Hot weather - its effects on grapes & wine
Summer - in the Northern hemisphere at least – means, or should mean, beautiful clear skies, sunshine and heat, all elements that you would imagine combine to make the perfect weather for wine grapes. But with climate change average summer temperatures are rising to such an extent that heatwaves are becoming more frequent - 2017, 2018 and 2019 all saw heatwaves affecting much of Europe with temperatures especially in continental Europe regularly above 40° Celsius. You might think that wine grapes benefit from as much sunshine and warmth as possible but in those years the heat was reported to wreak havoc on many crops including grapes destined for wine. Is hot weather therefore beneficial for wine grapes? When is hot too hot for grapes and what impact does hot weather have on the wine produced?
Effects of hot weather on grapes
In order to produce decent wine, grapes need sunshine and warmth to ripen (to varying degrees depending on the style of wine). As grapes ripen the level of sugar within them rises due to photosynthesis; the level of sugar at harvest is one crucial element in the sweetness but also in the quality of the resulting wine.
At the time of harvest if sugars are too high the wine will be high in alcohol and may taste of cooked fruits, even jammy fruits, and that can make the wine seem unbalanced. And as regular readers of my blog will know balance is crucial for good quality wine.
The sun brings another danger as grapes, like humans, can suffer from sunburn. It is thought that burnt grapes can also affect the fruit characters of a wine and so they are generally discarded, at least by the better winemakers.
Excessive heat and drought especially in late summer at peak ripening time is catastrophic as they effectively shut down the grape, causing photosynthesis and therefore ripening to stop. The temperature at which this is believed to happen is 'only' about 32° Celsius so not excessive for some wine-producing countries in the height of summer. Bitter flavours can develop in grapes suffering from heat stress, grapes are likely to be smaller and therefore yields lower and the vine's leaves which normally provide shade can drop off.
However it is the fall in acidity that is arguably the biggest issue for wine produced in a heat wave vintage. As sugar rises acidity in the grapes falls - see my blog on acidity in wine.
Even in years with a more usual weather pattern wine producers constantly measure acidity levels to make sure they do not fall too much. In excessive temperatures the need to monitor acidity is crucial as a drop is much more likely and more acute. Grapes that are low in acidity will produce wines that are heavy, jammy, lacking in freshness and flat in taste or 'flabby' in wine terminology. To make sure that wines are fresh, fruity perfumed and elegant, it is imperative to harvest before acidity has fallen too much.
It is all a fine balancing act for wine producers especially as it is believed that grapes are affected more than any other fruit by differences in climate.
Effects of hot weather on wine
The wines that are produced during very hot weather risk being:
high in alcohol, flat, bitter, jammy and unbalanced
- so not too many positive features. In addition they are unlikely to be long-lived. Although high alcohol is one factor that can help wines age, low acidity and a lack of fresh fruit flavours don't.
To add insult to injury - or perhaps with the characteristics mentioned above it is a blessing - there is likely to be less wine produced. In many areas affected by heatwaves the harvest will be down substantially especially in any areas affected earlier in the year by spring frosts, mildew, long periods of drought and other climatic influences. In 2017 Spain for example reported that in some regions yields were down by as much as 50%. Consequently, the simple laws of supply and demand often see wine prices increase after hot vintages where yields have been severely affected.
What winemakers can do to mitigate the effects of a heatwave
All is not lost. To combat the problems of too much sun, heat and drought there are things that can be done in the vineyard and in the winery such as:
- providing a physical shade over the vines, using bush training for the vines or trimming leaves in such a way as to maximise shade for the grapes
- irrigating, though many areas are restricted from irrigation by the wine classification regulations or even by local government regulations
- crop thinning i.e. removal of the unhealthy fruit which helps the vine divert its resources to the remaining grapes producing more concentrated flavours although this process can speed up ripening further
- using organic and biodynamic principles as many believe these make vines more resistant to drought and excessive heat, though this is obviously not an instant remedy
- bringing forward the date of the harvest. In some parts of Italy such as Sicily the 2017 harvest started three weeks earlier than ever before
- harvesting as quickly as possible and at night to take advantage of cooler temperatures
- keeping the winery cool to prevent fermentation progressing too quickly
- adding acidity to the wine though the best producers who respect 'terroir' will not use this option or will only do this as a last resort and in some wine classifications it is not even permitted
It is also important to stress that mesoclimate is the key rather than the over-riding climate of the region - vineyards in excessively hot areas may see lower temperatures if they are higher up, on hillsides, north-facing, in windy areas or near large expanses of water. There's another reason to get to know more about individual wineries and their vineyards.
Are the 2017, 2018 & 2019 vintages write-offs?
The bad news is that there have been some poor quality 2017, 2018 and 2019 wines - and that we are likely to have to pay more for our wine after heatwaves. It's enough to drive you to pour a large glass of wine - chilled naturally...
The good news however is that not all wines from these years are write-offs. Stick to cool climate wines completely if you want to be sure; you can also look at wines from smaller but experienced wineries which will be doing what they can to avoid the problems caused by the hot weather - or trust in your wine merchant to seek out the best wines for you.
© Wines With Attitude Limited, www.wineswithattitude.co.uk
Lindsay Cornelissen DipWSET is passionate about good quality wine and set up Wines With Attitude to share that passion with other wine lovers.