Chapter 4

Vineyard Site Selection

Components of Climate

Many climate components are important to grapevines and grape production. Temperature, solar radiation, rainfall, humidity, wind, and evaporation tend to all play a role in defining daily weather and the climate. However, the most essential climatic component influencing viticulture is temperature. Temperature influences many different processes involved in grapevine development. It also influences the levels of certain elements within the berry, for example sugar and acidity. Understanding such relationships is important as variations and/or changes in climate/weather can greatly impact the ability to produce a given crop. Most of these components vary significantly in different growing seasons, so a thorough site evaluation could require many years of consistent data gathering.

Temperature

The overall optimum temperature grapevine growth is 77 and 90 degrees F (25 to 32°C). Any temperature below this optimum range causes vegetative growth to become limited. Temperatures above the optimum range reduce the grapevines photosynthesis rate due to the increase in respiration. Fruitfulness tends to be improved by high temperatures during early bud development in late spring and reduced by exposure to cooler temperatures near flowering. Budburst is sensitive to fluctuations in air temperature.

Diurnal Temperature Variation

Diurnal temperature variation is the difference between the highest temperature of the day and the lowest temperature of the night.

Maximum Summer Temperatures

Warmer temperatures in general tend to produce more consistent grape harvests. The heat also hastens ripening, producing grapes with bolder flavors, more sugar, and wine with more alcohol and short on taste and aroma.

Minimum Winter Temperatures

Cold hardiness is a major limitation to growing the highest quality European grape varieties in some areas. Mid-winter low temperatures coupled with wildly fluctuating temperatures in the fall and spring can cause significant injury to the grapevines. Injury may include death of overwintering buds, injury to the vascular tissues of canes, cordons, and trunks, or even complete vine kill. The nature and extent of winter injury is not entirely predictable for any given variety-site-weather combination.

Frost Potential

Depending upon the time of year, grapevines may be injured by either spring or fall frosts. Cold injury in the spring may damage or kill developing buds leading to a significant loss of crop for that season. Spring frosts generally do not kill vines because secondary buds will subsequently break and their shoots will provide sufficient foliage to support the vine; however, it certainly can hamper a vintage, upset uniformity, complicate canopy management, reduce the crop, and exacerbate winter injury problems.

Solar Radiation

High light intensity favors production throughout the entire season; however, it appears most beneficial during spring and the period leading up to and at véraison. Both light intensity and temperature are involved in the formation of fruitful buds. The number of bunch primordia increase with higher light intensity and the fruitfulness of new buds depends on the daily duration of high light intensity falling on the bud itself rather than on whole plant. In general higher levels of radiation, either intensity or duration, result in increases yield and/or sugar content (Jackson et al., 1993).

Rainfall

Long-term rainfall records for the area will provide insight to the selection of varieties and future management requirements. Lack of rainfall can be a severe influence on grape productivity in the absence of good quality irrigation water. Johnson and Robinson (2001) recommend a minimum level of rainfall/irrigation of 20 inches (500 mm), higher if the growing season is characterized by high evapotranspiration rates. Excess rainfall is also a problem and most quality wines are produced in regions where annual rainfall does not exceed 28 to 32 inches (700 to 800 mm) (Jackson et al., 1987).

Humidity

Humidity and relative saturation deficit may play a major role in fruit quality. Very high and very low humidity can adversely influence grape development.

Wind

Light breezes maintain air circulation around the berries stopping the buildup of humidity and maintaining an even temperature within the canopy. However, high winds can cause serious damage to grapevines, especially to vines in the spring and early summer, when shoots are tender and more easily broken.

Evaporation

Evaporation is based on various climatic factors such as temperature, day length, wind, vapor pressure and solar radiation which affects plant growth.

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