"The concept is known as deficit irrigation. You give the vines less than 100 percent of their actual water needs prior to veraison, or ripening," said Ed Hellman, Texas Cooperative Extension viticulture specialiast based in Lubbock. Hellman has a joint appointment with Texas A&M University and Texas Tech University.
"Deficit irrigation uses less water. When done correctly, it also influences the amount of canopy the vines produce."
A small canopy has several advantages. Vines require less water throughout the growing season. More sunlight reaches present and future cane buds, where vines flower and set grapes. Sunlight, in turn, promotes fruit quality of ripening grapes.
A small canopy requires less annual pruning and allows more air circulation in the vines, which reduces the potential for plant disease. Regulating irrigation prior to ripening may also promote cold-hardiness of canes that produce fruiting buds.
"There are more than 3,000 acres of wine grapes in Texas, but there has been little research aimed at generating irrigation guidelines," Hellman said. "Deficit irrigation is a new concept in Texas. It shows promise, but it will probably only work in low rainfall areas such as West Texas - where most of the plants' water needs are met by irrigation."
Hellman and research associate Ashley Basinger began evaluating regulated deficit irrigation at the Newsom Vineyard near Plains, Texas, in 2002. Their research is part of Basinger's graduate studies towards a master's degree at Texas Tech University. They also tried a deficit irrigation strategy known as partial root zone drying, which originated in Australia. With partial root zone drying, irrigation water is alternately applied to only one-half of a vine's roots.
"We tested regulated deficit irrigation against a luxurious irrigation rate in one trial, and partial root zone drying against uniform irrigation in another trial," Hellman said. "In both experiments, the amount of water we applied was based on the plants' thirst level - their evapotranspiration rates."
Basinger timed irrigations in both experiments by regularly checking the moisture status of vines with a hand-held meter. "It's very important to know the vine's moisture status," Hellman said. "You don't want too much of a moisture deficit, and you don't want the vine to get thirsty at all during certain stages of growth.".
At the end of each growing season Basinger and Hellman compared grape yield, fruit composition and the cold hardiness of buds in both experiments.
"Our results found no advantage in partial root zone drying. Where we applied a limited amount of water had no effect on yield, fruit quality or the vine's cold hardiness," Hellman said. "The real effect was in how much water we applied and when. "We saw a slight yield reduction, better cold hardiness of fruit-producing buds and lower pumping costs with regulated deficit irrigation. Vines that received all the water they needed and more had a slightly higher yield, higher pumping cost and were not as cold hardy."
The results are still out on whether deficit irrigation affects grape and wine quality. A comparison of wines made from deficit irrigation grapes and fully irrigated grapes, by vintner Craig Parker at Prairie Creek Estates winery, is pending.
Even so, this research puts growers a step closer to acceptable wine grape yields from cold-hardy vines that require less water. "Deficit irrigation looks promising, but it is not a beginner's tool. Growers need to be well-versed in managing a vineyard using full irrigation before they try it," Hellman said. "Deficit irrigation requires a higher level of management, but it can make a difference in areas with limited water."