So often in the wine press, we read articles and interviews that talk about low yields being the key to producing high-quality wines. It is repeated so often, in fact, that it seems to have become just one of those facts about grapes and wine that everyone knows to be true.
I think we need to debunk this “universal truth.” One of the essential tenets that we are taught as viticulturists is that the best fruit comes from “balanced vines.” What this means is that quality fruit is produced when there is an adequate amount of leaf area exposed to the sun to ripen a given amount of fruit. How much is that? While there isn’t one specific number for all grape varieties, studies generally point to a range of somewhere around 10 to 15 square centimeters of exposed leaf area per gram of fruit.
So how do tons per acre and leaf area per gram of fruit relate to each other? Certainly tons and grams of fruit are different ways to measure the same thing, so that’s seems OK. But the relationship falls apart when we try to relate exposed leaf area to an acre of land.
It’s very hard to measure leaf area directly, so we often use pounds of dormant cane prunings per vine, or pruning weight, as a substitute, which can be measured easily in the winter. Using pruning weight instead of leaf area, we generally consider a ratio of crop yield to pruning weight (in pounds) of five to 10 to be indicative of a balanced vine. This ratio is often referred to as the Ravaz Index.
The problem is that there is no such thing as an ideal pruning weight for all grapevines. So many factors influence how much shoot growth, i.e., leaf area, a vine develops each year — soil type, water availability, soil fertility, rootstock selection, and seasonal heat accumulation, just to name a few. Putting all of these factors together can result in vines with less than 1 pound of pruning weight, or more than 4 pounds.
Using the Ravaz Index, a balanced vine with 1 pound of pruning weight should have had at least 5 pounds of fruit the previous growing season, while a vine with 4 pounds of prunings should have yielded at least 20 pounds of quality fruit.
A vine is considered undercropped when there is less fruit on it than the amount of leaf area would dictate is appropriate in order for it to be balanced. Contrary to what many wine “experts” (and some winemakers, unfortunately) say, reducing yields to the point of undercropping does not always improve grape and wine quality.
Undercropped vines have higher shoot vigor because there is less fruit to compete with the vegetative growth. This leads to more shading in the fruit zone and canopy, which causes problems with disease management, reduces fruit exposure to the sun, and decreases bud fruitfulness.
In order to accommodate vines with higher vigor, a number of trellising options have been developed that allow a higher number of shoots and leaves to have sun exposure. These divided trellis systems, such as Geneva Double Curtain, Scott Henry, and Lyre, increase the exposed leaf area of high vigor vines. Because there is greater exposed leaf area, the vines can actually carry larger crops without sacrificing quality.
The concept of tons per acre as an indication of fruit and wine quality is one that needs to be removed from our vocabulary. The only time that tons per acre really needs to be discussed by winemakers and growers at all is for planning tank space at harvest. Hopefully then, the wine press won’t applaud low yields as the be-all and end-all of quality, but rather the grower’s ability to achieve balance in their vines and produce quality fruit, regardless of how many tons of grapes were produced from an acre of land.