Inside Knights Bridge


Degree Days

Degree Days

Environmental conditions greatly affect the grapevine and its fruit. Temperature is particularly important as it determines when the vine goes through its cycles, from bud break through veraison and harvest. Different varieties require different amounts of heat to grow more mature fruit, which is why certain varieties perform better in specific regions. The cumulative amount of heat available in the growing season is expressed as 'Degree Days'...

Degree Days

Environmental conditions greatly affect the grapevine and its fruit. Temperature is particularly important as it determines when the vine goes through its cycles, from bud break through veraison and harvest. Different varieties require different amounts of heat to grow more mature fruit, which is why certain varieties perform better in specific regions. The cumulative amount of heat available in the growing season is expressed as 'Degree Days'.

Degree Days are calculated by adding, for each day between April 1 and October 31, the number of degrees by which the average temperature is greater than 50°F. For example, a day that had a high of 90°F and a low of 56°F has an average temperature of 73°F which translates into 23 Degree Days. Fifty degrees Fahrenheit is considered the lower threshold because below this temperature the vine becomes physiologically inactive.  Although at temperatures higher than 98°F the vine shuts down and becomes inactive; Degree Days still are accumulated.  All varieties need at least 1,700 Degree Days to produce mature fruit. 

In the 1944, Professors A. J. Winkler and Maynard Amerine from UC Davis classified California’s growing regions into five areas according to the average Degree Days.  According to Winkler and Amerine, Region I (up to and including 2,500 Degree Days) is the coolest and is similar to regions like Champagne, Carneros and the Russian River Valley.  Optimal varieties for Region I include Chardonnay, Sauvignon Blanc, Riesling and Pinot Noir. Region II (from 2,501 to 3,000 Degree Days) is similar to Bordeaux, Chalk Hill in Sonoma County, and some areas of Napa.  Varieties include those in Region I as well as Merlot.  Region III (from 3,001 to 3,500 Degree Days) is equivalent to the Rhone and Alexander Valley.  Suggested varieties include Viognier, Sauvignon Blanc, Syrah, Grenache, Carignan and Cabernet Sauvignon.  Region IV (3,501 to 4,000 Degree Days) is similar to southern Spain, Fresno and Sacramento. Barbera, Emerald Riesling and Ruby Cabernet thrive in this region.  Region V (more than 4,000 degree days), the hottest region, most closely resembles North Africa, Madera, and San Diego.  Recommended varieties are Tinta Maderia and Verdelho.  Many growers in Regions IV and V plant other varieties, but with the intense heat crops often suffer from quick ripening, sun damage, and decreased varietal character. 

Knights Valley, the warmest Viticulture Area in Sonoma County, generally falls within Region III, having a historical average of 3,270 Degree Days.  Thus, Viognier, Sauvignon Blanc, Cabernet Sauvignon, Syrah, Grenache, Carignan and the Bordeaux varieties are best grown in this area.  To determine the exact Degree Days of the Knights Bridge property, we are tracking the daily temperature from the two weather stations.  The lower weather station is located in Chardonnay-Block 2, while the upper weather station is located in one of the hillside Cabernet Sauvignon blocks, Block 6.  For the 2007 season there was nearly a 700 Degree Day difference between the two stations (2645 for Block 2 vs. 3336 for block 6).  One possible explanation for this difference is that Block 2 is roughly an average 440 ft above sea level, whereas Block 6 is roughly and average of 550 ft above sea level – a 110 foot difference. Yet even with this relatively small change in elevation, these two areas have distinct micro-climates.  In fact, based on our calculated Degree Days for 2007, Block 2 would more aptly be placed in Region II.  Other potential differentiating features include aspect, soil types, and that the vines on the upper hillsides receive extended hours of intense afternoon sun while Block 2 has more afternoon shade.  Annual tracking of Degree Days will reveal if these 2007 data are indicative for the property and will help us refine our understanding of what constitutes optimal days for future harvests.

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Degree Days