Thoughts on thinning - how much does a cluster contribute to yield?

It's such an early season in Canterbury this year!

Just before the nets went on in the Lincoln University Vineyard (the earliest they've gone on in the 13 years I've been here!), we were able to squeeze in some fruit thinning. That prompted me to write the following for the Wines of Canterbury newsletter...

Despite all the heat and promise of a fantastic season, why go through all that trouble when it looks like it will be a year where you can leave all that crop on and get it ripe?

Well, the first thing to check is how much crop you've got! We looked at the Pinot noir and Pinot gris and found that cluster counts were around 20 per metre of row for both of them. Not only that, but the clusters looked pretty big, too, so some more investigation suggested that berry number per cluster was around 125. So figure about 1.1 grams per berry for the productive clones of Pinot noir, and more than that for the Pinot gris, and you're looking at crop loads of around 2.7 or more kilograms per meter of row. Too much!!

For the mythical balanced VSP vineyard (refer to Sunlight into Wine, by Smart and Robinson, 1992), a typical pruning weight per metre of row should be about 0.5kg. Using a yield to pruning weight ratio of about 3 (which, while less than the 5 to 10 figure used by Smart and Robinson, seems to be more appropriate for vines in the Canterbury region) each meter of row should be able to (nicely) ripen 3 multiplied by 0.5kg of fruit, or 1.5kg.

So by doing some quick cluster and berry counts, we can see that there is quite a bit of extra crop on there, to the tune of at least a kilogram per metre of row!

So we need to take off about 44% of the fruit that's on the vine, which could be thought of as slightly fewer than every other cluster.

However, it's important to note that not all clusters are the same. The basal cluster is usually the most advanced in terms of ripening, so it's beneficial to keep as many of those as possible. But if you're removing all the upper clusters, it's important to note that they are also usually smaller, to the tune of about 70%, than the basal clusters. So if a basal cluster is 100g, the cluster above it will be about 70g, and if there's a third cluster on there (which the Chardonnay clone UCD6, for example, has in spades this season), it will be about 50g (it might be a good idea at harvest to do some measurements of this in your vineyard and with your varieties to see if this holds true in your situation).

So if you remove just the top cluster, you're looking at a reduction in yield by about 23% - not a third if you think of the clusters as being all equal. If you remove all but the basal cluster, you've taken off about 54% of the fruit. So to reach the goal of taking off 44% of the fruit, you should be removing all the top clusters, plus most of the secondary clusters.

Whether or not this is an economically feasible thing to do is another matter!!