If you visit vineyards in some of the colder wine regions such as Ningxia, Eastern Turkey, Finger Lakes and Ontario in late autumn, chances are you may not see any vines at all.
Before the soil freezes, growers may choose to bury their vines so they can survive harsh winter temperatures without being frozen to death or suffer winter injuries.
‘We draw the line at -17°C,’ said Chinese wine authority and consultant Professor Li Demei in a previous DecanterChina.com column.
‘If the lowest temperature of a region regularly drops below this point, then it becomes necessary to take special measures to protect the vines,’ he added.
In China, that includes most of the wine regions in the northern half of the Mainland, including Ningxia, Xinjiang and Huanren of Liaoning Province to the Northeast – which has a growing reputation for producing quality ice wines.
How to put vines to ‘bed’
As an initial step towards a healthy hibernation, growers need to prune their vines hard and take the canes off the trellis. They also need to get rid of any fallen leaves and branches in the field to ensure the ‘bed’ is clean and disease free.
In arid inland regions like Ningxia and Xinjiang, growers water the vines thoroughly about 10 days before burying them, making sure there is enough water – but not too much, which could lead to rot.
In northeast China and some regions in Canada, that extra supply of water may not be necessary for vineyards that can regularly expect plenty of snow in winter.
Roughly two weeks before the temperature hits zero, growers are ready to put their vines to bed.
To do that the growers need to carefully lay the canes to the ground. Vineyards in China sometimes apply a soil ‘cushion’ at the foot of the vine so it doesn’t snap easily.
Where there’s no snow, growers would dig out the soil from beside the rows of vines, either manually or using a tractor, to ensure the vines are fully covered. How thick the cover needs to be depends on how extreme the region’s winter can get.
Throughout the winter, the vines remain under the earth, waiting to be dug out in Spring, when growers manually stand them up and tie the canes back to the trellis.
Problems and alternatives
Whilst burying vines allows growers to produce wines in some of the coldest areas in the world, the highly labour-intensive process can increase vineyard management costs by a third, said Professor Li.
Growers also have to reduce the density of their vineyards, in order to allow enough space and secure sufficient soil for vine burying.
It is also a common concern that the process of burying and digging can be physically damaging to the vine, which may lead to diseases or infections, reducing the lifespan of a vine.
In addition, the timing of burying and unearthing is critical to the protection and prevention of premature bud break, which ‘can result in bud mortality due to freeze injury from spring frost’, according to a 2014 research by the Cool Climate Oenology & Viticulture Institute of Brock University in Ontario, Canada.
Despite the disadvantages, ‘protecting the vines in winter is necessary (in Northern China),’ said Professor Li, ‘it’s just we haven’t found a more efficient method.’
But growers are certainly trying their best to perfect the process. In order to minimise the damage associated with burial and digging, vineyards in Ningxia commonly adopt a ‘厂’ shaped (single-cordon) pruning system, making it easier for growers to bend it down.
Vineyards in Quebec are also experimenting with geotextiles as an alternative coverage material, according to the Brock University study.
Domaine St. Jacques is said to have adopted the method since 2006 by covering geotextiles over the trellis with a tractor in a ‘long tent-like’ fashion.
‘The materials can be placed on the vines independent of soil conditions so even if the ground is frozen they can be used,’ the study says. Though it also added that ‘there is a greater capital cost with geotextiles so durability and reuse is also a concern.’
At the same time, viticulturists in China are experimenting with hybrid and indigenous grape varieties with better resistance to low temperatures.
The indigenous vitis amurensis is one that’s been attracting attention.
With protection from snow, the freeze-tolerant species can survive in the cold north-eastern corner of China (up to 46 degrees north latitude), and produce ice red wines.