Ice ice baby
In the depths of winter, dormant grapevines can tolerate some winter cold, but the time of greatest risk is actually in spring, as the buds burst into new growth. A late spring frost hit the headlines in April 2017 causing devastation in many vineyards across France, Germany, Italy, Switzerland and the UK – where just a few hours of arctic weather destroyed a whole year's work.
Dubbed Black Thursday by some winemakers and described by others as the worst winter frost for at least quarter of a century, the effect on yield was devastating for some, particularly those unlucky enough to have been hit by frost in 2016 too. Crop losses of anything from 20 to 90% have been reported and this will inevitably have a knock-on effect on both wine availability and pricing in the near future from some of these popular wine regions.
In the deep mid-winter
In the dormant season, grapevines can cope with temperatures down to around -15°C or more (depending on the variety), but much colder and there is risk of freezing damage to the permanent woody tissue of trunks and shoots. This can split the bark, bringing additional risk of infection by a nasty bacterial disease called crown gall.
In continental grape-growing areas where winter cold is a common problem (such as Russia, Ontario and many of China's winemaking regions), a technique called earthing-up is used to protect vines. Soil is actually a good insulator so heaping the soil up around the lower parts of the grapevine can protect it.
In China, it is common for the whole grapevine to be removed from the trellis and buried. The downsides include risk of damage to the whole vine and vineyards have to be planted at lower density so there is enough soil to cover the vines. This is also expensive – it may cost up to a third of the total vineyard management costs in a year.
Ice can be good
Winter cold is not always negative but can helpfully kill off populations of damaging insect pests and potential pathogens. Even better, it can also provide the opportunity for making the luscious golden liquid that is ice wine (or Eiswein in German). Canada and Germany are perhaps most famous, though ice wine can be made in any region where it gets cold enough. Ripe grapes (typically not infected by noble rot) are left on the vines after the normal harvest (usually netted to keep the birds off), in the hope that temperatures will drop to below -7°C. The grapes are picked while still frozen, so the water ice can be removed, leaving a thick sweet liquid to be fermented into ice wine.
It's in the spring, just after bud burst, when growers should be full of optimism for the year ahead, that spring frost can cause so much devastation. Grapevine buds are well protected from cold in the winter, but as the weather warms the new leaves start to emerge, quickly followed by embryonic flower clusters.
These young tissues are vulnerable to freezing as ice crystals disrupt and damage the plant cells.
Young shoots hit by frost will shrivel and brown, destroying the potential crop for the whole year. Black Thursday in 2017 saw temperatures drop in parts of France to -3°C and below in late April and even warmer regions like Languedoc-Roussillon in the south reported severe frost damage.
Thinking long term
Producers do have a few options to reduce or avoid damage. First of all, careful site selection is important to avoid frost-prone areas. Slopes are useful as cold air falls, so the air drainage down a vineyard slope is of critical importance. Features like walls, hedges or road embankments can interrupt this airflow and result in frost pockets. I was once told in Piemonte that it is best to buy vineyards on a frosty morning so you can literally see where the cold spots are.
Choice of trellising method is also important, keeping the fruit zone high above the frosty ground can be helpful. Choosing later-budding varieties is useful in high-risk areas (riesling and cabernet franc for instance). Unfortunately, appellation rules often don't allow this. Pinot noir and chardonnay are early into growth, and thus particularly prone to ice damage, yet they are key varieties in frost-prone Chablis and Champagne.
The biggest threat in spring is radiation frost – associated with clear, still nights where the heat from the soil radiates away into space as there is no insulating cloud. Temperatures drop furthest close to the soil and create a so-called temperature inversion. Short-term protection options include warming up the vineyards using oil heaters, large paraffin candles or even setting fires using vine prunings or straw. And the smoke can help provide insulation too.
Huge fans and air blowers are a common choice in places like California and Ontario – used to mix up the warmer layers of air at higher altitudes with the cold air close to the soil. Helicopters are also sometimes used to force down warmer air though this is expensive. And in Bordeaux in 2017, it was reported that air-traffic control limits flying until after 6.30am – too late to prevent frost damage.
Huge fans in the vineyards in the Napa Valley used to circulate the pockets of warm and cold airHuge fans in the vineyards in the Napa Valley used to circulate the pockets of warm and cold air
Down among the vines
There are also a couple of vineyard management techniques that can be used if frost is a regular occurrence. Late pruning can delay bud-break by a week to 10 days and, strangely, removal of weeds in between vines can help because there is less surface area for cooling.
The population of bacteria in the vineyard can also have an effect as there are certain species of bacteria that act as frost nuclei and thus encourage formation of ice crystals. It seems these bacteria are more common on grass than on the vines themselves so mowing can help reduce frost damage.
Another surprising technique is the use of water sprinklers to protect vines as temperature starts to drop, a technique called aspersion. The water freezes around the shoots (releasing latent heat of crystallisation as solid ice forms) and protecting shoots from damage. This technique is only possible where pipes and sprinklers have been installed and there must be water available to run the system.
Sadly, once frost damage has happened, there is little that a grower can do. Damaged shoots can be removed in the hope that secondary buds will start into growth, but these are usually less fruitful and their later development means that there may not be enough time for the crop to ripen before autumn.
Frost is a cruel blow to grape growers who only have one chance each year to get their crop right; a clear reminder about what a direct agricultural product wine is. For the wine drinker, though there is still plenty of choice from other wine regions, so perhaps a good time to explore beyond the classics.