Thinning grape vines, reconsidered

Why thin?

Thinning presumably enhances the long-term health of the vine. However, I heard from a friend that the productive lifespan of St. Vincent vines is 7-10 years. Our vines (early summer, above) have been up since 2006, so what's the point of losing yield at this stage? Why not just go for it? Thinning would be like a couple in their 70's postponing a cruise until a better time later in life.

And then there's this, from Growing Grapes in Missouri, page 17, by Missouri State University-Mountain Grove:

St. Vincent is considered to be a chance hybrid of a French-American hybrid cultivar with an unknown parent. It is a red grape for wine with large berry size and small, loose clusters. It has high vigor and moderate to high degree of winter hardiness. The fruit matures late season. 

It does not require cluster thinning. 

Yield is high. The vine trains well to a cordon system with spur pruning. A good spray program is needed to control diseases. Loose clusters make it not susceptible to bunch rot. Wine quality is good. It is typically made into a dry, red wine, or used in blending.

Urban vineyard: Record harvest

This summer was a weird but prosperous time for the urban vineyard. We had a record harvest of 25 pounds (above), which will make 8-11 bottles, according to a range of estimates culled from the Internet.

First we got a lot of rain, then we got a little black rot:

I have never been a dedicated sprayer, so I blame myself for that. The arrival of the fungus came at the same general time when I thin the vines. I'm a reluctant thinner, and I usually wait much later than the recommended point in the season for doing so. This year, I confined my thinning to the diseased and destroyed clusters and hoped that the rot would not wipe out the whole crop.

Weirdly, black rot subsided, and the vines prospered:

In addition to the mercy of black rot, I see several reasons for the bounty:

1) Three of the five laggard vines began to yield in modest quantity this year, whereas they could not be counted to produce much of anything in past years.

2) During the weeks when we covered the vines with garden cover (known as the Shroud of St. Vincent, below), the weather was free of storms that could blow down the shroud. Because it remained intact, we had essentially no loss to birds. 

3) This year's thinning was less than in the past. Is thinning really necessary? For St. Vincent vines? See my subsequent post if you're interested. If not, click elsewhere. Cheers!