As anyone who has rushed into a liquor store looking for that last-minute house warming gift knows, the range of quality and price among wines can be overwhelming even when considering bottles from the same winery. While these differences have much to do with the age of the wine and the processes by which it is prepared, the distinction begins with the type of grapes used.
 

Daniel Bosch, a senior technician at the Mondavi winery in California’s Napa Valley, spends most of his days managing grapevines. He explains that in order to age into a fine wine, grapes have to be just right when they are plucked from the vine. "We want the grapes to ripen at the correct size and not too early or too late," he says. For many wines, including the Cabernet Sauvignon, big, watery grapes will generally produce poor color and a thin taste. Small and unripened grapes on the other hand make for an acidic wine. In general, the berries need to be small, at their peak in ripeness, and full of flavor.

Bosch says that one of the keys to controlling the quality of the berries is to carefully balance the vigor of the vine with the number of grapes the vine produces. "Vigor" is agricultural speak for the health of a plant and is the result of the type and depth of soil a plant is in, the water it receives, and the sunlight it gets. Unlike most staple crops, vigor in grapevines is not necessarily a good thing. Highly vigorous grapevines, which are characterized by especially dense foliage, have more energy to grow fruit. If a relatively small crop is grown on a leafy vine, then the grapes will be big and watery. On the other hand, a stressed vine with less foliage does not have the energy to produce a big crop. If too many grapes are grown off of such a vine, the grapes will not ripen at all. "In instance where a vine is vigorous, you want to grow more grape-producing shoots to expend the vine’s energy. The opposite holds true for vines that are stressed," says Bosch.

Subdividing Vineyards Based on Vigor
To manage the vineyards by vigor, vineyard technicians must first separate the vines based on their relative fitness. Of the many factors that can affect grapevine vigor, depth of the soil and the amount of moisture in the soil are the most important. Because hills erode and water settles at the lowest possible point, the deepest, richest soils can usually be found at the bottom of a slope. Consequently, the healthiest vines will grow at the base of an incline and progressively get less vigorous as elevation and slope increase.

Yet, even with this knowledge, demarcating the exact changes in vigor to determine how many shoots to grow on the vine still isn’t easy, Bosch explains. Vineyards are rarely flat, and hills intersect and follow many contours. The variations throughout a vineyard can be very subtle and the dividing lines uneven. "Even if we go over the vineyard row by row and taste the grapes and analyze the vines, it can be pretty confusing," Bosch says. "The variability isn’t always in a straight line."
 

To create the best wines, grapes need to be picked at the perfect time. The grapes must have balanced flavors—primarily acidity versus sweetness—and therefore they need to ripen enough to be sweet, but not overwhelmingly so. (Photograph copyright Robert Mondavi Winery)

For centuries, the French have managed to separate their vineyards by vigor and then manage each section to achieve an extremely high level of control over their grapes and quality of wine. Bosch explains the French reached this plateau by carefully studying the soil composition and topography of their wine-growing regions over many years. Some also monitor the leaves as they turn every fall. Vines with more vigor hold onto their leaves for much longer than vines under stress. Watching a hillside turn can give them a good idea of how to prune the crops from year to year.

Bosch points out that the problem with these techniques is that they take years and years to implement and require generations of experience, whereas the extensive wineries of the Napa Valley were only planted within the past 50 years. While the early Napa vintners chose a good area to grow grapes, they did not go to great lengths to subdivide the vineyard in terms of how vigorous the vines would grow. Instead they subdivided the vineyards based on road access and plot size. "When we started, we simply didn’t understand how subtle the variations could be from vine to vine," says Bosch. As a result they never have achieved the level of control over the grapes they harvest that the French have.

Until recently this lack of control didn’t matter so much, since the demand for premium wines from Napa Valley was not very high. But along with a growing reputation, a good economy, and a refinement of the American palate, the demand for reserve wines grew substantially in the early 1990s and drove the price up from around $35 per bottle for the most expensive wines, to well over $100. Several years ago, the wineries began to look for ways to detect the health of the vines and subdivide the vineyards by vigor, so that they could create larger stocks of reserve wines. They needed a method that wouldn’t require decades to implement.

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Vineyard managers carefully control the growth of vines to bring out the qualities in the grapes that they want to emphasize in their wine. Grapes from vines that are too vigorous tend to be dominated by “fruitiness,” and they lose the complex flavors imparted by the soil. (Photograph copyright Robert Mondavi Winery)