By Paolo Sabbatini
The question every grower asks, “Should I thin my vines?” is one of the most difficult questions Michigan wine-grape growers face. No grower enjoys walking through a vineyard and seeing healthy clusters lying on the ground, but every wine region of the world performs cluster thinning. Every removed cluster represents potential yield, months of vineyard work and fruit that will never reach the winery. However, in Michigan, the question is often partly rhetorical. The answer is: Yes, when the crop exceeds the vine’s capacity to mature it, the vines should be thinned.
For Michigan, the reason begins with dormant pruning. In Michigan, pruning establishes the vine’s potential crop, but it does not always establish the final number of clusters that the vine should carry to harvest. Growers commonly retain additional buds to compensate for uncertain winter injury, uneven bud survival, variable bud fruitfulness and possible spring frost damage. Under Michigan growing conditions, dormant pruning commonly retains approximately twice the number of buds required to achieve the target crop for a given cultivar, planting density and training system. This strategy is necessary in a cold climate, but it also means that vines frequently enter the growing season with more shoots and clusters than they can fully mature. A vineyard may appear well pruned in March but become clearly overcropped by July.
Once fruit set is complete and the crop becomes visible, the grower must answer a more useful question: Does this vine have enough healthy, functional canopy and enough favorable weather remaining to mature all the fruit it is carrying? As you will see later from data collected in Michigan, the answer is no. Leaving every cluster on the vine does not preserve crop value. It distributes the vine’s available resources among too many competing clusters. The result may be delayed or uneven sugar accumulation, retention of excessive acidity, limited color development in red cultivars, delayed flavor evolution and a greater risk that harvest will be determined by autumn weather, disease or frost rather than by fruit maturity. The clusters on the ground may not be visually satisfying, but they represent a deliberate investment in the maturity, uniformity and value of the fruit remaining on the vine.
Source : msu.edu