A general knowledge of grape growing is valuable to those who would like to understand wine. Considering the great number of grape varieties and the many types of wine, it is fortunate for the student of wines that it is not possible to grow every variety of grapes and produce every type of wine efficiently in every area. Certain guiding principles govern the choice. Weather is one of the most important factors.
Much research has been done in “climatology” (that branch of science which deals with climate and climatic conditions) as it applies to grape growing. An important concept to grasp here is something called “degree days”.
Grapes do not ripen below the temperature of 50 degrees Fahrenheit. So, if on a given day the temperature averages 65 F., that day is considered to have a 15 degree “credit” (65 – 50 = 15) towards the ripening process. If in a particular district the average temperature is 65 F. in June, the district earns 450 “degree days” for the month (15 x 30 = 450). Tallying up all “credits” throughout the four month growing season results in a “heat summation” (ranging from 1,700 to 5,200) for each locale evaluated.
There are five climatal zones which support vitculture:
Region I…62 – 65 F.average
1,700 – 2,500
Region II…65-68 F.average
2,501 – 3000
Region III…68 -70 F.average
3,001 – 3,500
Region IV…70 -74 F.average
3,501 – 4,000
Region V…74 -80 F.average
4,001 – 5,200
Southern Napa Valley and Santa Rosa as well as Europe’s Rhine and Champagne districts typify cool Region I. Middle Napa Valley, Sonoma Valley and Bordeaux are classified Region II. Northern Napa Valley and the Italian Piedmont are Region III. Davis, Lodi, Sicily and Central Spain are examples of Region IV. Fresno, Bakersfield, Algeria, and hot areas in Australia are rated Region V.
In Region I, due to low average temperatures and short growing seasons, only early ripening varieties can mature. In these areas obtaining sufficient sugar in the grapes can be a problem. Acidity in the juice tends to be high. Chardonnay and Pinot Noir do best in cool Regions I and II. Sauvignon Blanc and Cabernet Sauvignon fare better in Region II or III. At the other end of the spectrum, in warm Region V vineyards, grapes have a high sugar content with low acidity. These regions are ideal for dessert wine grapes.
Microclimates are also a factor. These are variations within a climate region that have their own weather characteristics. Establishing vineyards with maximum sun exposure or in valleys protected from the wind would be examples of microclimate conditions.