Vineyard Site Selection
The topography of a site plays important roles in grapevine growth and quality, and have interactive effects with climate elements. Topographic factors that exert the greatest influence on a site’s climate include elevation, slope, aspect, hill isolation, and how it affects air drainage and proximity to bodies of water.
Elevation influences the overall acceptability of a region for viticulture because of its influence on the minimum and maximum temperatures in a vineyard. Typically, lower elevations are preferable at high latitudes, and higher elevations are more desirable at lower latitudes. On average, the temperature falls 1.1 degrees F per 330 feet (0.61°C/100 m) of elevation, which means the growing season will be shorter increasing the possibility of frost at higher elevations.
In addition to the absolute elevation, the relative elevation of the site is an important consideration too. Poor relative elevation can significantly reduce the quality of an otherwise good site such as is the case with vineyards located within valleys. Although the vineyard may fall within the acceptable absolute elevation range, because of its location it may be prone to spring and fall frosts. Thus, in most mountain/valley complexes, it is usually better the vineyard is located at an elevation sometimes referred to as the “thermal belt.” The thermal belt refers to a mountainside zone where frost or freezing temperatures are less likely to occur than they are at either higher or lower elevations.
Grapevines are temperate-climate plants; the major viticultural regions of the world are concentrated between the latitudes of 30 and 50 degrees. In this zone the relatively large diurnal temperature range is optimal to produce the combination of sugar and acid levels that enable grapes to be made into quality wine.
The slope of a site refers to the degree of inclination of the land expressed as a percentage. For example, a 5 foot fall over a 100 foot horizontal distance would be a 5 percent slope. The ideal site for grapevines is on gently sloping land that will allow cold air to drain into adjacent lower areas thus reducing the risk of frost injury and cold winter temperatures. Good air drainage will also promote faster drying of foliage, which will reduce the duration and frequency of disease infection periods.
A vineyard’s aspect refers to the direction that the slope faces (e.g., east, southeast, etc.). Aspect affects the angle that the sunlight hits the vineyard and thus its total heat balance. Aspect is more important in higher latitudes where radiation is weaker, due to the angle of the sun, and light interception may be limiting to growth.
Vineyards with southern aspects (for the Northern Hemisphere) warm earlier in the spring and the vines may undergo bud break earlier than vineyards with northern aspects. The early bud break is desirable in locations that do not have a danger of spring frost because it translates into earlier bloom and harvest of the fruit.
Western-facing slopes are a popular choice for late-maturing varieties, such as Cabernet Sauvignon, promoting fruit ripening in the waning heat and daylight of fall.
Eastern-facing aspects receive the first of the morning radiation warming canopy and soil temperatures fastest when temperatures are generally at their lowest and most limiting.
In cool climates where summers are cool and growing degree days are low, northern slopes (for the Northern Hemisphere) should be avoided and southern aspects (S, SE, and SW) are preferred to allow maximum heat accumulation on that site to grow and ripen grapes.
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