There comes a point when more water does no good. The vines have made use of all they can and any extra does nothing for them.
“You’re essentially just wasting it,” Washington State University viticulture researcher Markus Keller told growers at the Washington State Grape Society meeting in Grandview, Washington, in November. That critical point where more water has no effect on juice grapes was one of Keller’s major points about irrigating wine and juice grapes.
In some ways, wine and juice grapes are the same. For both, temperature and canopy size drive water use. Also, both types of vines need most of their water between fruit set and veraison. After that, many characteristics about the crop have already been set, including berry size. No amount of irrigation can make up for berry size after the fruit turns color.
Back to that critical point where more is not more. Keller and his team made that conclusion after six years of irrigation trials at the WSU research vineyard, funded by the Washington State Concord Grape Research Council.
Reducing irrigation to 50 percent of evapotranspiration, or ET, before veraison resulted in a 15-percent smaller canopy, 10 percent fewer clusters, 8 percent smaller berries and a 14-percent drop in yield. However, varying irrigation levels between 75 percent and 150 percent of ET made no consistent differences in those same characteristics.
“This was quite illuminating for us,” Keller said. In light of his findings, he suspects some growers with overhead sprinklers and furrows may be wasting water.
Also, when Keller reduced water supply to 50 percent of ET after veraison, he saw no decrease in canopy size and yield. So, in a drought year, juice grape growers could save water with no yield penalty by applying 75 percent to 100 percent of ET before veraison followed by 50 percent of ET after veraison.