Lazy Hazy Growing Degree Days of Summer

I've been writing this blog for 3 years. I try not to return to the same topic if I can help it.

I'm making an exception for the weather. Why? Because it's the single most important thing in the life of a vineyard. Every year different. It can make or break a harvest.

And it's getting warmer. Much warmer.

Vineyards the world over define the warmth of a vintage by a measure called GDD: Growing Degree Days. For viticulture, it's a simple calculation. Vines start growing and photosynthesising at roughly ten degrees Celsius. You take the number of degrees that each day's average temperature exceeds 10C and that's your GDD value. Add it up over the growing season, from 1 April to 31 October, and that's your annual total.

GDDs tell you how warm a wine region is, and by extension what sorts of grapes will ripen (or over-ripen) there. There is a global system called the Winkler index that classifies wine regions in this way. In the very hottest "Region 5", like Australia's Hunter Valley or the vast outdoor bulk-wine factory that is the Central Valley of California, the GDDs can be well in excess of 2,000. At those temperatures, unless you are very careful or pick very early (as indeed they do in the Hunter Valley) you're cultivating raisins.

At the coolest end of the spectrum anywhere with under 1,390 GDDs is Region 1. The very coolest reaches, notably Champagne and Mosel, were at or a little below 1,000 in the mid 20th century. That's the limit for meaningful quality winemaking. In old atlases you'd see it marked out with a wavy line: "limit of vines" weaving from the Loire estuary through the Paris basin to the Ardennes, alongside the "limit of olives" further south and its little hillock going up the Rhone.

Where does that leave Southern England? For most of the history of English wine we were below the recognised GDD range. Our warmest sites struggled to ripen traditional vinifera but could eke out harvests of Germanic crosses and the hybrids now called PIWIs. That started to change in the 1990s, and it kept going. We are now regularly hitting numbers that were typical of Champagne in the 1970s. But more than that. Some of our warmer sights are now in the range of the traditional climates of Chablis and the Cote D'Or, and well within the bounds of traditional Muscadet.

Our site is a cool one. The annual count hovers around 900-1,000 in a normal year. We get some of the warmest daytime temps in the country but we are notably cooler at night and this diurnal range drags down the GDDs and slows down ripening. But our warmest season so far - the year of planting in 2022 - scored 1,027 which is higher than the growing seasons in the Marne Valley in 9 years since 2000.

Then we come to 2025. I'm writing this in early September, and already it is being hailed as the warmest vintage in English viticultural history. We've had several very good years in the last decade that have re-set the bar for the sorts of ripeness levels we should expect from English grapes, but unless we're cursed with an unusually cool and wet September and October the 2025 growing season will be leaving them all in the shade. And in any case by October almost all vineyards, though not ours, will already have taken in the harvest.

The unusually warm weather started in spring and has continued since. No record breaking heatwaves, no 40C this year, just relentless warmer than average conditions. Remember it takes a whole season to make a ripe bunch of grapes, not just a hot month or two. In the Met Office Central England Temperature series, every month since March has been at least 2C warmer than the long term average. It has also been sunny, and in places very dry.

As at the end of August I had clocked up 785 GDDs for the season so far. September and October should add another 200 or so. I think I'll end up around the 1,050 mark. The warmest so far, though not far off the 2022 number. But Kent has been by no means the hottest region of the country this summer. In fact it's been a laggard: with clear skies and calm cold nights my valley site has seen very sharp diurnal ranges keeping the totals lower than most others across the South, and the most remarkable numbers have come this year from those in Wessex and the Marches.

Back in mid July Stephen Skelton sent an email round the WineGB forum asking for GDDs for the season to date. I'm sceptical of the tropical numbers he got back from some participants, and I'm also aware of selection bias among those who choose to respond, but still. Many were beyond 550 already, especially those in Hampshire and the West. They must be close to 900 by now. No wonder so many are already bringing in their Siegerrebe and Bacchus before the wasps get it.

The West has had the driest weather too. Many places had almost no rain in July; we got a whopping 201mm. Our valley was lush and emerald green throughout August; other parts of the country were looking Tuscan.

2025 will almost certainly be another milestone in the evoluti0n of English and Welsh viticulture. A year when most vineyards in most parts of the South will be able to produce still wine from ripe grapes of traditional French varieties without tooth-stripping acidity. And the weather pattern that brought it was by no means exceptional. Synoptics that 50 years ago would have brought us a reasonable, slightly warmer than average spring and summer gave us something rather different this year.

This is all good news for our first commercial harvest. We don't have a huge yield, I can tell that from bunch counts, but it's hit veraison at least 3 weeks earlier than 2024 and the red berries are already tasting sweeter and less astringent than they did last October, with a month and a half left to go. Pinot Meunier that last year achieved at best a deep ruby colour has some bunches that now look as black as a Barossa Shiraz.

Yes, that's Pinot Meunier

We're planning to bring in the harvest on the 18th October, if the birds can keep their hungry little beaks away until then. We got a bit of bird netting up but it's so expensive, and such a faff, that all of the Pinot Meunier and Melon B remains unprotected and only the juiciest Pinot Noir gets the benefit. Look out on this blog for some shorter harvest updates as the date gets closer.

For the English and Welsh wine industry these warmer seasons can only be good news in the short term. We will get riper grapes, but then we can also start harvesting earlier with less risk of disease, less bird predation and more time for the vines to photosynthesise and put down carbohydrates after harvest. That should see yields rising.

Others are less lucky. Every year at least one European wine region is seeing crops suppressed by severe drought and sunburn, and this summer half of the Corbieres appellation was destroyed, scorched or smoke-tainted by the huge wildfires that swept across the hills. Just as we start to venture properly into the lower echelons of the Winkler index, other traditional regions are testing its upper boundaries.

2025 is the warmest growing season yet. I doubt that record will last long. *