Grape Harvest 2026: What the Hot Summer Did to the Grapes

This year the grape harvest did not start in September but in the middle of high summer. In the Bündner Herrschaft, the first grapes in Jenins were cut as early as the week of 24 August; in Valais, well-exposed sites started on the same day; and in Ticino, the director of a winery doubted that grapes there had ever been picked so early. In most Swiss wine regions, the harvest began around three weeks ahead of the usual date. The reason is written in MeteoSwiss's measurement series – and its traces are found in every single berry.

Summer 2026 in figures

With a national mean of 17.2 °C, June to August 2026 was the hottest summer since records began in 1864: 3.4 °C above the 1991–2020 norm and 0.4 °C above the previous record summer of 2003. In western and north-western Switzerland, and locally on the southern side of the Alps, the anomaly reached 4 °C or more. On 30 July, Basel-Binningen recorded 39.7 °C, and Locarno-Monti counted 50 tropical nights – nights in which the temperature did not drop below 20 °C. At the same time, the rain stayed away: western and north-western Switzerland and the eastern Central Plateau received less than 60 per cent of their usual summer precipitation, and the period from April to August was the driest and hottest since 1901 nationwide. Almost 40 per cent of all gauging stations registered new low-water records.

For the vine, this means two stresses acting at once and reinforcing each other: heat stress and water stress.

Water stress: how the vine manages scarcity

The grapevine (Vitis vinifera) is considered comparatively drought-tolerant. Its roots reach deep, and a mild water deficit is even desirable in quality viticulture: it slows shoot growth, so the plant invests its energy in the grapes rather than in new leaves. What matters is the degree. It can be measured as the leaf water potential before sunrise (Ψ_pd), when plant and soil have equilibrated overnight. According to the common classification (Deloire et al. 2004), values below −0.2 MPa indicate mild, below −0.4 MPa moderate and below −0.6 MPa severe water stress. How strongly the vine responds depends on variety, developmental stage and the duration of the deficit.

When water runs short, the plant produces more of the hormone abscisic acid (ABA). It closes the leaf pores, the stomata. The vine loses less water as a result – but also takes up less CO₂, and photosynthesis declines. At the same time, evaporative cooling is lost: leaves with closed stomata heat up more in the sun. Net photosynthesis of the vine leaf peaks at around 25 to 30 °C and drops markedly at higher temperatures. In a summer with maximum temperatures close to 40 °C, the leaf factory is running on low power precisely when the grapes should be storing sugar.

Grape varieties differ in how they cope. Some close their stomata early and keep their water potential stable (more isohydric), others let it fall further and keep photosynthesising for longer (more anisohydric). More recent research, however, sees this as a continuum rather than two fixed types (Gambetta et al. 2020).

Small berries: timing is everything

The grape berry grows in two surges separated by a lag phase. In the first phase after flowering, cells divide and expand – this is where the berry's final size is largely determined. With the change of colour, known as véraison, the second phase begins: the berry softens, stores sugar and grows again. Trials with Syrah showed that water deficit barely affects cell division but does affect cell expansion: the berries stay smaller because their cells stay smaller (Ojeda et al. 2001). If the deficit occurs between flowering and véraison, this reduction is irreversible – the cell walls apparently lose their extensibility, and later rain cannot undo it.

That is exactly what cantonal viticulture experts reported in August: the berries had turned out "very small because of the water shortage". Smaller berries have more skin relative to pulp. Because pigments, tannins and many aroma compounds are located mainly in the skin, their concentration in the must rises. At the same time, each bunch yields less juice – which is why the industry expects a smaller harvest in 2026.

Did you know? The Oechsle scale goes back to Christian Ferdinand Oechsle (1774–1852), a mechanic and goldsmith from Pforzheim. It is remarkably simple: if one litre of must weighs 1,090 grams instead of the 1,000 grams of water, it has 90 °Oe. The must hydrometer therefore only measures density – the reason it reveals sugar content is that sugar makes up by far the largest share of the dissolved substances in must.

High Oechsle readings: concentration, not extra production

In hot years, Oechsle readings climb quickly. That does not mean, however, that the vines produce correspondingly more sugar. Part of the increase is a concentration effect: the berry contains less water, so the sugar it holds is less diluted. Under severe water stress, sugar loading can even stall, because leaves with closed stomata supply hardly any assimilates. For the winery, the result is what counts: high must weights produce wines with more alcohol and put the yeast under osmotic stress during fermentation.

Acid loss: why hot vintages taste less fresh

A wine's freshness comes mainly from two acids, tartaric acid and malic acid. Tartaric acid remains largely stable during ripening. Malic acid, by contrast, is broken down by the berry from véraison onwards and used as a source of energy and carbon. These metabolic pathways run faster in warmth: trials with ripening berries showed accelerated malic acid degradation at elevated temperature (Sweetman et al. 2014). Warm nights matter particularly, because the berry respires at night as well. In a summer with 50 tropical nights in Locarno-Monti and 40 in Pully on Lake Geneva, the grapes keep losing acid even after dark.

Added to this is potassium, which accumulates in the ripening berry. Less acid and more potassium raise the pH of the must. The consequences go beyond taste: at higher pH, a smaller share of sulphur dioxide is present in its antimicrobially active, molecular form, the wine is microbiologically less stable, and colour as well as ageing potential can suffer. This is particularly delicate for a naturally low-acid variety such as Chasselas.

