Withholding water from this Columbia Valley Merlot vineyard desiccated the basal leaves by mid-July, eliminating the need for leaf removal.
Using deficit irrigation in Washington State on wine grapes, particularly red varieties, has many benefits—smaller berries, better fruit exposure, and more open canopies for spray penetration and air circulation. But the biggest advantage is cost.
"The main reason we should be practicing deficit irrigation is a savings in costs," said Dr. Russell Smithyman, director of viticulture for Ste. Michelle Wine Estates. Hand leafing, at $70 to $100 per acre, and mechanical leafing, at $10 to $20 per acre, are both costly, especially when done early in the season, he said. "If you remove leaves at bloom time, it can cause a reduction in fruit set, and, more than likely, you’ll have to come back and do it again later because the canopy is still growing."
Smithyman shared the philosophy, techniques, and concerns of deficit irrigation with growers attending the annual meeting of the Washington Association of Wine Grape Growers earlier this year.
Before following a deficit irrigation strategy, goals for canopy (average length of shoots, number of leaves per shoot, buds per shoot, canopy gaps, fruit exposure), quality, and yield must be developed for the vineyard.
The deficit irrigation philosophy that guides Ste. Michelle Wine Estates in their irrigation scheduling boils down to one early season irrigation prior to bloom to make sure there is enough water to grow the desired size of canopy.
"Then there’s no more irrigation until the canopy has reached the desired size and shoot growth has decreased," he said. Both criteria must be met before water is turned back on. He added that there may be slight tweaking of the irrigation scheduling depending on the site and variety.
"When the shoot tips are burned off, then we know that the canopy is not growing anymore," Smithyman explained.
"Where most of us err—and where the hardest aspect of sticking to the deficit philosophy—is we start water too early before the vines have really stopped growing," he said. Last year, with the vines developing late, the vines were still growing the first week of July when temperatures heated up. "We finally got the conditions of 100°F temperatures we needed to stress the vines, but growers were afraid to wait and began putting on a little water before it was time."
Once the two criteria of shoot length and no more shoot growth have been met, then growers must aim to keep a steady continuum of moisture in the soil profile, balancing the estimated vine ET (evapotranspiration values for vines are available on Washington State University’s AgWeatherNet Web site) with the amount of moisture available in the soil. Vine ET is a projection of how much water the vine will use in a given period and serves as a baseline, moving up or down depending on the weather forecast.
Deficit irrigation requires more management because water is being applied in a more precise manner, with less room for error, he noted. Growers need to be in the vineyard to "read" what the vines are telling them.
There are a number of devices that can help growers assess soil moisture status and water available to the vine—neutron or other types of soil probes, soil moisture sensors, pressure bombs measuring the leaf water potential, a new leaf Porometer that records plant water status by measuring stomatal conductance in leaves—and of course, the old-fashioned technique of squeezing soil and digging pits.
Smithyman said they do not use pressure bombs much anymore because of the time constraints in collecting data, but they do use soil moisture probes and vine ET values in determining how much water is needed each week.
While keeping the soil moisture at a steady number is important, Smithyman said, "Really, our overall goal is to open up the fruiting zone and get those basal leaves off the canopy, saving $70 to $100 per acre in doing so."
Not all vineyards adapt easily to deficit irrigation, he acknowledged. For some, reducing canopy growth through deficit irrigation is hard to achieve because the site has a high capacity to retain soil moisture. "They may have a deep soil profile, deep root system, with soils that are harder to dry out."
Often, you won’t see the effects in the first year, he said. "But in the second year, the vines tend to fall in line and not be invigorated. In one particular block, we saw huge improvement in wine quality the second year of following deficit irrigation and the vineyard moved into our reserve wine program."
He added that they have learned that deficit irrigation doesn’t work well for most white varieties because they need larger canopies and more water for the desired wine quality.
Growers unfamiliar with deficit irrigation are often worried or concerned about the health of vines. Are deficit irrigation strategies harmful to wine quality or the long-term health of the vine?
You will see smaller berries from deficit irrigation, which may equate to about 10 to 15 percent lower yields, Smithyman said. But retaining a few more buds during pruning can easily compensate for the loss of yield due to deficit irrigation.
Smithyman has conducted several deficit irrigation trials, with the first dating back to 1999. As a graduate student, he worked with former Washington State University viticulturist Dr. Robert Wample, who found that deficit irrigation worked well in Washington vineyards. In trials at Ste. Michelle, Smithyman has compared deficit irrigation with 70 percent ET after reaching the desired canopy growth with even further deficit irrigation of only 35 percent ET.
"From five years of research, we never once saw differences in cold hardiness among treatment vines," he said. During the trials, pruning weights and yields were similar each year without great fluctuations.
"Deficit irrigation is a sustainable practice," he concluded. "You can do this and not worry about having a spiraling decline of the overall health of the vine."
But if growers are still worried about trying deficit irrigation strategies, he suggested they alternate blocks within a vineyard if irrigation systems allow, irrigating some under standard and some under deficit.