Wines & Vines

110 WINES&VINES Collector's Edition GRAPEGROWING hourly readings at or near the daily high tem- peratures for several CIMIS weather stations in Northern California over a one-week period in July 2016. Cumulative high temperatures were maintained for a shorter period at Santa Rosa and Pleasanton, two areas known for variable summer marine intrusions, than at Point San Pedro, Brentwood and Modesto. Point San Pedro, at the immediate coast, has greater consistency in marine stratus layers than Santa Rosa and Pleasanton, which experi- ence fluctuations in coastal fog. Brentwood and Modesto, both inland from the coast, are known to be relatively free of coastal fog. Many areas transitioning from coastal to in- land may show GDD numbers similarly influ- enced by brief durations of high temperatures. Comparatively, similar GDD summations for areas at higher latitudes or greater elevations may show greater total hours near high tem- peratures or longer periods of solar radiation. Using average air temperature rather than daily mean temperature may create more ac- curate comparative numbers, assuming that the methodology is consistent across the areas being assessed. Variations in growing season A final consideration is the April 1 to Oct. 31 growing season used in GDD calculations. In areas of the West Coast, bud break typically occurs in March, sometimes as early as Febru- ary, while harvest dates in August and Septem- ber are more common than in October. Grape variety and microclimatic conditions in indi- vidual vineyards also play a role. A review of 50% bud break from 2013 to 2018 and harvest dates from 2013 to 2017 was conducted in seven vineyards in the Livermore Valley AVA, three planted to Chardonnay, three planted to Cabernet Sauvignon and one planted to Sauvignon Blanc. The average 50% bud break ranged from March 28 to April 5, depending on vineyard, for the three Cabernet Sauvignon vineyards, but the Chardonnay vineyards had average dates of March 14 to March 15. The Sauvignon Blanc vineyard ex- perienced 50% bud break around March 26. Harvests for Cabernet Sauvignon approxi- mated Oct. 8 to Oct. 21, depending on the vineyard. Chardonnay saw average harvest dates of Sept. 20 to Oct. 7. The vineyard with Sauvignon Blanc saw average harvest dates around Aug. 28. In many cases, bud break began prior to the April 1 growing season start, while harvest was most often completed prior to the Oct. 31 growing season end. The rationale for calculating heat summa- tions from April 1 through Oct. 31 are not clear, at least not in Amerine and Winkler's best- known works on the subject. The 50° F thresh- old is based on shoot growth, so April 1 may represent an approximate estimation of bud break, but harvest for many varieties occurred, even in the first half of the 20th century, prior to Oct. 31. Amerine and Winkler also calcu- lated heat summations from bloom to harvest and noted the role of heat summations in the 30 days preceding harvest, therefore not limit- ing their assessment to just April through Oc- tober. Heat summations based on bloom and harvest are only possible when grape produc- tion already exists within an area and would be difficult to use as a tool to match varieties to an area yet to see vines. Differential timing and lengths of growing seasons across North America further complicate the equation. Is there a better index? Other indexes also exist for assessing tempera- ture. The Heliothermal Index, also known as the Huglin Index (HI), was introduced by Pierre Huglin in 1978. HI is the April 1 to Sept. 30 sum of the mean of the daily mean temper- ate above 10° C and the high temperature above 10° C, multiplied by a coefficient indica- tive of the latitude. Other indices include Aver- age Growing Season Temperatures (GST) and Biologically Effective Degree Days (BEDD), along with several other formulas for calculat- ing heat summations. LIQUID NITROGEN DOSING regardless of your closure preference Minimize dissolved oxygen Extend shelf life Purge O 2 from empty bottles Purge O 2 from headspace Since 1958 4 Barten Lane, Woburn, MA 01801 T 781-933-3570 F 781-932-9428 sales@vacuumbarrier.com vacuumbarrier.com

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