As I've noted earlier, the amount of grapes a vine produces can have a direct effect on the quality of the grapes produced. If vines are allowed to produce too many grapes, the amount of sugar will be spread among those grapes and diluted too much to make good wine. In Europe, yield rates are legislated for each grape variety in each area. While there are no such laws in the New World, when good wine is the desired product, yield rates must be considered. Yields are usually expressed in weight or in volume. When expressed in weight, the yield will be in tons of grapes per hectare, and when it is in volume it will be hectoliters per hectare.
Here is a list of the different factors that can affect yields:
- The number of vines per hectare.
- The number of buds per vine, determined by pruning.
- The number of shoots, which depends on the soil and climate conditions, and the number of buds per vine.
- The number of clusters per shoot, which described the fruitiness of a vine, and is determined in the spring when the bud forms.
- The number of berries per cluster, which is determined by the flowering process, dependant on the weather during flowering. This is the most unpredictable phase.
- The weight of the berries. This is determined by the grape variety and how much water reaches the vines, either through irrigation or rain. The nutrient supply is also a factor.
- Green harvesting. This is a process in which people will go through the vines and hand-pick out excess bunches of grapes in an effort to reduce yield.
As you can see, there are many factors that influence the amount of yield that will come from a vineyard. Each one of these factors involves a decision on the part of the winemaker, and each of those decisions will increase or decrease the cost of production. Hopefully every decision that increases the cost will result in a better bottle of wine.