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News Headline July 2, 2010

Napa Grapegrowers Get Water Tips
Less irrigation, more monitoring could benefit vines and wine quality

by Paul Franson

Mary Maher

Mary Maher
Yountville, Calif. -- Although a wet winter has reduced the immediacy of saving water, for the long haul California growers can look forward to increasing restrictions on water usage and periodic shortages. To help growers deal with these issues, the Napa Valley Grapegrowers focused on irrigation at the latest of its sustainability workshops held June 29 at the new Community Center in Yountville. Steve Moulds of Moulds Family Vineyard, a director of NVG, set the stage for the seminar.

Five experts offered their views. Interestingly, one clear message was that many of today’s vineyard irrigation practices waste water -- sometimes to the detriment of grape yields and quality. “Growers typically overwater,” said Dr. Thibaut Scholasch of Fruition Sciences Inc. “We could irrigate much less, but we need to know how the plant is behaving.”

Another message was to water infrequently and deeply, taking conditions into account. One problem that many growers face is varied soils. Hillside vineyards clearly require a different treatment than deep valley soils, and clay with its high water-holding ability is clearly different than sandy soil or even the sandy loam or loam sand that is more common in Napa Valley.

Most of the speakers mentioned withholding water early to reduce berry size, although Mother Nature can ruin those plans.

Mary Maher, the viticulturist at Harlan Estate, was the first speaker. She mentioned that the old system was to water weekly, perhaps for eight hours at a time, without much adjustment for conditions. That’s changing, as vineyard managers attempt to be more selective.

Maher said that Harlan attempts to maintain early season water deficit: With Napa’s typical wet winter and spring, that can be difficult. She uses double irrigation lines to help compensate for different soils and uses cover crops to try to help even out water content in the vineyards early in the season.

“It’s tough to monitor soil moisture on hillside vineyards,” like those she farms, Maher admitted. Historically, growers depended on visual clues, but those can be misleading. Vines adjust to conditions, and they may be perfectly healthy even if the leaves seem to be drooping.

Of late, Maher’s been using five sap flow sensors and vine transpiration numbers to maintain an early season deficit.

She said it turns out they were watering too early at Harlan. “We had been irrigating rocky soils frequently and deep. We were watering twice a week. Now we water every 12 days, but for longer times.” She’s even stopped irrigating some blocks. At sister Napa Reserve estate, which is in deep soils, less irrigation is needed overall.
Steve Matthiasson

Steve Matthiasson
Another grower, Steve Matthiasson of Premier Viticultural Services, clearly regrets having to irrigate. “Less is more for terroir-driven viticulture,” he said. “Irrigation is like baby formula. It’s never as good as the real thing: vines getting deep water.” He added, “Every time I irrigate, I wince. Will it be right? We’re screwing with the plant, and it can make or ruin the wine.”

He pointed out that drip irrigation confuses the plant. Part of the roots get the water that tells them to open their stoma and suck in water, while dry roots would tell the stoma to stay closed and conserve the vine’s resources. Drip watering, especially if it’s shallow, encourages shallow roots clustered under the emitters. Many studies have been done on water transmission through the soil, but at best, in heavy soils the zone is no more than a couple feet wide. On the other hand, there’s no sense in putting water where there aren’t any roots.

Matthiasson said that a vine needs a certain amount of water for the season, and can stand a fixed deficit. “You have to distribute it at the right times.” For him, those times include veraison and at about 20° Brix. He added that the irrigation regimen can affect wine style, from austere to lush.

Tips from technical experts
Talks from three technical experts followed the growers’ perspectives.

Dr. Paul Anamosa of Vineyard Soil Technologies presented a tutorial on soils and plant water usage, important to better understand the best strategies for irrigation. One important practical hint: “Water at night. It saves water since the plant won’t turn on and immediately transpire the excess.”

Scholasch discussed means of measuring water, from soil moisture to pressure bombs to porometers to visual clues. He pointed out that all have their limits, since they may not measure the most important factors in a plant’s activity. He said that measuring actual sap flow, a relatively recently commercialized technique, is the best approach, although it must be combined with external data such as evapotranspiration.

Dr. Mark Greenspan of Advanced Viticulture in Sonoma concurred with the others: “Good water management means coming close to not irrigating at all, or at least as little as possible, especially in the valley floor. Irrigation can be detrimental.” He adds, “We actually live in a wet climate. It’s just that the water comes only in the winter. Irrigation is just a patch to get through the summer.”

Greenspan commented that irrigation wets only a small part of the root zone, which is undesirable. It can lead to a number of problems: excessive vigor and berry growth, leaching of nutrients in the soil and reduction in terroir.

He suggested delaying irrigation as long as possible. Once you start, don’t cut it off, because the vine likely has come do depend on it.

Most of all, Greenspan warned growers: “Get used to different conditions each year. Climate change means more variability. You will have to manage each year separately.”ShareThis