As we walked through the Levensohn Vineyard last week with our viticulturist, Zach Berkowitz, and our vineyard manager, Stan Zervas, of Silverado Farming, Zach cut into a green berry from one of our vines. He explained that we are now 50% of the way through the growing season for the 2014 vintage because the seeds in the grapes are now turning crunchy and hardening.
We expect harvest to occur three weeks earlier than our 2013 crop, which was picked on October 3. Since we replanted the vineyard in 2000, this may be the earliest Levensohn harvest yet (our latest was just before Halloween in 2005). A key factor impacting the rate of grape maturity is the air temperature during the growing season. In the wine industry, the number of Growing Degree Days (GDD) measures this.
According to Washington State University, “The progression of in-season grapevine development is strongly influenced by air temperature. As such, average heat accumulation is often used to compare regions and vine growing condition. This average heat accumulation is often refereed to as Growing Degree Days (GDD). The summation of daily GDD units can be used for a variety of things: comparing one region to another, comparing one season to another, and predicting important stages in vine development (bloom, veraison, and maturity). … GDD are calculated by subtracting 50 from the average daily temperature (°F). If the resulting value is less than 0, then it is set to 0. Thus, daily GDD units are always positive.”
Silverado Farming recently published the following report on GDD for 2014:
Clusters in early season varieties are starting to grow tight. This continues to be an early season, as bloom and fruit development are earlier than usual. Growing degree days are still a little behind last year, but we are catching up. A month ago, 2013 had the most GDD over the last 21 years. Today, 1997 is the leader. The coolest season was 2003, but now 1998 takes that category. We are at 972 GDD, which is 17 GDD, or about one day, behind 2013.
We are eight days ahead of the average, meaning 972 GDD was achieved 21 June in the average year. Finally, 2014 is has the fourth highest GDD accumulation, trailing only 2013, 1997, and 1996. The chart below shows accumulated GDD for four key years since 1994. These include this year, the year with the most accumulated GDD (1997), the year with the least accumulated GDD (1998), and the 21-year average. 2014 is clearly warmer than the average, but about six days behind 1997. The NWS Climate Prediction Center continues to show high probability for a hotter than normal summer, so above average GDD may well continue this season.
It’s possible, but unlikely in my view, that 2014 may have lower GDD than 2013. But we also have to consider the exceptionally mild winter we had and the unprecedented severity of the California drought in any assessment of how things are changing in the Napa Valley for grape growers and winemakers.We do expect an excellent harvest again this year, with high fruit quality for cabernet sauvignon. All signs are encouraging at this mid-point in the growing season. But it would not be surprising to see 2014 overtake 2013 as having the most GDD since 1994 and continuing the trend of well above average GDD. We don’t see this trend reversing.