With the growing season off to a cold start, I’ve been thinking about how to grow the best grapes in a cool season. Cluster thinning and leaf pulling can help by maximizing sugar production, in a grape vine, and by making the most of the sugar it produces. Good canopy management will bring out the best in a vine under all conditions, but it doesn’t work magic. Without enough heat, grapes won’t ripen well, and you can’t do anything about the weather. Or can you?

Scaling up a greenhouse

Jeff Chorniak inspired me to grow grapes in my suburban backyard and he used a collapsible greenhouse to extend the growing season. By sheltering his Cabernet Franc in this way, he made early spring and late fall just a tad warmer, and that can make all the difference when you’re growing grapes in Toronto. I’ve never done that myself, but the possibility of a cooler than normal season has me thinking about it. I’ve also wondered if the concept could be applied to a vineyard, by using plastic sheeting the way gardeners use row covers.

Some Puget Sound growers are doing just that. I’m hearing that overnight low temperatures, inside a tented vineyard row, are as much as 10F (5.5C) warmer than neighboring untented rows. On a clear day, under bright sun, it can get up to 15F (8.3C) warmer. “As much as” and “up to” are always tip offs that typical performance will be less, and that’s probably true here. Still, every degree helps and this might be a good way to cope with a cooler than normal year.

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.

This grape growing season, here in the Puget Sound, is off to a slow start. Most vine growth will occur with warmer temperatures in the summer, so the cool weather we’re seeing right now might not affect this year’s crop. Still, it’s got me thinking about making up for lost time – managing the vines to take advantage of every photon of sunlight that comes their way. One way to do that is leaf pulling.

Leaves in the shade: Like open windows with the AC on

The idea behind leaf pulling is that a leaf in the shade isn’t just idle, it’s holding back the vine by consuming sugar and nutrients that could be going to productive leaves or fruit. So if you pluck leaves that are in the shade, or those that shade other leaves, the vine will still get almost as much benefit from photosynthesis as before, but will not have the drain of non-productive leaves. Done correctly, this will mean better fruit. It might even mean earlier fruit.

Fruit in the shade: A more complicated trade off

Sunlight affects ripening fruit in different ways. Too much can burn, and not enough can delay ripening. Here, leaf pulling decisions are more tricky. What do you do about a leaf shading a grape cluster? If you pull it, the cluster will get more sun, but at the cost of reducing photosynthesis and sugar production. I don’t know that there’s a this-always-works-answer here. In cool seasons, the risk of burning may be less so you might be more inclined to pull leaves that shade fruit clusters. This might also be true later in the season, after the hotter July and August days are behind you. On the other hand, it’s those cooler seasons, or times of year, when you want to turn every photon of light into sugar. For what it’s worth, I’m planning to pull leaves for maximum photosynthesis through the hot summer, then open up the clusters to sunlight in late summer/early fall.

The flip side of maximizing photosynthesis is making the best use of the resulting sugar. I’ll talk about doing that with cluster thinning in my next canopy management article.

There are several species of grapes, but the great classic wine grapes are all Vitis vinifera. Maybe that’s because Norton isn’t well known outside the eastern and mid western US. It’s parentage isn’t known exactly, but it’s predominantly Vitis aestivalis with hints of Vitis labrusca and, despite being known as an all-American grape, Vitis vinifera. Growers treat it and Cynthiana as different varietals, though they are genetically indistinguishable, and call it “the Cabernet of the Ozarks.” When I hear something like that, it’s usually a less-worthy grape trying to piggyback on the stature of a noble grape. Not this time. Here’s what Jancis Robinson says about Norton in the The Oxford Companion to Wine, 3rd Edition, “Norton is undoubtedly underrated because of the entrenched bias against non-vinifera varieties … The grapes are acidic, but the wine is indistinguishable by taste from wine made from vinifera grapes.”

It gets better. Norton is tolerant of many fungal diseases, including the common ones in my neck of the woods, and phylloxera. Sounds like a grape I’d like to grow! Well there are at least two drawbacks. First, it’s difficult to propagate from hardwood cuttings. Ok, that would be a nuisance, but it is being grown and propagated, so I don’t think that’s a deal breaker. The deal breaker is that it requires a long growing season. There’s a lot to like about the Puget Sound, but we are limited in the wine grapes that will ripen here.