2015 vintage presented us with the most perfect Merlot grapes I have ever experienced. Cool dry days allowed us to pick at our leisure. Skin tannins ripened synchronistically with flavors and chemistry. One could easily fantasize about the resulting wine. As we were winding down the Merlot harvest, I had almost forgotten about the “Merlot half row” planted in a Chardonnay block years ago after a 1980s lightning strike took out part of a Chardonnay row. I decided to plant Merlot experimentally.
Aaron and I threw a dozen lugs in the back of the truck, and with great anticipation started finishing the Merlot harvest. It took about 30 seconds to realize that we had a problem. Masses of clusters were intertwined with each other requiring us to physically force them apart thus breaking berries. There were lots of pink berries lodged in the shaded sides of the clusters that never saw daylight. There was even some sour rot in a vintage where it hadn’t rained for weeks. Worst yet, the grapes had no flavor. They tasted like the table grapes one buys at the grocery store.
This half row of Merlot was also the “forgotten row.” We had overlooked it during cluster thinning back in July. It confirmed just how much of a difference thinning makes on wine quality.
Over the years I have had many approaches to cluster thinning. As with most new growers I first tried the usual “one cluster per shoot” method. I soon realized that this made no sense as cluster sizes varied enormously along with shoot size and vine capacity. I even attempted to leave a certain number of clusters per vine. In fact, I found that using numbers initially helped me make good wine, but it prevented me from making great wine. To grow great wine one has to respect the individuality of each vine and of each block. This requires an experienced staff that understands the goals and consequences of not only yields, but of cluster placement.
I find that the ideal time to drop clusters is at lag phase, which is about 6 or 7 weeks past bloom (mid to late July usually). At this time berries are less likely to compensate (enlarge) and the cluster architecture, position, and size is evident.
Young vines or stressed vines may be thinned earlier than lag phase. We remove clusters in baby “non-bearing” vineyards before bloom to conserve energy. An additional “fine tuning” green harvest for red-fruited varieties happens at about 90% veraison. Less than 5% of the crop (green/pink clusters) is removed at this time.
Our mantra in lag phase thinning is “aeration.” Any nesting clusters or parts of clusters are removed so that one can easily identify one cluster from another. Ideally no clusters touch one another. Wings or shoulders may or may not be removed depending on cluster architecture, position and crowding. If wings are behind in maturity they will be dropped during the final red grape 90% veraison thinning.
Small clustered varieties such as Petit Manseng, Riesling, Sauvignon Blanc and some Chardonnay clones need very little thinning. We spend the most time thinning in Merlot, Cabernet Sauvignon, clone #4 Chardonnay, and Petit Verdot.
Cluster thinning is considered fine-tuning in terms of yield control. Yield goals should have been addressed earlier during pruning and shoot thinning. If we find ourselves dropping a significant quantity of fruit, then we haven’t done a good job earlier in the season. An exception might be in the case of late ripening Petit Verdot and Cabernet Sauvignon. 50% veraison dates are recorded each year. If the current vintage is lagging way behind there could be difficulties in getting these varieties fully ripe. The decision could be made to drop more clusters so the vines can ripen a smaller crop faster. All these decisions are based on the history of each vineyard block.
In wet harvests aeration thinning can have a significant impact on avoiding rot. In any vintage it will improve quality as a result of more synchronistic ripening. Additionally, picking is a lot easier and faster with less bruising and berry breaking.
Grape Press, June 2016