm air above the inversion layer with colder air below, and cover wide areas of up to 10ha. Mobile wind fans, such as the Tow and Blow, are more common. These bring warmer air down from above the inversion layer, blowing it into the vineyard to push out colder air and replacing it with warmer air. While manufacturers suggest a coverage of 4ha, it can be closer to 3ha, depending on temperature, weather conditions and topography.
The Heat Ranger from New Zealand is said to cover 10ha, by pushing out booms of heat across the vineyard while rotating, but costs £125,000. There is also the FogDragon which claims to provide a blanket of smoke to stop radiation frost, it also deposits water on vine vegetation to release latent heat.
A cold air drain, which needs to be positioned in the lowest and coldest points in the vineyard, features a large fan that sucks cold air up and pushes it into the atmosphere. According to Alistair, these can be quite effective in the right location down to temperatures of -2C.
The FrostBuster and FrostGuard from AgroFrost use a process called phase transition. Biostimulants and polymer sprays seem to work in some situations and not others. The most well-known method is bougies, which are tried and tested and have been used for years but come with high labour costs.
An overhead-sprinkler irrigation system was deemed to be the most fool-proof system as it sprinkles water onto the vines, and then freezes, releasing latent heat. However, it uses 30,000 to 50,000 litres/ha/hour, which means that for an 8ha site and a five-hour frost, growers will need a water reservoir the size of an Olympic swimming pool. Manufacturers are now also producing electric heating wires, but for 1ha, you’d need approximately 100,000 kW of power.
Finally, growers can manage wind direction, either by having cold air flow through a gap in a hedge, or by putting up sheeting or barriers to prevent it flowing through the vineyard. “Those who don’t have protection in place, need to understand where cold air flows across their individual vineyard sites and where it accumulates. Then it’s important to consider what labour is available before working backwards to find the most effective form of frost protection for you,” said Alistair. “Lastly, growers need to use a good weather forecasting service and should also look at what insurance options there are for frost protection.”
Assessing damage and steps forward
The last speakers were vineyard managers Alex Valsecchi of Albury and Matt Strugnell of Ridgeview, who Stephen Skelton introduced as a man who’s “lit more bougies than most people have had glasses of wine”.
Alex began with video footage from one of the vineyards she manages near Horsham, West Sussex. Talking through the visual effects of the frost, Alex stressed that while some shoots were completely shrivelled and wilted, some vines appeared unaffected. “The damage is pretty random, and this is something that we’ve seen across all three vineyards that we manage,” said Alex. “Where we did have frost damage, not necessarily all the shoots along the cane were damaged, and not necessarily all the vines along the row, or even next to each other.”
As the advanced growth stage of the vines made the frost event particularly tricky to deal with, Matt said that growers needed to focus on assessing the severity of the damage across the vineyard first. “The advice is not to be tempted to go and start rubbing out affected shoots because, although you might be looking at a reduction in yield this year, you don’t want to remove or damage any secondary shoots that might come up, because they’re going to give you an option for pruning and tying down for 2021,” he said. “This late in the season, the best advice really is to is to leave them be.”
There was some positivity, with Matt pointing out that vines can “recover quite remarkably from something that looks quite ugly” and that growers may be pleasantly surprised to avoid “as dramatic a drop in yield as you might be anticipating right at the moment”.
As well as discussing the mixture of fruit from secondary shoots and primary shoots, and the issues that this can cause with harvest, particularly in white varieties, Alex and Matt also touched on what do with the regrowth from the crown and with newly planted vines.
Questions from participants then covered crown height; the cost of bougies, sprays and products such as fleeces; bare soil and sacrificial canes; NIAB EMR’s irrigation system; and varietal selection. Before closing the session, Stephen recommended that growers looking for more information read Cold Air Accumulation and the Grower’s Guide to Frost Protection by Steve Hammersmith.
The full webinar can be viewed in the members’ area on the WineGB website.