As in France, the Cape winelands could once again be sweltering under heatwave conditions come spring or summer. But what does an unexpected heatwave mean for vineyards – and how do winemakers cope? Anton Pretorius investigates.
The heatwave roasting parts of Europe has been warmly welcomed in Bordeaux where French wine producers say a sudden burst of sun after a wet spring could produce a superior vintage.
Temperatures in the Bordeaux region rose past 40 °C at the end of June, which, according to Philippe Bardet, head of the Bordeaux Wine Council, helped burn off any mildew caused by residual damp. “It’s very, very good for quality,” Philippe says.
Soon after Philippe’s comment, VOA News reported that after weathering record-breaking temperatures, parts of Europe are now gripped by a punishing drought that is shrivelling harvests, sparking water shortages and taking a toll on wildlife. Experts now warn that Europeans must prepare for today’s ‘new normal.’
In South Africa, a heatwaves can also have a detrimental effect on vineyards. Cast your mind back to October last year when the Cape Winelands experienced a sweltering heatwave. The mercury climbed to 42 °C in some parts – unusually hot for our late spring season. A changing climate means we can expect more of the same in the coming summer months, reducing pollination during the vines’ flowering season. It also means grapes – or berries, as wine farmers call them – are smaller than usual with less on the bunch.
But according to Vinpro’s harvest report of 2018, the smaller berry has less weight and that usually allows a better quality wine. “There’s a lot of flavour in the husks or skins, and the smaller the ratio between husks and juice, usually the higher the quality,” said Anton Smuts, chairman of Vinpro.
However, malic acid is sensitive to high temperature and drops dramatically when temperature rises above 30 °C. Acid is the main reason for most wines’ freshness. Without acid, the grape may lose its elegance and longevity, resulting in heavy, ‘jammy’ wines.
Associated risks
Dr Albert Strever, senior lecturer at Stellenbosch University’s department of viticulture and oenology.
What are the risks of an unexpected heatwave, and what should producers and winemakers take into consideration?
According to Dr Albert Strever, senior lecturer at Stellenbosch University’s department of viticulture and oenology, the biggest risk related to heatwaves is their occurrence during a sensitive stage of the berry’s development. “Extreme heat, drying, water stress – they all have an impact on the berry set,” says Dr Albert.
Later in its development, with the softer grape, heat damage (sunburn) is associated with side effects like “secondary infections, as well as the shrinking and mass loss of berries,” he says. “In red grapes specifically, colour and flavour degradation is a possibility.”
He says heatwaves can also lead to leaf damage and wilting. And if accompanied by water stress, chlorophyll can degrade and leaves might fall earlier. “This, of course, also has an impact on reserve build-up for the following season.”
Minimising the damage
Head of Winetech’s knowledge transfer, Lucinda Heyns.
Head of Winetech’s knowledge transfer, Lucinda Heyns, recently returned from a visit to France. She remains sceptical about the French’s positivity towards a heatwave.
“The phenological growth stage of the vineyard during a heatwave also plays a role in how much damage occurs,” Lucinda says. “We often forget that that the grapevine experiences various physiological processes throughout the season that doesn’t just affect the current season’s harvest, but that also affects the following harvests.
Dr Albert says the big difference between us and France is that we can irrigate to negate the negative effect. To soften the heat’s impact, some producers irrigate their vineyards prior to a heatwave. “In places like Upington, overhead irrigation is common practice for farmers in order to cool down their vineyards during the evening, to prevent respiration losses.”
“It’s important to break the leaves early in the season, and through proper canopy management, allow more natural shade for grapes to protect them against sunlight,” says Lucinda. “The late Prof Eben Archer would say if your soil preparation was done correctly and you’ve allowed the vines to develop a well-established root system, the vines will be more buffered against drought and heat waves.”
She adds that sugar levels are prone to rise quickly during a heatwave. “In some cases, producers wait before they pick their grapes and after a few days, the sugar level drops. But that’s not always the case.” During ripening, says Dr Albert, it may happen that berries temporarily lose moisture which is then refilled in the evening. “In a heatwave, the time a sugar sample is taken will also have a major impact on the concentration levels measured in the berry and extra care should be taken with sampling.”
Australian remedies
In Australia, some farmers use a combination of practices, including a trial of a kaolin-based clay sunscreen.
In Australia, wine grape growers are getting creative to protect their crops from the elements after months of extreme weather conditions in South Australia. Some farmers use a combination of practices, including a trial of a kaolin-based clay sunscreen which helps reflect direct sunlight or UV light, limiting the extent of berry sunburn and also leaf burn.
“The use of sunscreen has been extensively tested in Australia,” says Lucinda. “However, no trials have been done in South Africa yet.”
What about wine style? How does a heatwave impact the characteristics of a wine grape?
Dr Albert says the impact on wine style is largely cultivar-dependent and will also depend on the ripening period. “Pinotage – a cultivar that can be harvested earlier – might escape a heatwave in February for instance. But it’s more difficult with Merlot, which is also less tolerant to heat degradation than Shiraz or Cabernet Sauvignon.”
Healthy Chardonnay grapes (left) and sunburned Chardonnay grapes (right).
Quick tips for producers during a heatwave:
- Handpicking must be done as quickly as possible to avoid acidity decreasing, which may cause a loss in wine’s fresh character.
- Maximise transpirational cooling. Water loss through the open stomata of grapevine leaves has a cooling effect on the leaves and the surrounding environment. Transpiring grapevines will contribute to vineyard cooling.
- Minimise incoming radiation. A good canopy should be established early in the season through careful management of pruning, irrigation and nutrition. The canopy provides natural shading which reduces incoming radiation intensity (or bunch exposure).
- With limited resources such as reduced water availability or an irrigation system, prioritise varieties and/or blocks according to their value, sensitivity to damage (development stage), soil depth and texture, aspect, age and type of grapevine rootstock.
- There are many advantages to harvesting at night, for wines and workers alike. The cooler nights create better working conditions – not only is the temperature more tolerable, but bees and snakes tend to stay away at night. In some areas, grapes picked at night are firmer, making them easier to work with.
