I’M writing this in April and again hearing stories about spring frost creating havoc in France. The Loire Valley has been hit for the third time in four years. On the nights of April 26 and 27, 2016, there was frost in Burgundy that severely reduced the crop and saw the village of Marsannay lose 90 per cent of its fruit. In 2017 the right bank of Bordeaux was hit, and it is regarded as the worst freeze since 1991. Yields were cut by up to 40 per cent. Chablis was also hit and vignerons lit up 1000 anti frost candles to protect their vines. In Burgundy they burnt hay bonfires in the early mornings to block out the sun, which sounds a bit weird. There is a simple answer, the frost freezes the buds and when the sun rises and hits the frozen water droplets they act as magnifying glasses and burns the delicate buds beneath. So burning fires blocks the harmful sun rays and allows the ice crystals time to slowly melt. It is not all doom and gloom as, depending on the timing of the frost, there is hope that the vine throws a second bud and you can still get a reasonable harvest.
By definition, frost occurs at temperatures below 0C and commonly affects an area from ground level to around 2m. Above 1.5m can be a critical 1C warmer than at ground level. In Australia the areas most commonly affected by frost are inland regions such as Coonawarra and Padthaway in South Australia, and Grampians and Macedon Ranges in Victoria, but many cool sites across all states can be affected.
Avoiding frost starts in the vineyard planning process. Sites that are prone to frost are commonly on flat plains or where there is a slight undulation. Choosing sites near lakes or oceans deter frost by creating air movement/convection. Vineyards planted on slopes provide frost protection as cold air flows downwards and away from the vines. The higher the trellising the better when it comes to protecting the vine, as well as choosing the row directions to allow that cold air to drain away and not be trapped. Wind breaks may be essential to protect the vines from damaging winds, but can create a problem in trapping cold air.
Frost events can be controlled by fans or spraying water from overhead irrigation systems on to the vine when the temperatures are indicating frost is going to occur. This can be an expensive operation given that the water might be on strict allocation. The way the “aspersion technique” works is that sprayed water freezes around the buds and releases a small amount of latent heat which protect the vine from further damage by the frost. When wind fans are used they stir up the cold air at ground level by moving warmer air in, which is typically sitting above the cold frost. Fans can be extremely noisy so not popular close to residential areas. In some regions helicopters have been used which must also be unsettling for sleep-deprived residents.
One of my first touches with “Jack Frost” was brought about by my love of Tasmania’s Rochecombe wines and the story of its sad demise. Founded in the 1980s by Swiss vignerons, the business went into receivership due to frosts wiping out their crops three years in a row. At the time I remember feeling so sad for the owners. The legendary Czech migrant, master butcher, businessman, developer turned vigneron Josef Chromy OAM stepped in and bought the vineyard and he built a dam and irrigation system to protect it against frost. He then subsequently sold it on and today it is the site of Bay of Fires in Pipers River and the dam has a lovely family of platypus residing when I last visited.
At the other extreme, heat is another menace that can affect the vine in all sorts of ways. Sunburn damage can be done before, but more often, after veraison. This is when a grape starts to ripen and changes colour. Temperatures in the 40s are the most worrying with damage occurring after a short exposure of, say, five minutes, but heatwaves with temperatures in the mid 30s for consecutive days can be a treat. Semillon, chardonnay, cabernet sauvignon and merlot are susceptible to sunburn, while grenache and tempranillo can withstand the heat. The skins can be burnt brown and shrivel to the extent that they become more likely to be attacked by rot. The heat will also wilt the leaves which will increase the exposure of the bunches to sunburn. Leaf plucking and vertical shoot positioning (VSP) trellising is great in cool climates because it exposes the grapes to the sun, but in hot regions it can be a distinct disadvantage. To protect the grapes the vine must have adequate shade through its leaf canopy and be well hydrated in advance of hot days. Accurate weather forecasting is critical in predicting frost and heatwaves, which is why you see viticulturalists glued to their smart phones at certain times of the year.
Sunscreen for vines might sound ridiculous but has been used successfully in Australia and the US. Surround is a kaolin-based clay that has been trialled in the Hunter Valley on semillon. It is applied as a spray to the grapes and vine and keeps the vine cool and stops grapes getting sunburn. The protective film cuts out some the damaging wavelengths while still allowing enough through to continue photosynthesis that is critical for plant growth and sugar accumulation. In the Hunter Valley trial conducted by Wine Australia sunscreen-treated grapes had better fruit ripeness, colour and required less fining when compared with unprotected and heat damaged grapes.
The force of nature unleashing extreme weather events seem an ongoing news item across the globe in this millennium. But it could be argued that they are nothing new. Consider this, the winter freeze of 1956 destroyed a quarter of all vineyards in Bordeaux.