temperature at which development slows down or stops. In addition to the genetic factors that affect high temperature tolerance, I expect the level of root development plays a role. When evapotranspiration exceeds the water gathering ability of the plant, then stomata close and conserve water. When stomata close, there is no longer oxygen and carbon dioxide exchange, thus photosynthesis and vine development is inhibited. The bonus of high temperature is


that it is lethal to mildew spores. If misting or overhead sprinklers are used to drop the vineyard temperature, care must be taken to keep the temperature high enough to remain inhibitory to mildew. Perhaps more important than achieving high sugar levels is to retain a moderately low pH level. The major contributor to high pH is a dense canopy. Shading of leaves and fruit are associated with elevated potassium levels in fruit, which results in high pH. The other visible target for fruit maturity is to have the canes hardening off and cessation of cane elongation. These stages are associated with the development of the secondary metabolites we associate with varietal character. Secondary metabolites are developed after the vine stops growing and is preparing for dormancy. We identify them as fruity character, soft tannins in red wines, factors affecting good body, and good colour. Each stage of vine development has a


role in creating wine flavour. If the weather departs substantially from the norm, it may be difficult to bring vine performance back to an appropriate composition. Examine the figure which accompanies this article. Taken from these curves, the average for Summerland is slightly over 1,300 growing degree days, but this only tells part of the story. The steepness of the curve indicates the daily heat accumulation. Note that in some years the curve remains flat (a cool spring) until about day 130, almost mid-April. In other years the curve remains flat from about day 260, the beginning of September, a cool fall. Moderate fall temperatures in the upper teens assist with the development of fruity character, so target your harvest to the time in which that temperature is most likely to occur. No one said it was going to be easy. — Gary Strachan can be reached at gestrachan@alum.mit.edu .


Does climate change affect your winemaking?


By Gary Strachan


vintages. It makes great conversation, but is a headache when grapes arrive with a composition beyond acceptable standards. Immature grapes produce thin,


A


acidic, characterless and/or vegetal wines. Hot-season wines can be alcoholic, high pH, low TA, jammy, and have a short bottle life. It’s time for a little winemaking magic. I dislike correcting wine faults post-fermentation, so adjustment to the extraction and fermentation strategy is by far the best policy. The first on the list is to adjust


pH. The addition of citric, malic or tartaric acid will drop the pH by about 0.2 pH units for each g/L addition, but only tartaric acid will do this without raising the T.A. by 1 g/L. The potassium bitartrate byproduct is sparingly soluble in wine and will precipitate, leaving the T.A. unaffected while lowering pH. To raise pH of


joy of cool climate winemaking is the difference between


immature grapes, use potassium carbonate. This will also lower T.A., lower tartaric acid, and make the wine easier to balance with a subsequent malolactic fermentation. Over-ripe grapes benefit from


gentle extraction, perhaps whole cluster pressing for whites. A more aggressive extraction is required for under ripe whites. This could include pectinase


and cold maceration pre- pressing. For reds, pectinase and cold maceration pre fermentation can be supplemented by addition of complementary skins to the fermentation to assist co-pigmentation. When berry colour is low,


some winemakers withdrew a portion of the juice pre- fermentation to create a rose. If this done, it’s a good idea to


press the wine earlier rather than use an extended post fermentation maceration. Don’t forget that wine flavours


are created in the vineyard and extracted in the winery. The rest is only details.


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British Columbia FRUIT GROWER • Spring 2012


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