Vineyards reckon with unknowns during drought
ASHLAND — A recent study by an Oregon State University researcher found wine grape growers could cut water usage on their crop by half of what was previously recommended, but many Rogue Valley vignerons already have a grasp on how to irrigate sparsely, facing extreme drought conditions.
According to the study, published June 3, “wine grape water usage estimated by the U.S. Bureau of Reclamation’s AgriMet network for irrigation was 44% higher than necessary.”
The study was performed by assistant professor and viticulturist Alexander Levin through the Southern Oregon Research and Extension Center. Results offer local grape growers the opportunity to “optimize irrigation scheduling for improved fruit and wine quality,” Levin said.
AgriMet sets wine grape water needs at 20.2 inches annually in Southern Oregon — nearly double what Levin calculated is required, depending on the season.
From south of Eugene to the California state line, wine grapes represent more than 9,000 acres, 25,000 tons of grapes and $50 million in raw value, according to the University of Oregon Vineyard and Winery Report. Factoring in the market value of wine, grapes are worth about $238 million in Southern Oregon.
Keeping the industry — and the grapes — healthy is a delicate balance of intelligent irrigation, according to John Pratt, president of the Rogue Valley Winegrowers Association.
Between 2017-2019, Levin gathered data at two vineyards in Jackson County, using solar panels to estimate evapotranspiration — how much moisture a plant breathes during photosynthesis — and recalculate a crop coefficient for the area.
Industry-accepted crop coefficients used to calculate irrigation were developed in California, where different growth trends, water needs and cultural practices alter the input data, he said.
“As the growing season progresses, vineyard water requirements change with grapevine canopy growth and warmer, drier conditions,” Levin said. “For growers to establish sustainable irrigation practices, they need improved crop coefficients that take into consideration management practices and climate.”
Levin said the two vineyards he studied — one in the Applegate near Grants Pass and the other near Jacksonville — participated in other studies and adequately represented Rogue Valley vineyards as a whole. The Applegate site has sandier soils while the Jacksonville site is more clay-rich, he said. Yields at both sites edged slightly above the state average. Water table depths at each site were not measured.
A small number of sites with deeper soils and higher annual rainfall, such as toward the Illinois Valley, require less irrigation than in the Rogue Valley, Levin said.
“Of course, more would be better,” Levin said regarding the study’s sample size. “But given the disparate sites and their unique characteristics, I feel confident that the estimates are accurate. The methods we used to make the estimates are grounded in sound theory and are well established.”
Growers can optimize irrigation scheduling by implementing an irrigation management plan that is developed, executed and evaluated for necessary changes at the end of each season, Levin said.
In developing a plan, growers should set specific management goals for their crop, consider challenges such as water supply limitations, take inventory of resources and soil characteristics, outline a chronology of planned actions, create a list of contingencies depending on challenges and relevant environmental threats, and decide how to measure performance, he said.
Pratt said Levin’s presence and research in the valley is a “tremendous asset” to local winegrowers. Previously, high turnover in the research position left producers without an avenue to address major concerns for the region regarding viticulture, he said.
“Year in and year out, that’s our biggest challenge, is making sure that we are doing the right thing, conserving water and using water in such a way that it really maximizes the potential of our fruit to make good wine,” Pratt said. “It’s one of the biggest impact features of managing a vineyard that has a significant impact on the flavor of wine.”
In a region focused on high-quality wine and small-scale production, growers cannot afford to be lax when it comes to appropriate water usage, he said, especially facing limited access to Talent Irrigation District water.
According to TID, water access will likely end in early August, though an end date has yet to be set. As of Monday, the reservoir system sat low, with Emigrant Lake 25% full, Hyatt Lake 19% full, Howard Prairie 7% full and Agate Lake 69% full.
Ashland Public Works Director Scott Fleury said recent rainfall “is just a drop in the bucket of what is really needed” to replenish the system.
Roots in well established vineyards can reach more than 20 feet deep. Pratt’s 10-acre vineyard, located on a Medford hillside, features shallow soils and rocky subsoils about 18 to 36 inches down, impeding the plants from establishing deep and drought-resilient roots, he said.
If the leaves on his vines begin to yellow due to lack of water before the fruit finishes ripening — a 45- to 60-day process starting around mid-August, depending on the varietal — the plant is no longer producing the carbohydrates that ripen the fruit, he said.
“It will desiccate, it will dry, but if there’s not enough sugar in it and the flavors aren’t developed, then it’s not going to be harvestable,” said Pratt, a 15-year wine grape grower.
Crop insurance that vineyards purchased in response to prolonged wildfire smoke events may cover about half of potential crop losses, but it’s not a “panacea” for the problem, he said.
With drought concerns in mind, Pratt said he does not believe Rogue Valley wine grape growers are currently using too much water. Established farmers pay close attention to the latest irrigation management research and current drought conditions, he said.
“As an industry, we are very careful about how much water we apply, and we don’t apply more than is needed,” Pratt said.
Locally, growers predominantly use drip lines to irrigate their crop — the most water-efficient method, he said, far above sprinklers or flood irrigation.
Pratt uses a “pressure bomb” to test his plants’ hydration status and soil moisture probes that monitor different levels of earth. The amount of pressure required to produce water from a cut leaf through the pressure bomb serves as an indicator for when to start watering, he said. Most producers have not yet begun.
Watering does not begin until the soil is fairly dry, without allowing the plants to wilt. Overwatering encourages the plants to produce new vegetation, rather than concentrating photosynthetic energy to the fruit, Pratt said. As the fruit ripens, surrounded by mature leaves, there should be no new leaves on the vine.
“With this rain that we’ve had in the last few days, we won’t be turning [the water] on for a long time,” Pratt said. “The plants will have plenty of moisture and we won’t have to worry about irrigating our fields until well into July. At that time, we have to be really careful not to water too much.”
After testing evapotranspiration, Pratt replenishes about 60% of the water that the plants released. An ideal balance keeps the vines in a “semi-stressful position,” just above the wilting point, he said.
Evidence shows that allowing morning sunshine to reach the fruit helps develop colors and flavors in the wine, meaning the canopy cannot be too thick or cluttered, he said. Allowing sun and breeze to reach the plant reduces the likelihood of developing mildew — a consequence of overwatering and unchecked vegetative growth.
“No winery will accept fruit that has damage from mildew,” Pratt said. “One of the reasons that we limit the water, not only for the beneficial effect and concentration of flavors in the grape berry ... it prevents that huge vegetative jungle-like thing in the vine that leads to mildew.”
With the possibility of TID shutting off in early August, Pratt said, growers are stepping into the unknown. While grape vines have adapted to be highly resilient to various climate conditions, studies have yet to reveal what happens if irrigation ceases before grapes finish ripening, he said.
Some vineyards have access to ponds and wells to supplement water supply, but without TID, Pratt’s well provides just enough for the house and minor landscaping, not the 10-acre vineyard.
With standing contracts to deliver fruit come September and October, Pratt said he dreads the possibility he won’t have ripe fruit to give.
Contact Ashland Tidings reporter Allayana Darrow at email@example.com or 541-776-4497.