New Strategies in Deficit Irrigation Increase Yield and Quality

By Dennis Farrell, Student Writer

Researchers in the WSU Viticulture and Enology program are finding that what was common knowledge regarding irrigation strategies may not hold true when taking a closer look at grapevine physiology.

The industry-funded research focuses on regulated deficit irrigation (RDI), a common water conservation practice in Washington, in which plants receive less irrigation during a specific time of the growing season. In Europe, wine grape growers typically water early in the growing season and then stress the vines by drastically reducing or even eliminating irrigation later in the season—during ripening and close to harvest—to ensure small berries and higher concentrations of tannins and other desired compounds in the grapes.

Markus Keller, WSU professor of viticulture and leader of the research team studying deficit irrigation, said this European approach can cause growers to lose 10 to 20 percent of their yield due to grapes shriveling up. According to the Viticulture and Enology Program’s research results, this practice simply isn’t necessary to produce high quality wine grapes.

Grapes are studied at WSU IAREC in Prosser, Wash.
Grapes are studied at WSU IAREC in Prosser, Wash.

Grapes are studied at WSU Irrigated Agriculture Research and Extension Center in Prosser, Wash.

Keller said one of the goals of the late-growth-period stressing is to prevent the berries from getting too large, but in reality, the plants are able to transfer water back to the leaves from the berries to help maintain a fairly consistent size.

“We’re finding that when you add water late in the season, rather than berries increasing in size, it just prevents them from shriveling up,” Keller said.

For the best quality grapes while maximizing water conservation, Keller said the vines need to be stressed earlier in the growing season—before the fruit starts to ripen—and then alleviate some of that stress during ripening by applying water.

The growing season for grape vines is roughly divided into three growth periods, with the middle period being when the vines should be stressed. This is the time between the plant setting fruit and the beginning of ripening (veraison).

Keller said their findings have been met with some skepticism, but he and the other researchers believe the results.

“We’ve done the work, with Jim Harbertson at the Wine Science Center making the actual wines to see how they turn out, so we’re pretty confident that what we are finding is actually right,” Keller said. “We’ve done this for a number of years now.”

The European belief in vine stressing during ripening and right before harvest has become almost universal among winemakers there and they often tell growers not to irrigate as harvest approaches, Keller said. But this water loss can damage both the crop and the vines themselves as they go into winter with dry soil and cannot properly acclimate to cold winter temperatures.

Results of this research at WSU indicate that stressing vines in the middle of the growing season rather than at the end creates not only a better yield and healthier vines for the grower, but better grapes for the winemaker as well. This is because applying water makes the leaves more functional in producing sugar and increases movement of sugar into the berries.

“It will give you better quality, potentially, because grapes ripen by accumulating sugar and converting some of it to those other components that eventually make up wine quality,” Keller said.

However, the degree to which a vine should be stressed is not consistent across all wine grape varieties. For example, Keller said, the Viticulture and Enology Program has been testing RDI for white wine varieties.

“We’re working with Chardonnay and Riesling now more than ever before and we’re finding that maybe we should not stress them too much this early in the growing season,” Keller said. “Again, it comes down to irrigating according to the end use of the grapes.”

Keller said they are even finding differences with irrigation among white varieties but that is preliminary and research is continuing.