As it’s the end of the vine growing season and most grapes have been harvested, we are turning our attention to what happens in the vineyard during the cold winter months. There’s still lots of work to be done for grape-growers who can’t enjoy a winter slumber like their vines do.
No matter which produce grower (and hobby gardener) you speak to, they will tell you that winter is a period that has its own important role within the yearly cycle. The ground, trees, and plants can rest before their rejuvenation in springtime. Winter is quite simply a moment of peace for them. We can say that a good frost, up to a certain point, at this time of year is beneficial for the vines.
Naturally occurring work
Talking about frosts, they decompact and aerate the soil. This naturally occurring work is caused by a very simple process. When the temperatures drop below 0°C, the humidity in the earth transforms into micro ice cubes. The amount of space that water takes up therefore increases, causing any clumps of earth to break up. At the same time, clay particles will naturally dry out during a frost but will obtain water again when the temperatures rise.
This natural breaking up of the soil is invaluable as it facilitates water infiltration. Winter rain does not run off the fields, which in turn, limits soil erosion. Aeration allows micro-organisms to develop as well as helping roots to form that can wind their way deeper into the earth more easily.
Winter also protects the vine from common diseases such as mildew and powdery mildew. Despite mildew’s resistance to temperatures as low as -20°C or -25°C, extreme cold can destroy outbreaks, and cold weather in winter and spring will delay its appearance. Powdery mildew reacts to low temperatures differently depending on which region you find yourself in. In southern regions, negative temperatures have a real impact and can destroy the fungus. While in regions where powdery mildew is resistant to wintery temperatures, cold weather delays its springtime development. So depending on how cold the winter gets, winemakers can anticipate the risks of the disease and prepare the necessary treatments.
The vines themselves also need a quiet phase in their annual cycles. An intense cold snap forces the sap to descend into the plant’s roots. However, the cold can’t be too extreme or last for too long as the vine can die below -20°C. Frosts spread out over the winter will also weaken the vines by splitting the wood or making them vulnerable to trunk diseases such as esca or eutypiosis.
Protection against frost and cold weather
After the “Ice Saints’ days” which end on 13th May, there should be no more frost but any that do occur would be fatal for the new, green shoots. When there is high humidity, the buds can start freezing anywhere from -2°C and -3°C, while in drier conditions (<60% humidity), they can withstand the cold as low as -4°C or -5°C.
Spring frosts can destroy the harvest before the growing season has really started but they won’t kill the vine itself. Vineyards have found several ways of protecting against these devastating frosts. Grape-growers in the Champagne region have developed a technique for fighting against it by spraying water over the vines, which forms ice (at 0°C) that encompasses the young bud, protecting it from excessively cold temperatures. In Chablis, as well as spraying water, vintners use “heaters” placed among the vines to ward off the cold. However, the heaters and paraffin bougies pollute the atmosphere and take an awful lot of work to put in place and keep alight. In Canada, Central Europe and China, earthing up the roots of the vines is practised. There is also the technique of blowing air over the vines. This circulates the air, replacing the cold air around the vines with warmer air from above which can raise the temperature by 1°C to 4°C.
One thing we have yet to mention is ice wine, which also takes winemakers out into the vineyards in sub-zero temperatures. Ice wine is made from grapes left on the vines long after the other harvests have concluded. They are only picked once the temperatures fall below -7°C and the water content in the grapes freeze. Because of these stipulations, ice wine isn’t made every year and may become rarer with climate change. The grapes produce a concentrated, sweet wine with flavours of citrus fruits, honey and marmalade. Ice wine is produced in Germany and Austria where it is called Eiswein as well as Canada.