Of the many environmental factors which affect the productivity and quality of grape vines and their fruit, arguably the single most important variable is temperature during the growing season. Photosynthesis in Vitis vinifera is temperature dependant, and this in turn affects vegetative and reproductive growth. Different cultivars of V. vinifera have differing temperature tolerances, which largely drives the differing assemblages of cultivars observed in different wine growing regions.
Several measures have been proposed which seek to summarise the temperature records over a growing season into a single numerical value. These include the Huglin Index, Biologically Effective Degree Days (BEDD) and Growing Degree Days (GDD). Of these GDD, defined as ((Tmax + Tmin) / 2) – 10°C), is probably the most widely used and has proven useful in helping to understand differences in wine growing conditions in different parts of the world.
To fully interpret GDD data for a particular wine growing region, it’s necessary to examine how these values change both in space and time. The Marlborough Research Centre maintains weather stations at Blenheim in the Wairau Valley and at Dashwood in the Awatere Valley. Temperature data from these two sites has been integrated with information from other weather stations by the New Zealand National Institute for Water and Atmospheric Research (NIWA). This has allowed them to produce a grid of temperature data, which has been used to produce the GDD map. This map shows the median GDD values based on data derived over a number of years.
The map shows the strong influence that topography has on growing season temperatures, with the elevated areas being markedly cooler. Although on average the Awatere Valley is slightly cooler than the Wairau Valley, there is significant variation within each of these two subregions.
Looking at annual GDD figures at particular weather station(s) brings out the growing season temperature variations associated with particular vintages. Notable are the unusually high GDD values for 1997/98 and the very low values for 2011/12. The former is probably the result of an El Niño Southern Oscillation (ENSO) event which caused New Zealand to experience an exceptionally hot summer. By contrast, 2011/12 was described by Wine Marlborough as having a summer which “never really arrived”.
For 2012/13, growing season temperatures were significantly higher than the previous year. Gapes harvested from Marlborough in 2013 totalled 251,680 tonnes, up from 188,649 tonnes in 2012. Already being called an outstanding vintage by some winemakers, WineStats will be attending the London New Release Tasting to see if 2013 exhibits high quality as well as quantity.