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A year in the vineyard: December

Vineyard frost beats shoots and leaves. This year saw late-spring frosts decimate crops - Jonathan Hesford explains how

This year, many vineyards in France were damaged by frost, leading to the lowest volumes of wine made since the Second World War.

The same story unfolded in Italy and Spain, resulting in the lowest level of wine made in Europe for 60 years or so.

People talk about “A good year” in wine, which may mean large volumes, high quality across the board, or both. 2017 cannot really be considered a good year in most French regions.

The vine isn’t harmed by frost, as it can survive temperatures as low as -20C. Frost only causes damage if it occurs after the buds have burst and the shoots have started to grow, which happens in late spring.

The fast-growing shoots that emerge in March and April are very fragile. If the temperature drops below freezing, the sap inside the shoots can freeze, bursting the plant cells. This is similar
to putting lettuce in the freezer. Once that has happened, the shoot will die.

As grape bunches only grow on new shoots, that means the crop from the killed shoots is inevitably lost.

Vines are pruned in different ways in different regions. Those in areas prone to frost employ cane-pruning, which leaves one or two whole stems from the previous year. Each stem will have around 10 buds. Because the buds don’t burst all at the same time, an early frost will only damage some of the young shoots.

In warmer regions, where frost is rare, spur-pruning is the norm. Here several short stumps of last year’s stems are left, each having two buds. These tend to all burst at the same time, so a frost attack will destroy the whole crop.

Each bud contains a secondary shoot which will only grow if the first shoot fails. So a vine that has been attacked by an early frost may still produce shoots with grape bunches on them. However, these secondary shoots don’t always have bunches and they will not achieve the same level of ripeness because they started to grow later. The secondary shoots of spur-pruned vines tend not to grow any bunches.

So frost can result in a combination of loss of crop, unripe bunches and high variation in ripeness levels of the bunches at harvest time, depending on the region, grape variety and when the frost occurred.

Because of these risks, vignerons have developed many methods and techniques to avoid or reduce the problems. The first is to plant grape varieties that begin their growth after the risk of frost has disappeared from the region. 

The second is to choose sites that are unlikely to be attacked by frost. 

That is why, especially in northern regions, vines are planted on south-facing hillsides. They gain the advantage of the early morning sun and avoid the freezing cold air that descends and accumulates at the bottom of hills. It’s a very bad idea to plant vines in the bottom of a valley or to block the flow of cold air with bushes or trees. The Côte d’Or in Burgundy is a perfect example of this site-selection.

If frost is forecast, there are measures that can be taken. The traditional method is to light frost-pots between the rows to raise the temperature. These are little ovens of burning prunings. The sight of thousands of tiny braziers in the vineyards of the Loire and Chablis is quite magical.

In the New World, two more technical methods have been employed. The first is to use sprinklers to wet the vines. It may seem counter-productive but a layer of ice will keep the temperature of the young shoot at zero degrees, which isn’t low enough to freeze the sap. However, it only works if the shoots are very small and the frost doesn’t last long. Another method is to move the freezing air by use of helicopters or wind machines. 

Vignerons may also delay pruning if they think frost is likely. If a vine has a large number of buds, it won’t start to grow them until a little later. This way the buds that will produce the fruit are prevented from growing while frost is still a high risk. The excess buds and shoots will then be cut off when there is no more chance of frost. 

So frost is a big problem for vignerons. That’s why they have gone to great lengths to choose appropriate sites, grow the right grape varieties, prune their vines in a particular way and if all else fails, employ pretty extreme measures to save the crop and produce ripe, evenly-balanced fruit. 

Jonathan Hesford 

www.domainetreloar.com
If you have questions on this wine column, email me at info@domainetreloar.com

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