Video: cool weather, grape veraison and thinning fruit before harvest

by on August 24, 2010

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In last week’s video on leaf pulling, we mentioned that summer temperatures in Northern Sonoma County have been 10-20 degrees below normal. Veraison, when the green grapes change color and increase their sugar levels, started the first week in August. As a rule of thumb, harvest begins roughly six weeks after 100 percent veraison.

What does this mean in terms of the 2010 vintage? If you read some of the wine industry discussions, you probably think this year’s weather has been a big thorn in the side of every winegrower.


There is a lot of confusing information out there about cool weather and its influence on harvest. It’s important to keep in mind the following: First, heat does not ripen fruit. Solar radiation is what makes photosynthesis occur in grapes. (Grapevines convert carbon dioxide into sugars using the energy from sunlight during photosynthesis.) Also, photosynthesis slows around 90 degrees and shuts down after 95, protecting the plant from loss of water. Lastly, moderate temperatures in a growing season define classical vintages. This type of weather yields grapes with color, tannin and fruit concentration that are fully mature without excessive sugar levels.

But our current cool summer has led to a lack of early morning solar radiation, and the cool evening temperatures have slowed the relocation of saccharides (carbohydrates or sugars) in the plants that develop during photosynthesis. If this cooling trend continues post-veraison, our harvest dates will be later than recent vintages. It is, however, quite common for Alexander Valley to be bathed in fog up to 10 a.m., and those same weather patterns existed in our region 30 years ago. With harvest anticipated to begin 2-3 weeks later than last year, we need more moderately warm days without cold nights so the grapes continue their ripening under these almost ideal conditions.

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Veraison thinning (discussed in the above iPhone 4 video) is a critical practice every year for us. Two weeks ago, we began removing grape clusters at veraison on 100 percent of our estate vineyard blocks, dropping many clusters to the ground. (And our resident birds, rabbits and turkeys can’t enjoy them because they are still too acidic!) Though we had ideal spring weather conditions in 2010 — rapid bloom and very even fruit set — we still need to make sure every year that the vines are focused on the ripening the best grapes — and the right quantity of grapes. If there are too many grape clusters on a vine, the vine won’t be able to focus on ripening them fully. Cool temperatures are giving the grapes ample time to develop flavors, so 2010 could be a classic vintage.

If the fruit continues to ripen slowly and consistently, we can be picking grapes at a lower Brix, or sugar levels, rather than needing to drive the sugars higher to get flavor. Winemaker Rob Davis and I are very excited about this. The bottom line is that although this growing season is one of our cooler years (mildew pressures have been high all season), the possibility for producing a great wine this year is very high. Our first harvest in 1976 didn’t start until October 13; in 1977, we started picking on October 5, and in 1978, we started the last week of Sept. Unlike 2010, those were all draught years. And classics.

  • Medical Jobs

    My cousin recommended this blog and she was totally right keep up the fantastic work!

  • Lisa

    Glad you enjoyed our video. Cheers.

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