Colour and aroma: when ripeness drifts apart

Red colour sits in the berry skin in the form of anthocyanins, and their balance depends strongly on temperature. In a widely cited experiment, the anthocyanin content of red-wine grapes fell to less than half when the daily maximum temperature was 35 °C instead of 25 °C (Mori et al. 2007). The cause lay less in reduced synthesis than in increased breakdown of the pigments in the skin. The more stable malvidin compounds withstood the heat best.

This gives rise to the central dilemma of a hot vintage, the decoupling of ripeness: sugar accumulates quickly, while colour, tannins and aromas lag behind. Picking by must weight may mean harvesting grapes with unripe tannins. Waiting for phenolic ripeness risks too much alcohol and too little acidity. The aroma profile shifts as well. Methoxypyrazines, which give Sauvignon Blanc its green-pepper note, for example, remain lower in warm, sunny years. In early August, cantonal experts from Lucerne and Valais pointed out that Sauvignon Blanc and Müller-Thurgau lose freshness and aromatic intensity in such years, and that early-ripening Pinot Noir suffers because its ripening phase falls in the hot weeks of late summer.

Sunburn on the grapes

A berry in direct sunlight can heat up far beyond air temperature, because it barely cools itself through evaporation. Research distinguishes two types of damage (Gambetta et al. 2021). In sunburn browning, intense light, UV radiation and heat act together: the skin turns brown, but the cells survive. Sunburn necrosis, on the other hand, is mainly a matter of temperature: cells die, the berry becomes blotchy and shrivels. In experiments, surface temperatures of around 46 to 47 °C for more than an hour were enough to cause sunburn in young berries that had previously grown in the shade. Water deficit increases the risk further.

One countermeasure is not to remove leaves from the sunny side of the fruit zone in hot years, so that the foliage provides shade. In 2026, cantonal experts explicitly warned of burns caused by extreme heat and intense solar radiation.

Nitrogen, fermentation and an unwanted ageing note

Dry soil has a less obvious consequence: the vine takes up less nitrogen, because dissolved nutrients only reach the roots with soil water and mineralisation stalls in the parched topsoil. The must then lacks yeast-assimilable nitrogen. When low nitrogen meets high sugar, sluggish or stuck fermentations become a risk. In white wines, the "atypical ageing note" (known in German as UTA) is also associated with stress in the vineyard, including drought and nitrogen deficiency. The wine loses its varietal aroma early and develops notes reminiscent of acacia blossom, floor polish or mothballs; 2-aminoacetophenone is considered the key compound.

Not everything is a disadvantage

Drought also has an upside for viticulture. Downy mildew (Plasmopara viticola), one of the most important vine diseases in wet years, needs wet leaves for infection. When the rain stays away, disease pressure falls, and grey mould (Botrytis cinerea) finds hardly any foothold either. The harvested grapes are correspondingly healthy. Late-ripening varieties such as Petite Arvine, Humagne Rouge, Cornalin or Merlot can benefit from the extra warmth. And experience with earlier hot vintages is encouraging: after 2003, the industry feared negative consequences that did not materialise. For 2026, cantonal experts expect a smaller harvest, but one of very good quality.

Young vines are among the losers. Their roots do not yet reach the deeper, moister soil layers; with prolonged water shortage, their growth stalls and some plants die.

A vintage worth tasting closely

When the first 2026 wines reach the table, it is worth tasting more attentively: how much acidity has remained, how present is the alcohol, how deep is the colour? For a large tasting round with friends, the Set of 24 Wine Glasses with Gold Rim provides enough glasses for everyone – and because they are made of plastic, nothing breaks outdoors either. The Sangria Carafe & Coaster brings the wine to the table in style. Have you ever had a wine from a hot year such as 2003 or 2018 in your glass – and could you taste the difference?

Sources

• MeteoSwiss: review of summer 2026 (September 2026)
• Schweizer Bauer: "Weinlese beginnt rund drei Wochen früher als üblich" (24.08.2026)
• SRF News: "Weinlese im Hochsommer" (22.08.2026)
• watson.ch: "Trockenheit und Hitze bringen auch Vorteile für den Weinbau" (02.08.2026)
• Deloire, A., Carbonneau, A., Wang, Z. P. and Ojeda, H. (2004): Vine and water: a short review. Journal International des Sciences de la Vigne et du Vin 38 (1), 1–13.
• Gambetta, G. A. et al. (2020): The physiology of drought stress in grapevine: towards an integrative definition of drought tolerance. Journal of Experimental Botany 71.
• Gambetta, J. M. et al. (2021): Sunburn in Grapes: A Review. Frontiers in Plant Science 11.
• Meier, N. et al. (2007): Grape harvest dates as a proxy for Swiss April to August temperature reconstructions back to AD 1480. Geophysical Research Letters 34.
• Mori, K. et al. (2007): Loss of anthocyanins in red-wine grape under high temperature. Journal of Experimental Botany 58.
• Ojeda, H. et al. (2001): Influence of water deficits on grape berry growth. Vitis 40.
• Sweetman, C. et al. (2014): Metabolic effects of elevated temperature on organic acid degradation in ripening Vitis vinifera fruit. Journal of Experimental Botany 65.