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Wise use of deficit irrigation can help growers make the most of limited water supplies and pay off in concentrated flavors—at least in red grapes.

Dr. Markus Keller, a viticulturist at Washington State University in Prosser, discussed regulated deficit irrigation from the vine’s point of view at the British Columbia’s Wine Institute viticulture conference in Penticton, British Columbia, Canada.

Unlike partial root-zone drying, which stresses a part of the vine by limiting water to a portion of a vine’s root system, deficit irrigation reduces the total amount of water a plant receives to induce stress for a set period of time.
The practice aims to produce a specific goal, such as a particular canopy size or enhanced fruit characteristics.

The point at which a vine begins to experience stress will depend on several factors, Keller said. Since deficit irrigation focuses on withholding water, seasonal variations in climate such as the amount and frequency of rainfall and rate of evaporation will affect its actual practice. Soil conditions including the texture, amount of organic content, and depth roots can go in a given location will also play a significant role in determining how much water to withhold.

A vine’s exposure to sun and its age will also influence watering decisions. Vines become thirstier as they age, lowering the threshold at which the vine begins experiencing stress.

Timing

Timing is also an important factor, Keller said, urging growers to identify their goals so that vine stress occurs at the right time to achieve the desired effect.

The distinction is important because vines require different amounts of water at different points in the season. About 60 percent of a vine’s water uptake occurs between fruit set and veraison, Keller said, then 20 percent from veraison to harvest.

The later in a season stress occurs, the less effect it will have on yield and berry size, Keller said, but he also cautioned against inducing stress too early and inhibiting vine development.

Sandor Mayer, viticulturist and winemaker for Vincor International Corp.’s Inniskillin Okanagan Winery in British Columbia’s southern Okanagan Valley, said he typically withholds water from vines until a little after bloom.

“We don’t irrigate, unless it’s very necessary, until the last minute,” he said. “Last year, for example, we were able to hang the vineyard without applying water until bloom, and in fact, almost a little bit later than bloom.”

Mayer’s rationale is that the lack of water between bloom and berry development has a formative impact on the berries; afterwards, it’s simply a case of feeding the maturing berries.

Further withholding of water can produce smaller berries, but Mayer is cautious, noting that white grapes may lack the delicate or intense fruit characteristics that define Inniskillin’s wines if the berries are too small.

“You get smaller berries, yes, but you might also get overripe fruit,” he said. “So, you have to be very careful.”

Keller cited research by French, German, and Australian researchers for tips on the impact of deficit irrigation on specific varieties.

Cabernet Sauvignon, Grenache, and Tempranillo have so-called pessimistic vines whose stomata tend to close in response to vine stress, while Chardonnay, Syrah, and Sangiovese tend to continue growing until they collapse from lack of water.

Overall, Keller believes regulated deficit irrigation may be more appropriate for red varieties where rich, juicy flavors are desirable.