COLUMBIA — Under a beating afternoon sun, head winemaker Cory Bomgaars of Les Bourgeois Vineyards in Rocheport trudged through acres of grapevines and pointed out the drought-affected plants.
Some of the vines have hardened and turned brown from the lack of water. Stems that might have produced bunches of grapes have burned off in the extreme heat. Among the bunches that have formed, some of the grapes are shrunken, slightly shriveled and deep purple.
Bomgaars took a bite out of a shriveled vignole grape.
"Yeah, these are really bitter," he said, tossing the remaining morsel away. "That's not supposed to happen."
The recent heat wave — as well as the mild winter and spring — will have a complex effect on Missouri winemakers and their grapes this summer. Some may have to shift their harvest schedules to accommodate the ripening rates of different vines and grape varieties. But history suggests there's also a good chance that the stress caused by the heat and drought will produce an exceptional vintage.
In past years, dry summers have helped improve the vintage of the wine, which is the product of many factors that influence color, taste and general quality. Those factors include each year's unique climate, and the growing and winemaking practices of individual vintners, MU viticulture professor Anthony Peccoux said.
Peccoux said most hot, dry years produce good vintages. That's because heat puts stress on grapes and improves the biosynthesis of polyphenols, which make up the color and taste of the wine. This results in the wine having a better taste and aroma.
Peccoux cited a 2008 study by French researchers that showed mild drought can lead to higher quality vintages in red Bordeaux wines. The study also shows that more water deficit stress a few weeks before harvest season produces a better vintage.
"Usually when we have an advance in the season it's usually a good vintage as well," Peccoux said, referring to the favorable vine-growing conditions of late winter and early spring.
However, this summer's heat wave came about two months before the usual harvest season, so winemakers are curious to see exactly how this summer's drought will affect their grapes.
Just a few miles east of Hermann, the winemaking capital of Missouri, OakGlenn Vineyards and Winery sits atop a hill overlooking the Missouri River. Hundreds of acres of grapevines cascade down the hill into the river bottom.
While at first glance the myriad rows of vines don't appear to be suffering, a closer look reveals that the drought is causing trouble.
Carolyn Warnebold, who co-owns the business with her husband, Glenn Warnebold, surveyed the vineyard from her golf cart on a recent afternoon. While owning a vineyard is always hard work, it has been especially difficult this year, Carolyn Warnebold said.
"We were going to retire here," she said. "Now I just say we're tired here."
Warnebold pointed out the space between the rows of norton vines, which typically tend to grow into each other, thus making it impossible to drive a golf cart between them. Heat and lack of water, however, has stunted vine and trunk growth.
"The heat has slowed down the process as far as growth is concerned," Warnebold said.
OakGlenn is a dry-farm vineyard. It doesn't use an irrigation system to water the vines. Instead, it must adjust to the unusually searing summer.
Warnebold said that in addition to watering the vines individually, workers also have tried to keep weeds down at the base of the plants so the vines don't have to compete for water.
"We've had some high school boys come and water the vines 15 seconds each with hoses," Warnebold said. "Watering did do some good."
"But they're still suffering."
The age of the vines also plays into how well they survive. The roots of older vines stretch deeper into the soil, where they can absorb more moisture. OakGlenn has norton vines that date back to 1859 and to the original owner, George Husmann.
"One of the nice things about having old vines is that they have a heritage," Warnebold said. "You know they can survive through the years."
It also helps that nortons usually survive the stress of extreme heat. They require about half as much protective chemicals as some of the younger vines at OakGlenn, such as the chardonel vines, Warnebold said.
The younger chardonels, planted at OakGlenn in 1998, have yellowing leaves and shrunken berries. Also at the top of the hill, they are exposed to more sun.
It isn't just dry farm vineyards that are suffering, though. Irrigated vineyards such as Les Bourgeois have had trouble with the drought's effects on the younger plants.
"We're watering the younger plants almost double because of the less-developed roots," Bomgaars said.
The younger traminette vines require watering every five days; more developed vines get water every 10 days.
Although the drought is having its effect, winemakers don't anticipate enough damage to drastically affect the yield when harvest season arrives in a few weeks.
Bomgaars predicts a two-part harvest. He believes the vines that developed earlier during the mild winter and early spring will ripen first. Then he expects a delay before the harvest of grapes from vines whose growth slowed during the heat wave.
"It's all theory right now," Bomgaars said. "In a few weeks we will have flavor development," when samplings of the grapes will be collected and analyzed for acidity, sugar levels and other factors.
"We will have a better idea of what the chemistry looks like," Bomgaars said. "It's a little early in the game to feel for what it will do for the quality."