In the wild, Vinifera grapes can grow under very dry conditions with little water. But to produce high quality fruit for premium wine, when grapes receive water is as critical as the amount, says Washington State University’s Dr. Markus Keller.
Wine grapes are quite drought tolerant and can get by with less than 20 inches of water in a season, said the WSU viticulturist. Keller has observed ultrapremium red wine grapes grown on 12 inches of total rainfall and irrigation, without adverse effects.
But the plants need some water during eastern Washington’s hot, dry summers when temperatures climb. Most wine grape growers in the state use microirrigation (drip or sprinkler) to apply supplemental water at key times. Knowing when the vine needs and uses water can help guide irrigation strategies and schedules.
From bud break to bloom, the vine needs very little water, Keller said, but some water is needed from bloom to fruit set to ensure there’s an adequate crop. The majority of the water is needed from fruit set to veraison, before the fruit begins to ripen. Most of the water at this time is taken up by young roots that die quickly and must be constantly replaced by new root growth. “During fruit set to veraison, you’re not just feeding fruit, but you’re also feeding roots,” Keller explained.
As the leaf area of a canopy increases, so does its water use. A square meter of leaf area will use an average of two to three liters of water per day due to transpiration and water evaporation.
Tailoring water stress
One of the advantages of growing grapes in the semiarid environment of eastern Washington is the grower’s ability to meter out irrigation to better control canopy size, vigor, and berry size, compared with areas with abundant rainfall throughout the growing season. Research has shown that some stress can improve fruit quality, but when and how much should the vine be stressed?
“The vine can tell you a lot about the plant and water status if you’ll get out of the pickup and look at it,” Keller said. “Shoots and tendrils climb when they are growing,” he said, adding that if shoot tips are longer than the tendrils, the vine is under stress. Leaves will drop off the canopy when vines are under severe stress.
Not all wine grape varieties are alike in their water use. “Some are pessimists, and their stomata close, stopping transpiration, and the water status in the leaf remains stable,” he explained. When there’s just a little water stress, the vine thinks “Oh my gosh, I’m going to run out of water, and the stomata close and growth declines,” he said. Pessimist varieties include Grenache, Tempranillo, and to some degree Cabernet Sauvignon. Optimist varieties, ones that believe more water will come, so they keep their stomata open, include Chardonnay, Riesling, Syrah, Sangiovese, and Merlot.
“Water stress applied in the vineyard has to be tailored to your particular site and varieties,” Keller told growers attending an irrigation session held during the annual meeting of the Washington Association of Wine Grape Growers in February. “You have to make observations in the vineyard, know your varieties, walk the rows, and talk to your vines.”
And, the later the plant is stressed, the less effect it will have on the yield of the crop, he noted. Moreover, research has shown that repeated water deficit after veraison, to the point that leaves wilted, resulted in less sugar in the berry at harvest than when water was reduced before veraison.
Other research comparing regulated deficit irrigation in four treatments, applied before and after veraison, also found that fruit quality was significantly influenced by the timing of stress. “If you want to maximize the anthocyanins and skin tannins, you have to stress early in the season,” Keller said. “You can’t compensate for mistakes you made earlier by stressing during ripening.”
In general, moderate water deficit improves a host of fruit quality attributes, such as berry size, seed maturity, acidity, pH, tannins, flavonols, and color. But too much stress can negatively affect fruit quality, he emphasized.
In Washington, regulated deficit irrigation has been the strategy for most wine grape growers. However, Keller believes that partial root-zone drying, used extensively in Australia and Israel, may have benefit for some Washington growers. Both irrigation strategies impose some water deficit to constrain canopy growth, and control vigor and berry size, though they differ in their approach.
Regulated deficit irrigation imposes soil water deficit over time and is applied after fruit set. It results in a plant-water deficit and reduces berry size and yield.
Partial root-zone drying imposes soil water deficit over space and is applied throughout the season. With two drip irrigation lines running on both sides of the vine, the technique is more expensive, but by alternating which side receives irrigation, there is no plant water stress, but berry size and yield are controlled.
Keller suggested that growers struggling with yeast-assimilable nitrogen concentrations in their fruit could use partial root-zone drying to improve the nitrogen status in their fruit. Studies have found more nitrogen in fruit from partial root-zone drying than regulated deficit, with the same amount of water added, he said. Low levels of yeast-assimilable nitrogen in the must can cause problems during winemaking from slow or stuck fermentations, off flavors, and high levels of hydrogen sulfide.
Keep in mind that red and white varieties have different irrigation needs, Keller said. Berry size and sun exposure are more important in red varieties than in white varieties. He shared the following conclusions:
—Moderate water deficit is desirable for red wine grapes, but for whites, a mild deficit can improve flavors.
—Regulated deficit irrigation is useful in most red varieties, but avoid early stress during fruit set (can affect bud fruitfulness), midseason excess (can encourage large berry size) and late-season stress (can result in berry shriveling and reduced yield).
—Regulated deficit irrigation is less suited to white grapes than partial root-zone drying.
—More water might be beneficial during hot seasons or for warm sites with early white varieties and might delay ripening to help to retain acid and flavor.
—Impose water stress on white varieties with care. It is easy to overdo and negatively affect yield and quality. •