Cool Weather May Delay Grape Harvest in the North Coast

Lately, there has been discussion and worry about the cool temperatures being experienced in Northern California, particularly from grape growers in Sonoma and Russian River Valley.  At this time of year grapes should be well into veraison, the ripening stage of the development cycle where grapes begin to change color, sugars and berry size increase and acids begin to decline.  However, due to cooler weather, many vineyards are yet to see the onset of veraison or have only just begun to see changes.  It is well understood that cooler climates lead to improved color, higher acid and more varietal flavors all resulting in a higher quality wine.  However, the harvest window becomes much tighter for growers when determining the right time to pick as they wait for optimal sugars, acid levels and flavors to develop before the October rains arrive.

Jennifer Thomson, of Thomson Vineyards in Carneros, has this take on the 2010 season:

“We agree with what many are saying:

1. It will either be an epic season or the variables we can’t control will come into play and create the perfect storm and
2. Winemakers need to get comfortable with picking at lower Brix to get fruit in at the crush pad this year and miss the late October rains

We are recommending, and are fortunate to be working with several winemakers who have already requested, to pick at lower Brix in 2010.  This leads to less alcohol in the wine and getting back to the elegant and refined wines California became known for in the 80s and early 90s. Three winemakers we work with are on deck to pick at 23.5 brix: Bravium Wines, Kelham Winery and Black Sheep Finds.  The former two picked at 24+ last year. The latter at 23.1 brix (which resulted in a 13.5% alcohol in 2009 Hocus Pocus Thomson Vineyard Carneros Pinot Noir bottled yesterday). While all are happy with their respective wines, as growers, we award the 2009 Thomson Vineyards Gold Star to winemaker Peter Hunken of Black Sheep Finds for making the pick call three weeks prior to anyone else in 2009 leading to damn fine wine! We’re looking forward to picking early for him again this year!”

According to the Press Democrat, the average maximum temperature for July was 75.1 degrees F, more than 7 degrees below the 50-year average.  It was also the second lowest temperature average for July since 1960.  A quick look at the Growing Degree Days measurement for 2010 compared to 2009 shows the impact.  The degree day concept and calculation was developed by Dr. Winkler and Amerine in the 1930′s as a way to classify the different growing regions of California and is used as an indicator of ripening in a particular region.  Degree days is an accumulated value using the following daily calculation throughout the growing season (often from April 1 to October 31, but dependent upon the region):

(( high temp + low temp) / 2) – 50

Basically, it is the average temperature for the day subtracted by 50F, which is the point below which little to no photosynthesis occurs in the grapevine and therefore has no effect on ripening.  There are some shortcomings to this equation that I won’t go into here and it is still used as the standard.  WineBusiness.com has a page devoted to this type of information and can be found here.  Interestingly, the data they retrieved for Bennett Valley shows a much larger discrepency between this year and last year than my data below even though I retrieved it from the same source (http://ipm.ucdavis.edu – UC Davis Integrated Pest Management).  There could be some minor variations in the formula used accounting for the difference.  Instead, I decided to collect degree day data from another CIMIS (#103) station in Windsor, CA for the illustration below, which shows 2010 well below 2009 values throughout the entire season so far.

Using the WineBusiness.com site and cycling through previous vintages, I noticed that for Napa, 2010 is looking very similar to 1998 in terms of the degree days accumulation to-date.  In 1998, Napa had weather and disease pressure including late rains in May and June and unusually cool weather due to El Nino.  This caused delays in budbreak, early vine development and bloom.  However, the vintage “recovered” due to a warm and dry “Indian Summer” in October allowing the fruit to completely ripen with excellent flavor development.  In Sonoma, for the 2010 vintage to reach the same total degree days accumulated in 2009 (2673), the average daily temperature will need to be 64.8F between now and October 31st.

So, while the 2010 harvest is certainly delayed by a few weeks based on current information, there is still time for warmer weather to arrive and allow the fruit to mature as expected.  Furthermore, let’s hope the climate-induced slower ripening process combined with winemaker acceptance to possibly pick earlier than usuual results in maximum flavor development and quality.  Then again, I haven’t seen a negative vintage report yet so maybe another miracle is in order.  All I know is that it may extend my harvest internship much to John Holdredge’s dismay, but I will do my best to bring the heatwave with me from the South.