More on degree days and ripening
By Dave BuchananWith Colorado's grape harvest nearing its high point — most of the white-wine grapes already are in the fermenting tanks and some of the early merlot are starting to show up at wineries — questions about why some areas develop their grapes faster than others need more of an answer than merely "Oh, it's warmer over there." As viticulturist Horst Caspari of the CSU Orchard Mesa Research Center recently told this reporter, it takes more than simply a long series of hot days (which we have come to understand as summer) to bring a grape to desired ripeness. "We're a hot region but have a comparatively short growing season in relation to other grape-growing areas, Caspari said. "You find the same (grape) varietals behave differently under different circumstances." Caspari said some grapes can reach physiological ripeness, which isn't always the same as being ripe enough for a winemaker's pleasure, with a longer but cooler growing season. Colorado's high desert valley can get mighty hot but the growing season is short due to our late spring and early fall frosts. New Zealand, on the other hand, never gets this hot but it has a longer frost-free season, which gives cool-weather grapes such as savignon blanc and pinot noir plenty of time to ripen. "Here, once you get to spring, it gets warm and stays warm, while in New Zealand it takes longer to warm up and never gets quite so hot," said Caspari, who worked 11 seasons in New Zealand wineries. "What takes two months here to ripen takes three months in New Zealand." It's a matter of degree days, the amount by which the daily mean temperature exceeds 50 degrees during the growing season. It's used to compare climates and growing seasons and not all that complicated (well, almost not all that complicated). You can learn more about degree days and some of the studies done about how they affect grape growing here http://en.wikipedia.org/wiki/Climate_regions_of_California http://www.lavacap.com/degreedays.htm The Winkler degree-day system establishes climate regions for grape growing and Caspari said the Grand Valley might be a region 3.5-4 while Delta Country, which delights in growing cool-climate grapes, is a "low region 3." Sections of the Grand Valley (microclimates, if you will) have varying degree days, which is why some merlot is ready to harvest and other vineyards have another two to three weeks before maturity. That's also why some winemakers in isolated pockets on East Orchard Mesa can grow riesling, a nominally cool-weather region 3 white grape, while Ken and Kathryn Stubler of Red Fox Vineyards along the Gunnison River near Palisade have been remarkably successful with tempranillo, a Spanish varietal known for its love of hot, dry climates such as those in a region 5.