The extended period of rainy weather this spring has been lost on no one who has bothered to poke their head outside. The effect of this on local grape vines has been to significantly delay, by 3-4 weeks, the normal occurrence of bud break, that point in time when the new green shoots of a grapevine finally emerge (burst or break) from the buds on the cane. With this late start, what’s the effect going to be on the eventual wine grape crop?
This depends on a number of factors that will develop between now and harvest, the most important of which is the accumulation of something called degree days or growing degree days (GDD). It is widely assumed that grape plants are not actively growing unless temperatures reach at least 50 degrees Fahrenheit. A number of horticulturalists have posited variations of the theory that wine grapes must accumulate a certain amount of heat in order to ripen. They measure the accumulated heat by taking the daily mean temperature (the maximum for the day plus the minimum divided by 2) and subtracting 50. This gives one the number of growing degrees the plant has accumulated each day, which when multiplied by the number of days, yields the result, the GDD. Wine grape plants need somewhere around 1,500-1,700 GDD to ripen. So the main factor this year will be whether we can accumulate enough degree days before the weather turns cold again in the fall.
But like almost everything related to grape growing, it is just not that simple. Other research has shown that grape plants stop growing when temperatures get too high. They respire, which is more or less the opposite of photosynthesis. This inhibits the normal ripening process. As a result, if we have a number of days where temperatures are extremely high, above 90 or so, then that heat is not really making up for the slow start in the spring. Sunlight is another factor. There are a number of areas which are warm enough to produce quality wine grapes, for example east coastal Australia, but which do not because of cloud cover. Thus although there are an adequate number of degree days during the growing season, the photosynthesis occurring in the plant is affected and that results in lesser quality fruit. Naturally, rainfall also plays its part as common sense suggests that no plant does well unless it receives the right amount of moisture.
So, with all those factors still unknown for this growing season, it becomes less of an existential practice and more speculative to forecast the eventual merits of the vintage. But what one is left with is a better appreciation of why the growing of grapes is so complex and, as a result, so intriguing in its variety.
