Vineyard Site Selection
Temperature-based Metrics for Assessing Sites
There are a number of temperature-based metrics available for assessing site suitability for grape production. The following have been developed in order to best qualify and quantify the assessment:
Growing Degree Days
Grapes need adequate amount of warmth to grow properly and to ripen the crop. Grape varieties vary in the amount of warmth needed to ripen adequately. Winkler and Amerine, working at UC Davis, California, developed a simplified index to measure “heat summation” in an attempt to assess grape growing sites for their quality. The heat summation index, known as Growing Degree Days (GDD), is defined as the daily average temperature (the sum of daily maximum and minimum temperatures divided by two) of a site on a daily basis during the seven month growing season (April to October in the Northern Hemisphere, October to April in the Southern Hemisphere) minus a threshold of 50 degrees F (10°C) needed for vine growth and time.
Latitude Temperature Index
Although the GDD rating system is still recognized in California and other parts of the world, there are other regions where it is believed to have shortcomings, especially in cooler, less continental regions such as New Zealand. David Jackson and Danny Schuster developed the Latitude Temperature Index (LTI) that factored in latitude of the region and, therefore, the length of the day making it more useful in more marginal and more maritime regions.
Average Growing Season Temperature
Average growing season temperature provides a refined single index for identifying grape growing regions of cool and very cool climates. It is calculated by taking the average temperature for each month of the seven month growing season (October to April in the Southern Hemisphere, and April to September in the Northern Hemisphere), divided by seven. Jones (2006) determined “Grapevine Climate/Maturity Groupings” for some of the most widely cultivated varietals, using climate data from several different wine producing regions (See Figure 4.1).
The length of the frost-free season is important to the onset of bud break, flowering, and the timing of harvest. Decreasing fall temperatures reduce the capacity of the vine to synthesize sugar and ripen grapes. Ultimately, frost will kill leaves that have not naturally senesced, preventing further sugar accumulation in fruit and perennial portions of the vine.
Spring Frost Index
One method of evaluating a site’s risk of frost is the spring frost index (SFI) by Gladstones (2000), which is based on the range between a site’s average mean and average minimum temperatures for a given month.
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