published in the 1998
Minnesota Grape Growers Association Annual Report, updated February 2002Please take a look at Peter's list of trial vine varieties for New Brunswick
It is important to be able to characterize climate with relationship to the ability to bring various grape varieties to maturity before fall frost. The ability to more accurately compare grape growing climates would help us avoid expensive trials with varieties that are completely inappropriate, even though they appear to have possibilities based on indexes that are presently in use.
The gold standard has been the measurement of heat accumulated during the growing season. The base for this measure is 50 degrees Fahrenheit or 10 degrees Centigrade because grapevines are not physiologically active when the mean or average daily temperature is below this level. Mean temperature for a particular month is calculated by adding the mean maximum temperature to the mean minimum temperature and dividing by 2. So if the mean temperature for a particular 30 day month was 68 F or 20 C (68 minus 32, times 5, divided by 9 = 20), then the heat accumulation for the month would be (68-50) x 30 = 540 F Degree Days or (20 - 10) x 30 = 300 C Degree Days. Below I have chosen various grape growing areas to illustrate why this measure is not very useful.
This listing from the "poorest" to the "best" ranks Hinckley, which is 70 miles north of Minneapolis, and Geisenheim higher than Christchurch. This is not realistic as Christchurch has a greater ripening ability and will mature Cabernet Sauvignon and Sauvignon Blanc, neither of which are successful at Hinckley or Geisenheim. Also to have the Bordeaux locations, which consistently ripen the Cabernets and Merlot well, ranked below Vineland is unrealistic.
The book, Production of Grapes and wine in Cool Climates by David Jackson and Danny Schuster (1994, Gypsum Press, ISBN 0-86476-064-7) suggests that the Degree Day measure is not adequate because it does not deal properly with the length of season. These authors show that when areas have the same amount of heat accumulation in Degree Days, the one with the longer season will have the greater ripening ability. They suggest that continental climates tend to have rapid heat accumulation over a short season as is the case for many areas in the American northern tier. Jackson and Schuster attempted to address season length by using latitude and the mean temperature of the warmest month (July in the north, January in the south). Their Latitude Temperature index (LTI) is the mean temperature of the warmest month (in degrees C) times 60 minus latitude. Fredericton at latitude 46 with a mean July temperature of 19.1 has an LTI of 19.1 x (60-46) = 267, while Hinckley at the same latitude with a mean July temperature of 20.2 has an LTI of 283.
Again ranking the locations from the "poorest" to the "best":
This ranking does not serve very short season areas well because latitude is a rather indirect indicator of season length. Fredericton and Christchurch have similar LTI but Christchurch ripens vinifera grapes that Fredericton can only dream about. Summerland and Vineland have similar ripening abilities but vastly different LTI. Hinckley, which is borderline in ability to ripen any of the common vinifera grapes, is near the "best" end of this ranking. Again to have Vineland ranked above the Bordeaux locations is not realistic.
On the supposition that Jackson and Schuster correctly identified season length as a factor of primary importance concerning the ability to ripen grapes, I tried an index that considers mean temperature of the warmest month (as does LTI) and the number of days in the growing season that are above 10 C (50 F). I named this trial measure the Time-Heat index. The graph presented here shows how the number of days with mean temperature above 10 C (50 F) can be approximated by plotting mean temperatures for each month. These mean temperature data are usually available for most locations with climate records.
This ranking is better in that Fredericton and Hinckley are at the poor end of the ripening spectrum. However the placement of Summerland near Geisenheim is troubling. Summerland ripens varieties such as Cabernet Sauvignon and Riesling much better than Christchurch does. To have Vineland, which often struggles to ripen the Cabernets, ranked better than Bordeaux is unacceptable.
Jackson and Schuster have also made reference to the fact that areas with otherwise similar climates can be differentiated by the amount of rainfall, although they did not attempt to incorporate this parameter into their index. Rainfall is important because areas with more precipitation have less full sunlight (more days overcast). Diffuse light coming through cloud cover is much less effective for plant growth and sugar production than is full sun. Compared to other areas Summerland is semi desert. I tried modifying the Time-Heat Index by subtracting annual precipitation (one inch = 25.4 mm) and I called the resulting measure the Time-Temperature-Precipitation Index (TTP Index) which ranks the seven locations as follows:
This Time-Temperature-Precipitation (TTP) index appears to place all six areas in what I believe is the proper order. It should allow us to choose grape varieties, for new growing areas, that have ripened elsewhere with similar grape ripening ability based on this more definitive climate descriptor. For instance it would not be prudent to attempt to grow varieties in Fredericton that are considered to be late season ripeners at Hinckley, while those considered early or mid season ripeners at Hinckley might be worth a try. Similarly varieties considered very early ripeners at Summerland and Vineland (whites: Pearl of Csaba, Siegerrebe, Zefir, Schonburger and Ortega; reds: Agria, Fruehburgunder, Dornfelder and Castel 19637) might be considered for trial planting at Hinckley and Fredericton. However, the proximity of the Bordeaux locations to Vineland, which ripens vinifera less consistently well, is disturbing.
We have had new growers who have seriously considered atempting to grow the hybrid VIDAL here in New Brunswick because it produces excellent ice wine in Ontario. Vidal is one of the latest ripeners in Ontario and it has no chance of ripening here. The use of the Time-Heat-Precipitation (TTP) Index may help to avoid wasting efforts on such chancy endeavours.
Peter Salonius