This difference affected the way Ruel managed the vines this year. Anticipating a smaller crop, his crews did minimal cluster thinning in the blocks of Chardonnay and other earlier-blooming varieties to leave more fruit. However, because of the improved set in the Cabernet Sauvignon and Merlot, those clusters were thinned at the usual rate, he says.

Meanwhile, the on-going drought limited berry and cluster sizes this season, Ruel adds. Although his records show that his vineyards received 82 percent of the normal rainfall between July 1, 2014, and June 30, 2015, almost all of that fell in December of last year.

“Even with our drip irrigation system, we can’t replicate the full-saturated soil profile that results with natural rainfall,” Ruel says. “This year our cover crops and weeds had already been taking up water from the December rains. So, by the time the vines started growing in the spring, the soils were pretty dry.”

This year’s smaller berries, which reduce the amount of juice to make wine, are a disappointment for Chardonnay growers, he notes. But, he considers undersized fruit a blessing for Cabernet Sauvignon and Pinot Noir fans. “The tannins and other phenolics are all in the skin and seeds,” he says. “So the smaller berries make for very strong and concentrated flavors in the wine.”

Since 2005, when their last block of vines was put in with vertical shoot positioning trellises, all new plantings at Trefethen Family Vineyards have featured horizontally-divided canopies. It’s part of a long-range plan of adjusting Trefethen’s viticultural practices to deal with a warming climate.

Despite the 2013 and 2014 seasons when Napa Valley enjoyed favorable summer weather for ripening grapes, he cautions against becoming complacent about mild summers.

“We need to remember that we can get some very strong heat spells in the Valley,” he says. “This summer was definitely warmer than the past two. In both the first part of this month and in the middle of last month, we had serious heat spells, when temperatures here rose above 100 degrees.

“Under such conditions, ripe, soft grapes are quite vulnerable to dehydration, especially with a VSP trellis, which exposes some fruit to sun damage. Using a quadrilateral cordon to divide the canopies horizontally can reduce the risk of dehydration.”

Choice of root stock also plays a role in Trefethen’s preparations for a warmer, drier climate. Now, instead of planting low-vigor root stock, like 101-14, with its shallower root system that requires more frequent irrigation, Ruel is planting deeper-rooted 110R and St. George rootstock that can handle drought conditions better.

This year, in keeping with Trefethen’s practice of minimizing use of herbicides, Ruel replaced a worn-out implement, which uses a subsurface knife to cut weed roots, with a new, updated version. He uses the tractor-drawn tool to control weeds mechanically between and within the vine rows. It features a hydraulic arm that automatically retracts the knife when it detects a vine to prevent damaging the vine.

This year, he and other Napa Valley growers have been on the alert for signs of Pierce’s Disease in their vineyards. This follows a flare-up of Pierce’s disease in some Napa Valley vineyards last year and even more reports this season, Ruel notes. The typical management regime includes controlling the blue-green sharpshooter, historically the most important vector of this bacterial disease in the Napa Valley and other coastal areas of California.

“Due to the extent of Pierce’s disease we’re seeing around the Napa Valley, we don’t know if we’re fighting another vector of if drought or some other factor has changed the behavior of the blue-green sharpshooter,” Ruel says. “But, we’re no longer finding the disease just near riparian areas. Now, we’re also seeing it in the middle of vineyards.”