The heat wave arriving for the Labor Day weekend and the week beyond comes at a critical point for the valley's vineyards, many of which are well on their way down the final stretch toward harvest.
The untimely bout of intense heat is forcing growers, wineries and vineyard workers to adapt quickly to meet the demands of their rapidly ripening fruit and the high expectations for the year's vintage.
The string of expected 100-plus-degree days comes as yet another one-off in a growing season punctuated by weather oddities, from drought-quenching amounts of winter rain to a freak hailstorm in June, and an earlier heat wave this summer that also left the state sweltering.
“We haven’t had anything that I can remember in recent memory of this intensity and this duration,” P.J. Alviso, director of Estate Viticulture at Duckhorn Vineyards in St. Helena, said Wednesday.
At this point in the growing season, grapes have progressed through veraison – the process of ripening – and their Brix – a measurement of sugar levels in the berries – continues to rise, until the optimal level of sugar is reached, at which point the grapes are harvested.
But a prolonged heat wave now makes things “tricky, to say the least,” Alviso said.
For starters, the ripened fruit is particularly susceptible to environmental extremes this late in the season.
Especially at risk are red varietals. As the grapes ripen and become darker, their stomata – or pores – become inactive, thus hindering the grapes from cooling themselves, which can lead to burnt clusters.
Brittany Pederson, viticulturist with Silverado Farming Company, recalled a shorter heat wave earlier this year, followed by two weeks of cooler weather. Such stretches, dubbed “hang time days,” allow tannins and flavors to develop in the grapes, Pederson said.
“But when it’s that hot for that long, it begins to ripen the sugars pretty quickly but without allowing the acids and tannins and flavor profiles to really develop … The sugar levels will get too high and force people to pick.”
Another concern, Pederson said, is the lack of water available now for irrigation. “Even the vines themselves, when it gets this hot they’ll start to slow down,” she said, noting that when a vine becomes dehydrated, “it’s just depleting its resources and there’s so much energy that’s gone into the fruit at this point in the season.”
While the vines at Duckhorn are being irrigated around the clock and “are holding OK,” Alviso said the continued heat spike will “definitely” speed up the time until this year’s harvest, hastening grape growers’ schedules for picking.
“Definitely it’s speeding up and advancing things whether people are really ready and want to pick at that time or not,” Pederson said.
Having gone into the heat wave already at a high Brix, Duckhorn’s merlot is now ready to be picked, Alviso said. The winery began plucking its first merlot Thursday, effectively kicking off its harvest of red grapes.
But despite the heat, the valley’s signature grape, cabernet sauvignon, is still several weeks out from harvest, both Alviso and Pederson said, owing in part to the grape’s genetic penchant for coping well with higher temperatures.
As for the vineyard workers doing the actual harvesting, to escape the unforgiving heat expected in the coming days, companies like Renteria Vineyard Management have altered their picking schedules, with the majority of crews now working through the cool night hours.
At this time of year, owner Oscar Renteria will normally divide his workforce, with half of his crews harvesting at night and half during the day. “If it wasn’t for this heat we’d be doing half and half,” he said Wednesday. Instead, “90 percent” of Renteria’s workforce is now harvesting at night.
Picking beneath night lights -- Renteria bought more last week anticipating the shift to night work -- and beginning as early as midnight, workers harvest through the night for seven to eight hours.
Beginning before midnight is currently not an option, Renteria said, as the prime picking window dictates a berry temperature of around 65 degrees, which can take several hours to reach after a 100-plus degree day.
But as Renteria begins its hillside harvests next week, the steep terrain will prohibit the company's use of night lights, pushing start times ahead to 4 a.m. Workers will then pick until around 10 a.m., shortening the amount of work hours from those spent night harvesting.
Thus, "Wineries will need to be patient to receive their lots in smaller doses over a series of days," Renteria said.
Echoing Alviso and Pederson, Renteria noted that the spike in heat also "condenses the season," shortening the window of time left before grapes must be harvested and requiring more labor more immediately.
"Usually, we’re gradually getting into picking," he said. "Now we’re running at capacity."