November 2007 Issue of Wines & Vines

Off-Dry Farming

Low water input is the sustainable standard

by Larry Walker
Low water input is the sustainable standard
The wine that took top honors at the famed Paris tasting of 1976 and helped put Napa Valley on the world's wine map, Warren Winiarski's 1973 Stag's Leap Wine Cellars Cabernet Sauvignon, was dry farmed, Winiarski confirmed to Wines & Vines. He recalled that the field-grafted rootstock was given some water the first year, then nothing. The vineyards were later put on drip irrigation.

"We have recently been cutting back dramatically on water use," he added. Winiarski said they have learned that when pressure bombs and other means of measuring moisture are used, the vine is often given what it wants, not what it needs to set quality fruit. He said water use in the past two years has been cut from as much as 75 gallons per plant to 35 or 40 gallons. The cutback in applied water included additional leaf dropping as a way to restrict water use.

"We found that by giving the vine all the water it wanted, we were just adding to leaf growth and vigor, and extending the growing season," he said.

Using less water for vineyard irrigation is generally seen today as one of the standards of sustainable viticulture. Even growers who don't feel they can dry farm, are looking at ways to decrease water use. Some are doing both.

Jean-Pierre Wolff (see Dry, Dry Again) is dry farming a Chardonnay vineyard, but taking innovative approaches to lower water input irrigation on the rest of his vineyards. One of the keys he uses to measure moisture need is evapotranspiration (ET) of the vine. (Evapotranspiration is the water lost to the atmosphere by two processes--evaporation and transpiration. Evaporation is the loss from open bodies of water, such as lakes and reservoirs, wetlands, bare soil and snow cover; transpiration is the loss from living plant surfaces.) Wolff considers it an excellent measure of water stress in the vine.

"One hundred percent would mean the vine was totally unstressed. As the rate drops, the rate of stress increases," Wolff said. "One hundred percent would be like watercress. The evapotranspiration rate is really what deficit irrigation is all about," he added. "One thing we tend to forget is that the vine is a wild plant. It is hardy. It's pretty hard to kill a vine once it is established. But the point is not to just keep the vine alive, but to make quality wine."

Wolff said the ET rate on his dry farmed vines is usually around 15%. "They would get that much moisture from the fog. We are only a few minutes from the Pacific."

Low water input is the sustainable standard
Jean-Pierre Wolff
Wolff uses a combination of soil sensors and weather station data to measure the ET rate. "The cost of that kind of technology is dropping," he said. "You need that information to establish a good base."

He said that assuring an accurate delivery system for the water is important. He uses pressure-compensated drip emitters that are built into the line, and a variable speed pump in order to maintain constant pressure. "You just have to apply these things to the site, then tweak and adjust," he said.

Wolff also uses the standard pressure bomb to measure moisture in the leaf. "However, that method is extremely dependent on the skills of the technician who takes the measurement. It is also sensitive to when you do it--is it a foggy morning, a sunny afternoon? The pressure bomb is just another tool, not the silver bullet."

Wolff is still looking for the ideal weather station. "I want the data to be available to my vineyard manager in his office or home, and I want a wireless system that can be used for mildew spray control."

He has recently changed his use of cover crops. "I'm using native grasses now and even in a drought year like this, they are working very well. They are not so aggressive in sucking up moisture, they require less mowing and are effective in erosion control."

Finally, Wolff uses his eyes. "I look at the shoot tips and tendrils. You can see if the vine is showing signs of dehydration or stress if they begin to look wilted."

Dr. Cliff Ohmart is research/IPM director for the Lodi-Woodbridge Winegrape Commission and a Wines & Vines columnist. Ohmart said that it is hard to lay down a firm definition for deficit irrigation, because it varies from site to site, region to region. "Deficit irrigation is technically any irrigation that is less than the full water use demand from a vine," he said.

Ohmart said Lodi growers overall have cut annual irrigation use by about 35% using deficit irrigation techniques. "We normally get about 16 to 18 inches of water a year in Lodi, and full water use for a vine here requires adding another 18 inches or so via irrigation," he said.

Again, it is clear that it is necessary to go back to the specific site to determine irrigation needs--or perhaps decide to turn off the water tap altogether.

Ohmart added, "Probably the biggest reason Lodi growers do not dry farm is that the yield would be too low.

"As you are well aware, Lodi growers do not get the amount of money per ton of grapes that the quality deserves, and as a result must weigh tonnage issues with quality issues. It is a real catch 22, and dry farming is just not economic."
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