Grapes turn from bright green to red during veraison at Barboursville Vineyards, near Charlottesville. (Dave McIntyre)

Third in an occasional series about the 2015 vintage at Barboursville Vineyards in Virginia

Wine is the product of a grapevine’s instinct to survive and reproduce, and of the vigneron’s effort to control and ultimately hijack that process.

“Vines can reproduce in two ways,” Luca Paschina, chief winemaker and general manager at Barboursville Vineyards, explained to me in an impromptu biology lesson during a recent visit to the winery, northeast of Charlottesville. “If a tendril hits the ground, it can set new roots — very effective. But if tendrils don’t reach the ground, the vine ripens grapes to attract birds, which eat the grapes and spread the seeds for the vine to propagate.”

That’s why wineries invest in bird netting to protect the grape clusters on their vines after the grapes start ripening, a process called veraison that in the Northern Hemisphere typically occurs in mid- to late July. Veraison is when the vine turns its energy from photosynthesis and producing chlorophyll in its leaves to increasing sugar content in the grapes. The berries turn from bright green to amber green in white varieties, such as chardonnay and sauvignon blanc, or to dark purple in red varieties, such as cabernet sauvignon and nebbiolo. Acid levels drop and sugars rise, making the berries sweet and tasty for the birds.

“They sit in the trees or on power lines and dive-bomb the vines,” Paschina said. Birds pecking at ripening grapes can damage them, leading to rot that can spoil an entire cluster and spread to others. That netting is crucial.

Barboursville’s vineyard manager, Fernando Franco, carries a small cap pistol in his pickup truck that can shoot a firecracker into the air to scare birds away, but it’s not effective. “They figure it out soon enough,” he said.

There are other predators, of course.

“Birds, deer, raccoons, groundhogs, bears,” said Franco, counting off his foes on his fingers. “They all want the grapes.” The enemies list also includes Japanese beetles, which were particularly heavy this year in their onslaught on vine leaves, inhibiting photosynthesis. And fruit flies, which can damage the grapes and promote spoilage. And hail, which can strike during the capricious vengeance of a summer thunderstorm. Anything that can puncture one berry can ruin a cluster.

New predators always seem to emerge. The 2013 vintage is known in central Virginia as the Year of the Squirrel. A record shortage of acorns the previous autumn sent swarms of the hungry varmints into vineyards, looking for nourishment. Luckily, they were after the protein in the seeds and never developed a taste for the sweet carbohydrates in the grapes, Paschina said.

The squirrels have not been back. “Grape seeds are more work than acorns; otherwise we’d be pfft,” he said.

The vigneron is the ultimate predator of the grapes, protecting them from other enemies in order to keep them for himself. Franco’s job is to encourage the vines’ survival instinct and coax the clusters to maximum ripeness, then to harvest the grapes and make wine, not new vines. The 2015 vintage began in January, when Barboursville’s crew of 16 seasonal workers arrived from Mexico and El Salvador and began pruning the vines, positioning them to grow top-quality fruit. Through flowering and fruit-set in April, Franco and his team have tried to coax about 330,000 vines over 182 acres to produce about four pounds of fruit each, their formula for maximum quality. That involves pulling leaves and hedging the tops off the vines to control vigor, an important concern during this year’s rainy spring and early summer. Through “canopy management,” a grower can maximize sun exposure on the grapes to help them ripen while ensuring airflow through the leaves to dry them off after a rain and minimize the threat of diseases such as downy mildew.

The day before I arrived in mid-July, Franco’s crew had cut several clusters of viognier grapes from each vine, discarding them between the vine rows, a practice known as “green harvesting.” It’s another example of how a vintner’s desire to make the best possible wine conflicts with nature’s imperative for the vine to reproduce.

“It’s throwing money on the ground,” Paschina said, “but if you don’t do it, the wine is not as good.” And with red grapes, many vintners drop clusters that turn color unevenly during veraison. That’s because once all the grapes turn color, it will be difficult to tell which clusters were ripening slowly and might be under-ripe at harvest. When you hear of a wine made with “low yields,” that means the vintner is trying to maximize quality at the expense of quantity. And, of course, price.

Veraison happens out in the vineyard, but as a harbinger of harvest it sparks activity inside the winery. At Barboursville and wineries throughout the Northern Hemisphere, crews were busy blending and bottling wines from previous vintages, while cleaning and sanitizing stainless-steel tanks to prepare for this year’s harvest. Out with the old, to be ready for the new.

The dog days of summer might seem like a sleepy time for vintners as they wait for their grapes to signal harvest. But for Franco, trying to coax the berries to ripeness while protecting them from rival predators, midsummer can make or break a vintage.

“People always say to me, ‘Harvest must be busy,’ ” Franco said. “No. Harvest is a load off my shoulders. I’m busy now.”

As we spoke, the chimes of the Knight’s Chapel Church of the Brethren, a tiny place of worship on the edge of Barboursville’s vineyards, echoed over the hillsides. Franco looked out over his the planted rows and said, “I love these guys.”

He could have been referring to his vineyard crew or the vines themselves. I didn’t ask.

McIntyre blogs at dmwineline.com. On Twitter: @dmwine.