Do grape vines know what they’re doing?

Left to themselves, grapevines will push out more clusters of fruit than they can ripen well. Making wine from such grapes will be disappointing: not enough sugar, poor flavor, too much acid. The problem is that a given vine can only pull in a finite amount of nutrients through its roots, and only produce a finite amount of sugar in its leaves. Divide that finite amount by an increasing number of grape clusters, and each one will have less and less. At some point, “more” becomes “too much” and you’ll get a large crop of poor grapes that will never produce anything but poor wine.

Yes, and that’s the problem

Grape vines will do this because they’re not trying to make wine. All they want to do is produce something that animals find more appealing than a nearby twig or blade of grass, and they want to produce as much of it as possible. Ok, they’re not self aware, and therefore aren’t really trying to do anything, but we are. We’re trying to make wine, and the tendency of the vines to produce as many grapes as possible gets in the way of that. So, part of the reason we prune them is to reduce the yield down to something that will ripen well. Sometimes, even after pruning, the vine will push too many clusters, and that’s when we need to thin. Another approach would be to prune less aggressively than normal, then thin to keep the vine from overbearing.

A specific deadline

You can’t just lop off nearly ripe clusters of fruit in the fall and call that “cluster thinning.” To get the most benefit from this technique, you need to thin the clusters before the flowers open. Its probably true that there will be something to gain from a “better late than never” approach, but it won’t be much. Timing is critical here, so monitor your vines closely after bud break and make sure you thin before the flowers open.

And a fuzzy target

If you don’t like deadlines like that, then you’ll be happy to know that the idea of how much to thin is a lot more vague. I’m going to have to punt here and say that “it depends.” It depends on the varietal, your location, the year to year climate variability, and your own goals. You may be able to find advice, from other growers in your area or from books, about how many high quality clusters this or that varietal can produce in your area. In the end, though, nothing can substitute for experience with living vines in your vineyard. Get to know your vines by trying fewer clusters this year to see if that improves the quality, or advances ripening. Maybe try a few more clusters to see if that boosts yield without harming the wine. Knowledge like that is priceless and will help you make better wine from your own grapes.

Further reading

Cluster thinning is aimed at efficient use of sugar in the vine. My last article on canopy management was about leaf pulling, to promote efficient sugar production in the vine. These two goals are opposite sides of the same coin, so if you missed the first article, you might want to click here and check it out. To make sure you don’t miss future articles, subscribe to this blog. Its simple, its free, and you’ll see every article without having to check back manually.

I’m still learning about canopy management. I’ve gotten a lot out of Jeff Cox’s From Vines to Wines. It’s where I first read about cluster thinning, and I still refer back to it. What makes it a great book for the home winemaker who wants to grow his own grapes is that it covers both topics really well. The first part of the book covers the viticulture, from which vines to plant, site selection, pruning and training, and pest management right through harvest. The second part picks up right where the first part left off and talks your through making wine from the grapes you just harvested. From primary to finishing and ageing with troubleshooting information and a section on tasting.

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