Napa Valley Grapegrowers look Beyond Hang Time
Posted by Paul Franson on March 12, 2007
Jim Verhey, moderator of the seminar and president of Silverado Premium Properties, set the tone for the other speakers: "According to long-term trends, we should be entering a glacial (cooling) period, but human activity is changing the climate and making it warmer."
Speakers from UC Davis agreed that global warming is occurring. Dr. Rick Snyder from Davis said flatly, "The evidence is strong that global warming is occurring."
But he and Dr. Deborah Elliott-Fisk also suggested that the impact on Napa Valley may not be catastrophic as the popular media suggest.
To start with, Dr. Elliott-Fisk noted that the soils and subsurface environment of the vines won't change in the next century. "Chemicals can only affect the first few inches, which don't affect the vines much, and even deep ripping can only change a few feet." She adds, "The soils in Napa Valley are 100,000 years old." Climate isn't likely to change them soon.
She also says that global warming may not have a huge impact on Napa. She thinks the degree-days of the Valley floor may shift slightly south, making more of the valley warmer, but the mountain tops may not change much due to the moderating effect of San Pablo Bay (northern San Francisco Bay). In fact, the area may get foggier than it is now, which would reduce the impact.
On the other hand, lower snow pack is forecast, and that, along with changing rainfall patterns, might make irrigation more critical.
She suggests that California overall might be about 3 degrees Celsius (5 degrees Fahrenheit) by 2055 to 2075, and there might be 15 more days with temperatures over 90 degrees Fahrenheit.
She says growers can likely adapt to the changes with the proper viticultural practices.
In fact, the weather in Napa Valley has been improving in some regards for at least 90 years. Dr. Snyder has analyzed records since 1917 from Napa and finds that January maximum temperatures haven't changed much in 90 years, but the minimums have risen 5 degrees on average. Likewise, September's maximums are up 6 degrees.
He feels similar changes should not be a big problem. "It's the extremes that hurt you, not averages," and Napa has actually had fewer extremes in the last few decades than in the past.
Likewise, flooding has declined from the past.
Nevertheless, he says the climate models predict an acceleration of changes including warming, partly as it encourages the evaporation of water vapor from the oceans, a potent component of the atmospheric layer that retains heat.
Snyder suggests that proper irrigation of the vines might be a key component of maintaining quality, as it fights damage from heat spikes.
The remainder of the seminar largely dealt the related issue of "hang time."
Growers have complained that wineries have been demanding that grapes stay on the vines to ensure fully ripe flavors, and that results in lower revenue to them - and not to better wine, but alcoholic "fruit bombs."
The evidence pretty much supports them. A study by Napa ag extension agent Ed Weber found average Brix at picking rose from 22.5 in 1976 and 25.5 now.
He also conducted a study that determined that a 7 to 21 percent lower yield results from 4 weeks hang time past 26 degrees Brix. And any increase in Brix is due to dehydration, not continued maturation. "Sugar accumulates to 23 to 25. Above that. It's dehydration." And there's a 5 percent average drop in yield for each degree over 26.
Viticulturist Pete Richmond notes that grapes need 120 to 135 days past bloom, and that's the end of their development. "The game is over by Nov. 1," says consulting winemaker Larry Brooks, who also noted that tests at Enologix should that desirable tannins and anthocyanins don't necessarily continue to increase significantly with higher Brix, but can actually decrease.
Many winemakers insist on long hang time and no late irrigation thinking they will get better quality, but all they're doing is dehydrating the grapes. "Many wineries harvest at 23 to 24 Brix and make exceptional wines," notes Ed Weber. Others remove alcohol or add water to restore a balance that could be better maintained with moderate irrigation.
So it appears that the same things growers can do now to maintain quality - and yield - are the things they should do to prepare for global warming.
Maintaining high quality and vine health, especially in light of global warming, may involve trellises that better shield the grapes, different rootstocks and other steps, but clearly irrigation is vital. Even now, a number of speakers warned that cutting off water early as is trendy can cause irreversible impact on this year's crop (and perhaps the future). "As water stress occurs, roots die or become less functional," cautions Steve Matthiasson of Premiere Viticultural Services.
There's clearly more at stake than growers' pocketbooks. Picking at high Brix doesn't necessarily result in better wine - just more alcohol and greater concentration - a result Europeans sometimes achieve by removing water from their musts artificially, just as we add it.